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 UNIVERSITY OF CALIFORNIA. 
 
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 FLAM MARION'S POPULAR 
 ASTRONOMY 
 
 Translated from the French by J. ELLARD GORE, F.R.A.S. 
 With 3 Plates and 288 Illustrations. 
 
 " The six books into which the book is divided give a very lucid and accurate 
 description of the knowledge which has been acquired of the moving bodies of space, 
 both as respects their motions and physical constitutions. Of the translation we can 
 only speak in terms of praise. Not only does it well represent the original, but 
 Mr. Gore has added useful notes for the purpose of bringing the information up to 
 date, and has also increased the number already very considerable of the excellent 
 illustrations, so that the work is likely to become as popular in England as it has 
 been in France." ATHENAEUM. 
 
 "The work which Mr. J. E. Gore has translated into English has made for itself 
 a name and reputation in France . . . and has gone into general circulation to the 
 number of a hundred thousand copies. This last fact is proof how well within the 
 bounds of possibility it is to make the latest discoveries of science comprehensible 
 and fascinating to the common mind. M. Flammarion has attained this triumph 
 through the grasp of his knowledge, the lucidity of his style, and his power of 
 bringing home the most stupendous and complicated of the things revealed to us in 
 the depths of space. M. Flammarion's pages should find almost as great acceptance 
 in this country as in his own. Simplicity of arrangement and of statement are part 
 of his charm and of his success." SCOTSMAN. 
 
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 well take a high place in the rank to which they belong. It is full, lucid, and, thanks 
 to Mr. Gore's careful revision, well up to date. . . . Mr. Gore's edition is so carefully 
 brought abreast of the latest discoveries that the English student may now con- 
 gratulate himself on being in an even better position than the countrymen of 
 M. Flammarion." DAILY CHRONICLE. 
 
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 wonderful and fascinating of all sciences will find precisely what they seek in 
 M. Flammarion's eloquent and poetic chapters. . . . There are many illustrations 
 in this able and attractive treatise." SPEAKER. 
 
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 in popular language with some of the most interesting of the discoveries and specu- 
 lations of astronomers." DAILY NEWS. 
 
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 laborious service to the science, and he possesses a valuable faculty of popular 
 exposition. . . . The volume is profusely and well illustrated, some of the best plates 
 making here their first appearance." SATURDAY REVIEW. 
 
 " A high place must be accorded to Flammarion's ' Popular Astronomy.' Never 
 before has the science of the heavens been treated with such fulness and interest as 
 in this fascinating book ; for Flammerion is a man of letters as well as a man of 
 science a man of letters, too, endowed with the wondrous gifts of lucidity and 
 charm which distinguish the best French writers. . . . Flammarion's book is much 
 more absorbing than most novels, more romantic than most romances, more poetic 
 than most poems, yet strictly and scientifically accurate." LUDGATE MONTHLY. 
 
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 assists the reader to grasp astronomical theories a task in which less popular writers 
 often fail when they make the attempt." LITERARY WORLD. 
 
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 ... As a manual for those who wish to obtain a good general knowledge of 
 astronomy this work will be found unsurpassed." SCIENCE GOSSIP. 
 
 LONDON : CHATTO & WINDUS, in ST. MARTIN'S LANE, W.C. 
 
Crown 8vo, cloth, 2s. net. 
 
 THE STELLAR HEAVENS : 
 
 AN INTRODUCTION TO THE STUDY OF THE STARS AND NEBULA. 
 BY J. ELLARD GORE, F.R.A.S. 
 
 " The volume will be a very useful help for directing the observer's attention to 
 the various more conspicuous objects in the sky, and although it does not pretend 
 to take the place of that well-known friend of amateurs, namely, Webb's ' Celestial 
 Objects for Common Telescopes,' it will prove a serviceable guide." NATURE. 
 
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 opinion in regard to the stars generally. ... It is a very good book . . . the outlines 
 of a great subject in a concise form." BUILDER. 
 
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 a valuable help to students and observers who desire to think and understand for 
 themselves." DAILY NEWS. 
 
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 date. ... It is a compact and useful little volume ; and a list of the constellations 
 now in use, along with other lists of notable stars given in the Appendix increase 
 its value." SHEFFIELD DAILY TELEGRAPH. 
 
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 also full of interest to those who are only students of the labours of others, yet desire 
 to keep up with the progress of this specially important department of the sciences." 
 ATHEN.UM. 
 
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 those who are beginning direct observation of the wonders and beauties of the stellar 
 heavens and to those who may desire to possess in a handy form a good deal of the 
 latest detailed information respecting various objects of special interest. . . . Mr. 
 Gore is well known as one of the best among those writers who, while themselves 
 original workers in astronomy, undertake to popularise this great science, and 
 facilitate its investigation." GLASGOW HERALD. 
 
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 125 small pages an amount of information that should enable an intelligent student 
 to answer an ordinary examination paper on the subject with some approach to 
 completeness. . . . There is hardly anything in his little book that lends itself to 
 adverse criticism, while there is a great deal to commend, both as regards the 
 information supplied and the manner of supplying it." LITERARY WORLD. 
 
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 of value. . . . The work will be useful to astronomical students." BIRMINGHAM 
 POST. 
 
 "To those who wish to make themselves acquainted with the information that has 
 been acquired respecting the number and distances of the stars and nebulae, this 
 book by Mr. Gore is to be strongly recommended. ... A very interesting little 
 volume. " FIELD. 
 
 " To those who would have some real knowledge in regard to the stars, may I 
 recommend a little book which has just been published ' The Stellar Heavens ' ? 
 STIRLING SENTINEL. 
 
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 knowledge. The tyro in astronomy will find everything made intelligible to him, 
 and to the adept also the book will be serviceable. . . . His notes on the various 
 stars are good, and the book has a useful index." CHRISTIAN WORLD. 
 
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 student and the young observer." GUARDIAN. 
 
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 free from needless technicalities ; and the descriptive catalogues of the more interest- 
 ing double and variable stars will be found most helpful to the student who has 
 opportunities for telescopic observation." DUBLIN EXPRESS. 
 
 LONDON : CHATTO & WINDUS, in ST. MARTIN'S LANE, W.C. 
 
STUDIES IN ASTRONOMY 
 

THE GREAT NEBULA IN ORION. 
 
 From a Photograph by W. E. Wilson, F.R.S. 
 
STUDIES 
 IN ASTRONOMY 
 
 BY 
 
 J. ELLARD GORE 
 
 F.R.A.S., M.R.I. A. 
 
 ASSOCIATE OK THE ASTRONOMICAL SOCIETY OF WALES 
 CORRESPONDING FELLOW OF THE ROYAL ASTRONOMICAL SOCIETY OF CANADA 
 
 1 Willst du ins Unendliche straiten, 
 Geh' nur in Endlichen nach alien Seiten." 
 GOETHE 
 
 LONDON 
 CHATTO *f WJNDU'S 
 

 " Old Horace */ will strike? said /if, 
 
 ' The stars with head sublime J 
 But scarce could see, as now we see, 
 
 The man in Space and Time. 
 So drew perchance a happier lot 
 
 Than ours, who rhyme to-day. 
 The f-res that arch this dusky dot 
 
 Von myriad-worlded way 
 The vast sun-clusters' gathered blaze. 
 
 World-isles in lonely skies, 
 Whole heavens within themselves, amaze 
 
 Our brief humanities." 
 
 TENNYSON. 
 
PREFACE 
 
 MOST of the articles in the following pages have 
 been published during the last few years in The 
 Gentleman' s Magazine, Knmvledge, The Observatory, 
 etc., and my thanks are due to the editors and 
 publishers of these periodicals for permission to 
 re-publish them. The articles have been carefully 
 revised and partly re- written, and the information 
 brought up to date. The following articles have 
 not been previously published, " The Ring Nebula 
 in Lyra," "The New Star in Perseus," and "The 
 Coming Comet." For the illustrations my best 
 thanks are due to Professor Barnard, D.Sc. of the 
 Yerkes Observatory (U.S.A.), M. Henry, of the 
 Paris Observatory, and Dr. W. E. Wilson, F.R.S. 
 
 J. E. G. 
 
 June, 
 
 221662 
 
CONTENTS 
 
 CHAPTEB PAOl 
 
 I. THE SIZE OF THE SOLAR SYSTEM ... ... 1 
 
 II. JUPITER AND ITS SYSTEM ... ... 7 
 
 III. GIANT TELESCOPES ... ... ... ... 17 
 
 IV. THE DISTANCES OF THE STARS ... ... 32 
 
 V. THE SUN'S JOURNEY THROUGH SPACE ... ... 47 
 
 VI. THE STORY OF GAMMA VIRGINIS ... ... 59 
 
 VII. THE PLEIADES ... ... ... ... 67 
 
 VIII. GLOBULAR STAR CLUSTERS ... ... 75 
 
 IX. THE SUN'S STELLAR MAGNITUDE ... ... 85 
 
 X. THE SUNS OF SPACE ... ... ... 91 
 
 XI. STELLAR SATELLITES ... ... 103 
 
 XII. SPECTROSCOPIC BINARIES ... ... UJ 
 
 xiii. "THE DARKNESS BEHIND THE STARS" ... 130 
 
 XIV. THE NEBULAR HYPOTHESIS ... ... 138 
 
 XV. STELLAR EVOLUTION ... ... ... 154 
 
 XVI. THE CONSTRUCTION OF THE VISIBLE UNIVERSE 166 
 
 XVII. THE SECULAR VARIATION OF STARLIGHT ... 177 
 
 XVm. THE HERSCHELS AND THE NEBULJE ... 187 
 
x CONTENTS 
 
 CHAPTER PAGE 
 XIX. A CHAPTER IN THE HISTORY OF ASTRONOMY ... 207 
 
 xx. MESSIER'S NEBUUE ... ... ... 215 
 
 XXI. THE RING NEBULA IN LYRA ... ... 246 
 
 XXII. A GREAT BELGIAN ASTRONOMER ... ... 251 
 
 XXIII. SOME RECENT ADVANCES IN STELLAR ASTRONOMY 268 
 
 XXIV. THE NEW STAR IN PERSEUS ... ... 295 
 
 XXV. THE COMING COMET ... ... ... 304 
 
 XXVI. IMMENSITY AND MINUTENESS ... ... 308 
 
 XXVII. LIGHT, ELECTRICITY, AND THE ETHER ... 313 
 
 APPENDIX ... ... ... ... 327 
 
 INDEX ... ... ... ... 329 
 
ILLUSTRATIONS 
 
 TO FACE PAGE 
 THE GREAT NEBULA IN ORION ... Frontispiece 
 
 PHOTOGRAPH OF THE MILKY WAY NEAR MESSIER 11 ... 137 
 
 THE SPIRAL NEBULA, 51 MESSIER ... ... 152 
 
 STARS IN CENTAURUS (AL-SUFl) ... ... ... 183 
 
 THE GREAT GLOBULAR CLUSTER IN HERCULES (M. 13) 221 
 
 THE DUMB-BELL NEBULA ... ... ... ... 225 
 
 THE SPIRAL NEBULA, 33 MESSIER TRIANGULI ... 228 
 
 THE STAR CLUSTER 38 MESSIER IN AURIGA ... 230 
 
STUDIES IN ASTRONOMY 
 
 I 
 
 The Size of the Solar System 
 
 A] my readers are doubtless aware, the 
 solar system consists of a number of 
 planets revolving round the sun as a 
 centre, and of subordinate systems of satellites 
 revolving round the planets, or at least round 
 some of them. Our own earth is one of these 
 planets, the third in order of distance from the 
 central luminary, which forms the common source 
 of light and heat to all the members of the 
 system. In addition to the planets and satellites, 
 there are also some comets which form permanent 
 members of the solar system. Some of these 
 comets revolve round the sun in very elongated 
 orbits, while the planets and satellites revolve 
 in nearly circular orbits. A consideration of the 
 absolute size of this planetary system and its 
 relative size compared with that of the universe 
 of stars, or at least the universe visible to us, 
 may prove of interest to the reader. 
 
 B 
 
^STUDIES IN ASTRONOMY 
 
 To determine the size of the solar system, it is, 
 of course, necessary in the first place to ascertain 
 the dimensions of the planetary orbits with refer- 
 ence to some standard, or unit of measurement, 
 as it is termed. The unit of measurement adopted 
 by astronomers is the sun's distance from the 
 earth. As the earth is the third planet in order 
 of distance from the sun, the distance is, of course, 
 an arbitrary unit. We might take the mean 
 distance of Mercury from the sun as the unit, but 
 as we refer all our measurements to terrestrial 
 standards, and the diameter of the earth is used 
 in the measurement of the sun's distance, it is 
 found more convenient to take the earth's distance 
 from the sun as the standard of measurement for 
 the solar system and the distance of the stars. 
 
 The relative distances of the planets from the 
 sun have been determined by astronomical obser- 
 vations, and are represented approximately by 
 the following figures, the earth's mean distance 
 from the sun being taken as unity : Mercury, 
 0-387 ; Venus, 0'723 ; the earth, 1-0 ; Mars, 1-523 ; 
 the minor planets, 1-946 to 4-262 ; Jupiter, 5'2028 ; 
 Saturn,9-538 ; Uranus, 19-183 ; and Neptune, 30-055 ; 
 or taking the earth's mean distance from the sun 
 as 1000, the distance of Mercury will be repre- 
 sented by 387 ; Venus, 723 ; Mars, 1523 ; the minor 
 planets, 1946 to 4262 ; Jupiter, 5203 ; Saturn, 9538 ; 
 Uranus, 19,183 ; and Neptune, 30,055. These are 
 the mean or average distances, the orbits not 
 
THE SIZE OF THE SOLAR SYSTEM 3 
 
 being exact circles, but ellipses of various eccentri- 
 cities, that of Mercury among the large planets 
 being the most eccentric, and that of Venus the 
 least so. Among the minor planets the eccentri- 
 cities vary from 0, or a perfect circle, to 0'38, the 
 value found for a small planet discovered by 
 Stewart in August, 1901. 
 
 The first scientific attempt to determine the 
 sun's distance from the earth seems to have been 
 made by Aristarchus of Samos. His method was 
 to note the exact time when^fce moon is exactly 
 half full, and then to measu^Pthe apparent angle 
 between the centres of the sun and moon. It is 
 evident that when the moon is half full, the earth 
 and sun, as seen from the moon, must form a right 
 angle with each other, and if we could then 
 measure the angle between the sun and moon, as 
 seen from the earth, all the angles of the right- 
 angled triangle formed by the sun, moon, and 
 earth would be known, and we could deduce at 
 once the relative distances of the sun and moon 
 from the earth. This method is, of course, per- 
 fectly correct in theory, but in practice it would 
 be impossible, even with a telescope, to determine 
 the moment when the moon is exactly half 
 full, owing to the irregularities of its surface. 
 Aristarchus had no accurate instruments, and no 
 knowledge of modern trigonometry, but by means 
 of a tedious geometrical method he concluded 
 that the sun is 19 times further from the earth 
 
4 STUDIES IN ASTRONOMY 
 
 than the moon. This result is now known to be 
 far too small, the sun's distance from the earth 
 being in reality about 388 times the moon's 
 distance. 
 
 In modern times the sun's distance has been 
 determined by various methods. The most recent 
 results tend to show that the sun's parallax, as 
 it is termed, cannot differ much from 8*80 seconds 
 of arc. The solar parallax is the angle subtended 
 at the sun by the earth's semi-diameter. A 
 parallax of 8'80 seconds implies that the earth's 
 mean distance from the sun is about 92,800,000 
 miles. Multiplying this number by the figures 
 given above, we find that the mean distances 
 of the planets from the sun are as follows, in 
 round numbers : Mercury, 35,913,000 miles ; Venus, 
 67,094,000 ; Mars, 141,334,000 ; the minor planets, 
 193,024,000 to 395,513,000; Jupiter, 482,838,000; 
 Saturn, 885,126,000; Uranus, 1,780,182,000; and 
 Neptune, 2,789,104,000. This makes the diameter 
 of the solar system, so far as at present known, 
 about 5578 millions of miles. Across this vast 
 distance light, travelling at the rate of 186,300 
 miles a second, would take 8 hours 19 minutes 
 to pass. 
 
 But vast as this diameter really is compared 
 with the size of our earth, or even with the dis- 
 tance of the moon, it is very small indeed when 
 compared with the distance of even the nearest 
 fixed star, from which light takes over four years 
 
THE SIZE OF THE SOLAR SYSTEM 5 
 
 to reach us. The most reliable measures of the 
 distance of Alpha Centauri, the nearest of the fixed 
 stars, places it at a distance of 275,000 times the 
 sun's distance from the earth, or about 9150 tunes 
 the distance of Neptune from the sun. If we 
 represent the diameter of Neptune's orbit by a 
 circle of 2 inches in diameter, Alpha Centauri 
 would lie at a distance of 762 feet, or 254 yards, 
 from the centre of the small circle. If we make 
 the circle representing Neptune's orbit 2 feet in 
 diameter, then Alpha Centauri would be distant 
 from the centre of this circle 9150 feet, or about 
 If mile. As the volumes of spheres vary as the 
 cubes of their diameters, we have the volume of 
 the sphere which extends to Alpha Centauri 766,000 
 million times the volume of the sphere containing 
 the whole solar system to the orbit of Neptune. 
 If we represent the sphere containing the solar 
 system by a grain of shot ^ inch in diameter, 
 the sphere which extends to Alpha Centauri 
 would be represented by a globe 38 feet in 
 diameter. 
 
 It will thus be seen what a relatively small 
 portion of J space the solar system occupies, com- 
 pared with the sphere which extends to even the 
 nearest star. But this latter sphere, vast as it is, 
 is again relatively small compared with the size of 
 the sphere which contains the great majority of 
 the visible stars. Alpha Centauri is an exception- 
 ally near star. Most of the stars are at least ten 
 
6 STUDIES IN ASTRONOMY 
 
 times as far away, and possibly many a hundred 
 times further off. A sphere with a radius a 
 hundred times greater than the sphere containing 
 Alpha Centauri would have a million times the 
 volume, and therefore 766,000 billion times the 
 volume of the sphere which contains the whole 
 solar system ! 
 
 From these facts it will be seen that, enormously 
 large as the solar system absolutely is compared 
 with the size of our own earth, it is, compared 
 with the size of the visible universe, merely as a 
 drop in the ocean. 
 
 The sun is usually considered as a body of 
 enormous size. And so it is. It is over 1,300,000 
 times larger than the earth, and nearly 1000 times 
 the size of the "giant planet" Jupiter. Yet, 
 compared with the sphere containing the solar 
 system, its volume is insignificant. Taking its 
 diameter at 866,000 miles, I find that the volume 
 of a sphere having the same diameter as the orbit 
 of Neptune would be over 267,000 million times 
 the sun's volume ! 
 
II 
 
 Jupiter and its System 
 
 JUPITER has been well termed the "giant 
 planet " of the solar system, exceeding as it 
 does, both in volume and mass, all the other 
 planets put together. Its volume and mass 
 are variously stated in works on astronomy. I 
 find that the volume given in different books 
 ranges from 1200 to 1387 times the volume of the 
 earth. This discrepancy probably arises, in some 
 cases at least, from assuming the planet to be a 
 perfect sphere, whereas it is considerably flattened 
 at the north and south poles much more so than 
 our globe. Recent measures by Prof. Barnard, of 
 the Lick Observatory, make the equatorial dia- 
 meter 89,790 miles, and the polar diameter 84,300 
 miles. Assuming these dimensions for Jupiter, and 
 for the earth those given by Harkness, namely, 
 7926-248 and 7899-844 miles respectively, I find 
 that assuming both bodies to be oblate spheroids 
 the volume of Jupiter's globe is 1369-4 times the 
 volume of the earth. According to Harkness, the 
 sun's mass is 327,214 times the mass of the earth, 
 
8 STUDIES IN ASTRONOMY 
 
 and 1047-55 times that of Jupiter. Hence it 
 follows that Jupiter's mass is 327,214 divided by 
 1047-55, or 312-36 times the mass of the earth. 
 Its density or specific gravity, compared with 
 that of the earth, will therefore be 312-36 divided 
 by 1369-4, or 0-228, that of the earth being 1. 
 Assuming the earth's density at 5*576, as found 
 by Harkness from a discussion of various measures, 
 the density of Jupiter will be 1/27 (water = 1), or 
 a little less than that of the sun, which is about 
 1-40. 
 
 From the measures given above, the mean 
 diameter of Jupiter is 87,045 miles, or eleven times 
 that of the earth, and about one-tenth of the 
 sun's diameter. 
 
 Taking the earth's mean distance from the sun 
 at 92,796,950 miles, as given by Harkness, the 
 mean distance of Jupiter from the sun will be 
 482,803,970 miles. The eccentricity of its elliptical 
 orbit being 0-04825, its distance from the sun 
 at perihelion is about 459,507,760 miles, and at 
 aphelion 506,100,180 miles. Between its greatest 
 and least distance, therefore, there is a difference 
 of 46,592,420 miles, or about one-half the earth's 
 mean distance from the sun. The inclination of 
 Jupiter's orbit to the plane of the ecliptic being 
 only 1 18' 41" or less than that of any of the 
 large planets, with the exception of Uranus the 
 planet never departs much from the Ecliptic, and 
 hence it was called by the ancients " the Ecliptic 
 
JUPITER AN 7 D ITS SYSTEM 9 
 
 planet." Its period of revolution round the sun 
 is 11 years 314*8 days. The inclination of the 
 axis of rotation being nearly at right angles to 
 the plane of its orbit, there are practically no 
 seasons in this distant world, and the only vari- 
 ation in the heat and light at any point on its 
 surface would be that due to the comparatively 
 small variation in its distance from the sun 
 referred to above. Its mean distance from the 
 sun being 5*2028 tunes the earth's mean distance 
 from the sun, it follows that the heat and light 
 received by Jupiter is twenty-seven times (5*2 
 squared) less than the earth receives. Thus it 
 will be seen that the amount of heat received by 
 this planet from the sun is very small, and were 
 it constituted like the earth, its surface should 
 be perpetually covered with frost and snow. Far 
 from this being the case, the telescope shows its 
 atmosphere to be in a state of constant and 
 wonderful change. These extraordinary changes 
 cannot possibly be due to the solar heat, and they 
 have suggested the idea that the planet may 
 perhaps be in a red-hot state, a miniature sun, 
 in fact, glowing with inherent heat. The great 
 brilliancy of its surface, the "albedo," as it is 
 called, and its small density less than that of 
 the sun are facts in favour of this hypothesis. 
 As the attraction of Jupiter's enormous mass 
 would render the materials near its centre of 
 much greater density than those near its surface, 
 
10 STUDIES IN ASTRONOMY 
 
 the latter must be considerably lighter than 
 water, and may possibly be in the gaseous state. 
 
 It has been objected to this hypothesis of 
 inherent heat in Jupiter that the satellites totally 
 disappear when they pass into the shadow of the 
 planet and are eclipsed, as they frequently are, 
 and that if they received any light from Jupiter 
 they should remain visible in large telescopes 
 when eclipsed. This objection is not so plausible 
 as it may at first sight appear. The light afforded 
 by Jupiter to his satellites when eclipsed may 
 be small, although the heat of the planet may be 
 comparatively great. Red-hot iron, although at 
 a very high temperature, does not give much 
 light, and at the great distance the satellites 
 are from the earth the light they receive from 
 Jupiter might very well be quite imperceptible 
 even in a large telescope. Possibly the "dark 
 side" of Jupiter may appear to his satellites as 
 our moon does when totally eclipsed and show- 
 ing a ruddy light; and we know how little 
 light the moon gives us during a total eclipse. 
 There are several observations on record which 
 seem to favour the hypothesis that the surface 
 of Jupiter glows with inherent light in addition 
 to the light it reflects from the sun. The famous 
 observer, Cassini, once failed to find the shadow 
 of the first satellite when it should have been on 
 the disc. The shadow of the same satellite was 
 seen grey by Gorton on one occasion. The shadow 
 
JUPITER AND ITS SYSTEM 11 
 
 of the second satellite has been seen very indistinct 
 by Buffham, Birt, and Grover, and it was seen 
 grey by Flammarion and Terby in March, 1874. 
 The well-known astronomer, Captain W. Noble, 
 speaking of the chocolate colour of the second 
 satellite's shadow when in transit in 1892, says, 
 " The only feasible explanation of this appearance 
 which occurred to me was that the portion of 
 the planet's disc from which all sunlight was shut 
 off was in a red-hot or glowing condition." The 
 third satellite has been frequently seen in transit 
 as a black spot, although fully illuminated by 
 sunlight. This may be partly due, as has been 
 suggested, to dark spots on the surface of the 
 satellite ; but the phenomenon of a " black 
 transit " cannot be wholly due to this cause, for 
 were the spots on its surface so very dark as this 
 hypothesis would imply, they would also diminish 
 the brightness of the satellite when seen on a 
 dark sky. This is apparently not the case, for 
 the third satellite is usually the brightest of all 
 when observed outside the planet's disc. 
 
 In a paper on Planetary Atmospheres, by the 
 Russian physicist Rogovsky, recently published, 1 
 he computes the temperature of Jupiter to be 
 very high between 1320 and 4060 of the Centi- 
 grade scale. He says, " All the observations con- 
 firm this. Thus Bond found the emissive power of 
 its surface to be twice that of the best white-lead. 
 1 Astrophysical Journal, November, 1901. 
 
12 STUDIES IN ASTRONOMY 
 
 It is, therefore, probable that Jupiter adds to 
 the reflected sunlight light of its own. Bredi- 
 khin, on the basis of his many years' observation 
 of the surface of Jupiter, Lohse, J. Schemer, and 
 others have enunciated the opinion that Jupiter- 
 is a glowing body." And again, " The dense and 
 opaque atmosphere hides its glowing surface from 
 our view, and we see, therefore, only the external 
 surface of its clouds. The objective existence of 
 this atmosphere is proved by the bands and lines 
 of absorption in its spectrum." 
 
 The four well-known satellites of Jupiter were 
 discovered by Galileo on January 7, 1610, and 
 were some of the first-fruits of the invention of 
 the telescope. They are usually known by the 
 numbers I., II., III., IV., counting from the planet. 
 Their distances from the centre of Jupiter are 
 266,400 miles, 423,800 miles, 676,000 miles, and 
 1,189,000 miles respectively, and their approxi- 
 mate diameters 2400, 2100, 3430, and 2930 miles. 
 Satellite II. is therefore about the same size as 
 our moon ; I., a little larger ; III., intermediate in 
 size between Mercury and Mars ; and IV., about 
 the size of Mercury. Their periods of revolution 
 round Jupiter range from l d 18 h 27j m to 16 d 15 h 32 m . 
 Their density is small, that of I. being only a 
 little greater than that of water ; that of II. and 
 III. about 2 (water = 1) ; and that of IV. about 
 1-47. 
 
 A fifth satellite was unexpectedly discovered 
 
JUPITER AND ITS SYSTEM 13 
 
 by Prof. Barnard with the great Lick telescope 
 on the night of September 9, 1892. This is a very 
 faint and difficult object, shining only as a star 
 of the 13th magnitude. Its distance from the 
 centre of Jupiter is about 112,500 miles. It is 
 therefore only 67,600 miles from the surface of the 
 planet, round which it revolves in the short period 
 of ll h 57 m 22 s , with a velocity of about 16| 
 miles a second. It is too small for direct measure- 
 ment, but, judging from its faintness, its diameter 
 does not probably exceed 100 miles. 
 
 The rapid motion and change of phase of these 
 satellites must form a most interesting spectacle 
 in the sky of Jupiter. The rapidity of their 
 motion is easily explained by the great mass of 
 Jupiter. Were they to move as slowly as our 
 moon does, they would soon fall on to the body of 
 the planet. All the satellites, with the exception 
 of the outer one, IV., are eclipsed at every revolu- 
 tion. Owing, however, to a remarkable relation 
 which exists between the motions of the three 
 other larger satellites, it follows that they can 
 never be all eclipsed at the same time, so that 
 there will always be more or less moonlight in the 
 Jovian sky. The diameters of the discs of the 
 large satellites, as seen from Jupiter's equator, 
 will be 36, 18|, 18, and 8| minutes of arc respec- 
 tively. Satellite I. would therefore show a disc 
 somewhat larger than our moon, and the others 
 smaller. Barnard's satellite would have a disc of 
 
14 STUDIES IN ASTRONOMY 
 
 about 5 minutes in diameter. The combined light 
 of the satellites, however, as seen from Jupiter, 
 is, owing to their great distance from the sun, 
 considerably less than that we receive from our 
 solitary moon. But Jupiter himself must afford a 
 considerable amount of light to the satellites at 
 night. From satellite I. he appears as a disc of 
 about 19 degrees in diameter ; from II., about 12 
 degrees ; from III., over 7 degrees ; and from 
 satellite IV., over 4 degrees. Jupiter's light on 
 all the satellites, especially on I. and II., must 
 therefore much exceed that of our full moon. 
 But if the illumination of their nights is good, 
 their daylight is not quite so satisfactory. Total 
 eclipses of the sun by Jupiter are of almost daily 
 occurrence, those seen from the first satellite last- 
 ing nearly 2| hours, and from the fourth over 4J 
 hours. 
 
 Seen from Barnard's satellite, I find that Jupiter 
 would show an enormous disc, of which the equa- 
 torial diameter would be about 47 degrees, and 
 the polar about 44 degrees. From the proximity 
 of this little satellite to the surface of Jupiter, 
 and the great velocity of its rotation round the 
 planet, we may deduce some curious and interest- 
 ing facts connected with it. In the first place, as 
 the satellite is comparatively so close to the sur- 
 face of the giant planet, and revolves round it 
 nearly in the plane of Jupiter's equator, the 
 satellite will not be visible from higher Jovian 
 
JUPITER AND ITS SYSTEM 15 
 
 latitudes than about 65 degrees north and south. 
 Residents 1 in Jupiter nearer to the poles would 
 therefore know less about this tiny satellite than 
 even we do, except, perhaps, by the reports of 
 those who had visited lower latitudes. Again, 
 the period of Jupiter's rotation on its axis being 
 about 9 h 55 m 37 8 , and the period of revolution 
 of the fifth satellite about 12 hours, it follows that 
 five revolutions of the satellite are nearly equal 
 to six rotations of Jupiter. From this relation, I 
 find that the satellite will remain above the 
 horizon of any spot on Jupiter's equator for about 
 23 hours, and remain below the horizon about 37 
 hours. During the time, therefore, that the satel- 
 lite is visible in the sky of Jupiter it makes nearly 
 two revolutions round the planet's centre, and 
 will go twice through all its phases, which are 
 similar to those of our moon. This apparent 
 anomaly is due to the fact that the motion of 
 revolution of the satellite and that of the rotation 
 of the planet are in the same direction, namely, 
 from west to east, and that the periods do not 
 differ very much in length. 
 
 Some idea may be gained of the relative size of 
 the giant planet and its tiny acolyte by supposing 
 the satellite to be represented by a grain of shot 
 of one-tenth of an inch in diameter. Jupiter will 
 then be represented by a globe 87 inches, or 7 feet 
 
 1 That is, were life possible on Jupiter, which, of course, is not 
 the case. 
 
16 STUDIES IN ASTRONOMY 
 
 3 inches, in diameter ! As the volumes of spheres 
 vary as the cubes of their diameters, the volume 
 of Jupiter will be about 680,000,000 times the 
 volume of the little satellite. 
 
 The light afforded by Jupiter to its little satellite 
 must be very considerable. Taking the diameter 
 of Jupiter as seen from the satellite at 45 degrees, 
 the area of its disc would be about 8000 times the 
 apparent area of our full moon. Of course, the 
 intensity of sunlight on Jupiter is 27 times less 
 than that of sunshine on the moon, but, on the 
 other hand, Jupiter's " albedo," or light-reflecting 
 power, is very high, probably about four times 
 that of the moon. The brightness of Jupiter's 
 surface will therefore be ? of the brightness of 
 the moon's surface. Hence we have the light 
 afforded by Jupiter to the little satellite equal to 
 8000 x , or over 1100 times the light of our full 
 moon. 
 
Ill 
 
 Giant Telescopes 
 
 THE invention of the telescope is ascribed by 
 Borelli, a Dutch mathematician, to Zach- 
 ariah Jaiisen and Hans Lipperscheim, 
 spectacle-makers, residing in Middleburgh, Holland, 
 about the year 1600. The news of the invention 
 did not spread rapidly, and was unknown to 
 Galileo until the year 1609. In that year the 
 famous Italian astronomer, having learned the 
 principles of its construction, set to work and 
 succeeded in making one which magnified three 
 times about the power of a modern opera-glass. 
 He afterwards succeeded in constructing one which 
 magnified thirty times, and the reward of his 
 efforts was, as is well known, the discovery of the 
 satellites of Jupiter, the phases of Venus, the spots 
 on the sun, etc. Galileo's telescopes were made on 
 the principle of the opera-glass and binocular field- 
 glass, namely, with a convex object-glass and a 
 concave eye-piece, both being single lenses. The 
 great objection to this form of telescope, with 
 single lenses, is due to what is called " chromatic 
 
 c 
 
18 STUDIES IN ASTRONOMY 
 
 aberration," which produces a fringe of colour 
 round the objects viewed. This colouring inter- 
 feres greatly with clear vision. Take an inferior 
 opera-glass, or cheap hand telescope, and look at a 
 range of hills projected against a background of 
 white clouds. Along the " sky-line " of the hills 
 will be seen a rainbow-tinted fringe, which pre- 
 vents the outline of the hills being seen sharply 
 denned as it would appear in a really good 
 telescope. This defect, annoying as it is with 
 terrestrial objects, is especially so when we view 
 celestial objects like the moon and planets. To 
 get rid of this imperfection at least, to some 
 extent the old telescope-makers had recourse to 
 instruments of enormous length. The famous 
 Hevelius, the astronomer of Dantzic, constructed 
 one of 150 feet in length, the tube, or rather 
 skeleton tube, being made of planks, and suspended 
 by ropes to a strong mass fixed in the ground. 
 By a very ingenious system of ropes and pulleys, 
 he succeeded in keeping this unwieldy affair 
 tolerably straight and steady. He suggested that 
 it would be a better arrangement to have the 
 apparatus attached to a revolving tower, but want 
 of means prevented him from carrying out this 
 plan. Campani, of Bologna, "constructed a similar 
 telescope of 136 feet long in 1672, and Huygeus 
 one of 123 feet, which is still preserved by the 
 Royal Society. Bradley measured Venus in 1722 
 with a telescope 212 feet long, and Auzot is said 
 
GIANT TELESCOPES 19 
 
 to have constructed one of 600 feet, which, how- 
 ever, he could not use, owing to its enormous 
 length. These huge instruments were, however, 
 gigantic only in length, their diameter being only 
 a few inches. One of Carnpani's, preserved by the 
 Royal Astronomical Society, has an object-glass of 
 only 2 inches in diameter. A modern telescope 
 6 feet long would probably be superior in every 
 way to the largest of these old instruments. 
 
 Sir Isaac Newton made several experiments 
 with a view to the improvement of refracting 
 telescopes, but came to the conclusion that it was 
 impossible to get rid of the chromatic aberration 
 produced by lenses. He then turned his attention 
 to the construction of telescopes with metallic 
 mirrors first suggested by James Gregory, a 
 Scotchman, in 1663 and succeeded in making 
 several which gave satisfactory results. In this 
 form of telescope the image, being formed by 
 reflection, is free from colour. Newton's telescopes 
 were, however, very small, and only a few of any 
 size were constructed for about a hundred years, 
 when Sir William Herschel took up the subject, and 
 succeeded in constructing several reflecting tele- 
 scopes of considerable size, his largest being no 
 less than 4 feet in diameter. This great instrument 
 was finished in the year 1789, and with it the 
 illustrious astronomer discovered the two small 
 satellites of Saturn, Mimas and Euceladus. In 
 after years a reflecting telescope of 4 feet in 
 
20 STUDIES IN ASTRONOMY 
 
 diameter and 40 feet long was constructed by 
 Mr. Lassell, who took it to Malta, and with it 
 discovered numerous nebulae. 
 
 These telescopes were, however, soon exceeded 
 in size by Lord Rosse's famous instrument of 6 
 feet in diameter, completed in 1845. This giant 
 telescope, which is still the largest in the world, 
 is 52 feet in length. The tube, 7 feet in diameter, 
 is formed of wood, strengthened with iron hoops. 
 There are two mirrors, one weighing 3~ and the 
 other 4 tons. The metal of which they are made 
 is an alloy of copper and tin in the proportion of 
 126 parts copper to 57 J of tin. As the telescope 
 is fixed between two high walls running north 
 and south, observations can only be made when 
 objects are near the meridian. 
 
 These large metallic mirrors, although of great 
 light-grasping power, are deficient in definition, 
 and are said to " bunch bright stars into a cocked 
 hat ! " A German astronomer, having looked 
 through Lord Rosse's telescope, afterwards said, 
 " They showed me something which they said 
 was Saturn, and I believed them." Another objec- 
 tion to these telescopes is that the metallic mirror 
 rapidly tarnishes, and has to be repolished. It 
 may be imagined that this operation, in the case 
 of a mirror weighing 4 tons, is a matter of no 
 small difficulty. 
 
 Metallic mirrors have, in recent years, been 
 superseded by mirrors made of glass. The glass 
 
GIANT TELESCOPES 21 
 
 disc is first carefully ground to the proper curved 
 surface. This surface is then covered with a thin 
 coating of silver by a chemical process, and this 
 silver film is then polished. These mirrors reflect 
 much more light, and give much better definition 
 than the old metallic mirrors. They are, of 
 course, liable to tarnish also after being some 
 years in use, but they can be re-silvered and 
 polished with very little expense and trouble. 
 These " silver-on-glass " mirrors have recently 
 come into great favour, and, being much cheaper 
 than refractors of equal power, they are very 
 popular among amateur astronomers. Some very 
 large telescopes of this kind have been con- 
 structed in recent years. One of 3 feet in diameter 
 was made by Calver in 1879. It is now at the 
 Lick Observatory, and with it some fine photo- 
 graphs of stars and nebulae have been taken. 
 There is another of 4 feet diameter in the Paris 
 Observatory, constructed by Martin. One of 5 
 feet in diameter was made by the late Dr. Common, 
 and proved very satisfactory. This telescope is 
 probably equal, if not superior, both in light and 
 power, to Lord Rosse's telescope. Another of 5 
 feet aperture, made by Mr. G. W. Ritchey hi 1902, 
 is at the Yerkes Observatory (U.S.A.). Larger 
 telescopes of this class are contemplated, glass 
 mirrors of even 8 and 10 feet being now spoken 
 of as possible in the near future. 
 Although Sir Isaac Newton despaired of any 
 
22 STUDIES IN ASTRONOMY 
 
 improvement in refracting telescopes which would 
 get rid of the chromatic aberration, the problem 
 was not abandoned as hopeless, and in the year 
 1729 two years after Newton's death Mr. 
 Chester More Hall, considering the construction 
 of the human eye, succeeded in obtaining a com- 
 bination of lenses of different kinds of glass which 
 gave an image free from colour. This was the 
 origin of the achromatic telescope, as it is called, 
 which has made such rapid progress in recent 
 years. The combination of lenses now employed 
 was devised in 1758 by the famous optician, John 
 Dollond, and to him is often ascribed the inven- 
 tion of the achromatic telescope, but the credit of 
 the invention is really due to More Hall. In 1765 
 John Dollond's son, Peter Dollond, discovered that 
 the chromatic aberration could be further reduced 
 by a combination of three lenses instead of two. 
 This form of object-glass is still sometimes used 
 in binoculars, but for large telescopes two lenses 
 only are generally used. 
 
 Notwithstanding this great improvement in the 
 construction of refracting telescopes, many years 
 elapsed before telescopes of any size were con- 
 structed on this principle. Even in the year 1825 
 the largest telescope of this kind was one of only 
 9J inches in diameter, constructed by the famous 
 optician, Frauiihofer, for the Dorpat Observatory, 
 Russia. M. Struve, the director of the observatory, 
 wrote with reference to it, "I stood astonished 
 
GIANT TELESCOPES 23 
 
 before this noble instrument, undetermined which 
 to admire most the beauty and elegance of the 
 workmanship in its most minute parts, the ap- 
 propriateness of its construction, the ingenious 
 mechanism for moving it, or the incomparable 
 optical power of the telescope, and the precision 
 with which objects are denned." Astronomers of 
 the present day would hardly call a telescope of 
 this size " a noble instrument," refractors of 8 to 
 10 inches in diameter being now comparatively 
 numerous. Struve, however, did excellent work 
 with this instrument, and discovered and cata- 
 logued hundreds of double stars, a good example 
 of what has been said with reference to telescopes 
 in general, that the work done with any instru- 
 ment " does not depend so much on the diameter 
 at the big end as on the man at the small end." 
 
 Gradually, however, refracting telescopes in- 
 creased in size. In 1834 an achromatic of 11| 
 inches aperture and 19 feet in length was con- 
 structed by Cauchoix, and mounted in the Cam- 
 bridge Observatory. This is known as the 
 Northumberland Equatorial, and was so named 
 after the Duke of Northumberland, who presented 
 it to the observatory. In the same year a 
 refractor of 13^ inches aperture and 25| feet long, 
 by the same maker, was mounted at the observa- 
 tory, Markree Castle, Ireland, by the late E. J. 
 Cooper. There is also a refractor by Cauchoix of 
 11*8 inches aperture at Dunsink Observatory, 
 
24 STUDIES IN ASTRONOMY 
 
 Dublin, and there are several of from 12 to 13| 
 inches in the United States and elsewhere. At 
 the Poulkova Observatory, Russia, there is a fine 
 refractor of 15 inches aperture and 22| feet focus, 
 the work of Merz and Mahler. The weight of this 
 instrument is 7000 Ibs., or over 3 tons. It has a 
 series of eye-pieces, the highest magnifying about 
 2000 times. The Harvard College Observatory, 
 U.S.A., has a telescope of the same size and by the 
 same makers as the Poulkova telescope. The Paris 
 Observatory has also a refractor of 15 inches 
 diameter, but it is not a very good one. The 
 following observatories also possess refractors of 
 about 15 inches aperture : Milan, Stony hurst, 
 Rio Janeiro, Madrid, Brussels, Edinburgh, and 
 Tulse Hill (Dr. Huggins). The Harvard College 
 telescope at one time shared with the Poulkova 
 refractor the honour of being the largest refractor 
 in the world. With it Bond, the famous Ameri- 
 can astronomer, discovered, in September, 1848, 
 Hyperion, the 8th satellite of Saturn (discovered 
 independently, a few days later, by Lassell in 
 England). With it, also, Bond discovered the dark 
 or "crape" ring of Saturn, which was also in- 
 dependently discovered by Dawes in England. 
 Bond's great drawing of the Orion nebula was 
 also made with this telescope. 
 
 Dr. Cerulli has a refractor of 15^-inch. aperture 
 at his observatory, Peramo, Italy, and there are 
 others of about the same size at the Washburn 
 
GIANT TELESCOPES 25 
 
 Observatory (U.S.A), and at the Meudon Observa- 
 tory. Mr. Warner, of " Safe Cure " fame, has one 
 of 16 inches at his private observatory, Rochester 
 (U.S.A.). Professor Max Wolf has one of 16 inches 
 at Heidelbergh, and there is another of about the 
 same aperture at the Goodsell Observatory (U.S.A.). 
 Telescopes of 16| inches aperture are at Zi-ka-Wei, 
 Vienna, and Nice, and 18-inch refractors at the 
 Royal Observatory, Cape of Good Hope; the 
 Vander Zee Observatory ; the Flower Observatory 
 (U.S.A.) ; the Lowell Observatory, Mexico ; and 
 the National Observatory, La Plata. 
 
 At the Dearborn Observatory, Chicago, there 
 is a telescope of 18^ inches aperture, with which 
 Bumham has done such excellent work among 
 the double stars. It was with this instrument 
 that Alvan Clark discovered the companion to 
 Sirius before the telescope left his workshop. 
 This was for ten years the largest refractor in the 
 world. 
 
 The observatory at Milan has a refractor of 
 19*1 inches, by Merz, and there is another of 
 about the same size at the Imperial Observatory, 
 Strassburgh. 
 
 There is one of 20 inches aperture at the Manila 
 Observatory, and another at the Chamberlin 
 Observatory, Colorado. 
 
 We now come to refractors of over 20 inches 
 aperture, and the following list includes all those 
 at present in existence : 
 
26 STUDIES IN ASTRONOMY 
 
 1. Refractor of 20'5 inches aperture, private 
 observatory of M. Porro, Italy. 
 
 2. Refractor of 21'2 inches, constructed by 
 Buckingham and Wragge for Mr. Buckingham's 
 private observatory. 
 
 3. Refractor of 21 -8 inches, by Merz, at the Etna 
 Observatory. 
 
 4. Object-glass of 22 inches, at the Edinburgh 
 Observatory. 
 
 5. Refractor of 23 inches, constructed by Alvan 
 Clark for the Halsted Observatory, Princeton 
 (U.S.A.). 
 
 6. Refractor of 23'6 inches, National Observa- 
 tory of Paris ; constructed by Brothers Henry and 
 Gautier, 1891. 
 
 7. Refractor of 24 inches, by Alvan Clark (1896), 
 Lowell Observatory, Mexico. 
 
 8. Photographic refractor of 24 inches, Royal 
 Observatory, Cape^ of Good Hope ; the work of 
 Sir Howard Grubb, and another by the same 
 eminent maker at the Radcliffe Observatory, 
 Oxford. 
 
 9. Photographic refractor of 24 inches, by Alvan 
 Clark (1893), at the Harvard College Observatory 
 (U.S.A.). The focal length is only 11-3 feet. It 
 was presented to the observatory by Miss Bruce. 
 
 10. Refractor of 24*4 inches, National Observa- 
 tory, Meudon ; made by Henrys and Gautier (1891). 
 This is also a photographic telescope. 
 
 11. Refractor of 25 inches aperture, made by 
 
GIANT TELESCOPES 27 
 
 Cooke of York (1870) for the late Mr. Newall. 
 Now at the Cambridge Observatory. 
 
 12. Refractor of 26 inches, made by Alvan Clark 
 (1881) for the private observatory of Mr. Leander 
 McCormick at Virginia (U.S.A.). With this in- 
 strument numerous measures of double stars have 
 been made by Messrs. Leavenworth and Muller. 
 
 13. Refractor of 26 inches, also constructed by 
 Alvan Clark (1873), for the Washington Observa- 
 tory (U.S.A.). With this "noble" instrument 
 Professor Asaph Hall discovered the two satellites 
 of Mars in 1877, and has made numerous obser- 
 vations of double stars. In the object-glass of 
 this telescope the thickness of the crown lens is 
 1-88 inch, and that of the flint lens O96 inch. 
 
 14. Photographic refractor of 26 inches, at the 
 Royal Observatory, Greenwich; the work of Sir 
 Howard Grubb. The focal length is 26 feet. 
 
 15. Refractor of 27 inches, at the Imperial 
 Observatory, Vienna ; constructed by Sir Howard 
 Grubb. This telescope has been chiefly used in 
 the search for minor planets between Mars and 
 Jupiter. 
 
 16. Refractor of 28 inches, Royal Observatory, 
 Greenwich ; also made by Sir Howard Grubb. 
 
 17. Refractor of 28*9 inches, at the National 
 Observatory, Paris ; made by Martin. 
 
 18. Refractor of 30 inches, constructed by 
 Alvan Clark (1888), for the Poulkova Observatory, 
 Russia. 
 
28 STUDIES IN ASTRONOMY 
 
 19. Refractor of 30'3 inches, at the Bischoffsheim 
 Observatory, Nice; the work of the Brothers 
 Henry and Gautier. 
 
 20. Photographic refractor of 31 '5 inches, "at the 
 Astrophysical Observatory, Potsdam; made by 
 Steinheil and Repsold, 1899. 
 
 21. Refractor of 32'5 inches, at the National 
 Observatory, Meudon; made by the Brothers 
 Henry and Gautier, 1891. 
 
 22. The great telescope of the Lick Observatory, 
 California. This magnificent instrument has an 
 object-glass of 36 inches aperture and 57'8 feet 
 focal length, the work of Alvan Clark and Sons, 
 the mounting being constructed by Warner and 
 Swasey. The Lick observatory was founded by 
 the late Mr. Lick, a retired piano and organ 
 maker, of Baltimore, who made an enormous 
 fortune by land speculations, most of which he 
 left for public purposes. The observatory is 
 situated on the summit of Mount Hamilton, at 
 a height of 4200 feet above the level of the sea, 
 and about 60 miles south-east of San Francisco. 
 The tube of the great telescope is 57 feet long, or 
 about 5 feet longer than Lord Rosse's giant re- 
 flector. The telescope is fitted with " finders " 
 of 3, 4, and 6 inches aperture. The largest of 
 these would have been considered a fairly large 
 telescope at the beginning of the nineteenth 
 century- The telescope is sheltered by a dome 
 of 75 feet in diameter, weighing nearly 89 tons, 
 
GIANT TELESCOPES 29 
 
 and resting 011 a brick wall 25 feet high. Sur- 
 rounding the pier which carries the telescope is 
 a floor which is raised and lowered by hydraulic 
 power to suit the varying height of the eye-piece. 
 Although this moving floor weighs over 22 tons, 
 it can be raised in nine minutes. In addition to 
 the object-glass, the telescope is supplied with a 
 photographic lens of 33 inches aperture and 49'2 
 feet focus, with which some fine photographs of 
 the moon and other celestial objects have been 
 made. Although this giant telescope has only 
 been in use for a few years, much excellent work 
 has been done with it. By its aid Barnard dis- 
 covered the fifth satellite of Jupiter, which. is so 
 faint that its existence was never suspected with 
 any of the large telescopes with which the planet 
 has been frequently observed. Numerous measures 
 of close double stars have also been made with 
 it by Burnham, who has added so much to our 
 knowledge of these wonderful and interesting 
 stellar systems. On the death of Mr. Lick, the 
 founder of the observatory, the coffin containing 
 his remains was built into the masonry pier which 
 carries the telescope, the great instrument, with 
 its surrounding dome, thus forming a fitting 
 monument to his memory. 
 
 A refracting telescope of even larger dimensions 
 than that of the Lick Observatory has been 
 finished and mounted in a new observatory 
 founded at Wisconsin, not far from Chicago, by 
 
30 STUDIES IN ASTRONOMY 
 
 Mr. Yerkes, a wealthy American. The object- 
 glass, which is 40 inches in diameter, with a focal 
 length of 62 feet, was made by Alvan Clark, and 
 has proved satisfactory. The convex lens of 
 crown glass that nearest the object is about 
 3 inches thick in the centre and about | inch at 
 the edge, and weighs about 200 pounds. The 
 concave or flint glass is about 1J inch at the 
 centre and 2f inches at the edge, and weighs about 
 300 pounds. The mounting for this giant instru- 
 ment was constructed by Warner and Swasey, 
 and was exhibited at the Chicago Exhibition. 
 The tube is of sheet steel, and weighs about 
 6 tons! The total weight of the telescope and 
 mounting is about 75 tons! The driving-clock 
 alone weighs about 1| ton ! The dome covering 
 the telescope is 80 feet in diameter ! 
 
 With reference to the largest-sized refractor 
 which can be made, it appears that we have not 
 yet reached the limit for this form of telescope. 
 Mr. Clark expressed his opinion that, notwith- 
 standing the absorption of light due to the in- 
 creased thickness of the lenses necessary in these 
 large telescopes, their light-grasping power has 
 hitherto increased in proportion to their size. 
 He considered that the 30-inch object-glass which 
 he made for the Poulkova Observatory is " vastly 
 superior" to the 26-inch Washington refractor; 
 that the 36-inch Lick telescope is "certainly 
 superior to the 30-inch ; " and he had " every reason 
 
GIANT TELESCOPES 31 
 
 \ 
 
 to suppose that the 40-inch will be superior to 
 the 36-inch." If this be so, we may expect in- 
 teresting celestial discoveries with the great 
 40-inch Yerkes telescope. Mr. Clark's anticipa- 
 tions have already been partially realized. 
 
IV 
 
 The Distances of the Stars 
 
 THE determination of the distance of the stars 
 from the earth has always formed a subject 
 of great interest to astronomers in all 
 ages. The old astronomers seem to have con- 
 sidered that the problem was incapable of solu- 
 tion. In later years the famous astronomer 
 Kepler, judging from what he termed "the 
 harmony of relations," concluded that the dis- 
 tance of the fixed stars should be about 2000 
 times the distance of Saturn from the sun. At 
 that time Saturn was the farthest known planet 
 of the solar system. But the distance of even 
 the nearest star, as now known, is about 14 times 
 greater than that supposed by Kepler. Huygens 
 thought the determination of a star's distance 
 by direct observations to be impossible, but made 
 an attempt at a solution of the problem by a 
 photometric comparison between Sirius and the 
 sun. By this method of estimation he found that 
 Sirius is probably about 28,000 times the sun's dis- 
 tance from the earth. Modern measures, however, 
 
THE DISTANCES OF THE STARS 33 
 
 * 
 
 show that this estimate is also far too small, 
 the distance of Sirius, as now known, being over 
 500,000 times the sun's distance, or about 18 times 
 greater than Huygens made it. 
 
 When the Copernican theory of the earth's 
 motion round the sun was first advanced, it was 
 objected that if the earth moved in an orbit 
 round the sun, its real change of place should 
 produce an apparent change of position in the 
 stars nearest to the earth, causing them to shift 
 their relative position with reference to more 
 distant stars. Copernicus replied to this objection 
 and we now know that his reply was correct 
 by saying that the distance of even the nearest 
 stars is so great that the earth's motion would 
 have no perceptible effect at least, to the naked 
 eye in changing their apparent position in the 
 heavens. In other words, the diameter of the 
 earth's orbit round the sun would be almost an 
 imperceptible point if viewed from the distance 
 of the nearest stars. This explanation of Coper- 
 nicus was at first ridiculed, and even the famous 
 astronomer, Tycho Brahe, could not accept such a 
 startling hypothesis. This celebrated observer 
 failed, indeed, to detect by his own observations 
 any annual change of place, but he fancied that 
 the brighter stars showed a perceptible disc like 
 the planets a fact which, if true, would imply 
 that, if the distance was so great as Copernicus 
 supposed, the real size of the stars must be 
 
 D 
 
34 STUDIES IN ASTRONOMY 
 
 enormous. Tycho Brahe estimated that stars of 
 the 1st magnitude have an apparent diameter of 
 120 seconds of arc, those of the 2nd magnitude 
 90 seconds, and that stars of even the 6th magni- 
 tude would have a diameter of about 20 seconds. 
 But this delusion of Tycho Brahe has been dis- 
 pelled by modern observations, which show that 
 even the brightest stars have no perceptible disc. 
 This fact was also proved by Horrocks and Crab- 
 tree, who noticed that in occupations of stars by 
 the moon the stars disappeared instantaneously, 
 a fact which proved beyond a doubt that the 
 apparent diameter of the stars must be a very 
 small fraction of a second. We now know that 
 the apparent diameter of even the nearest star, 
 a Centauri, cannot exceed the y^th of a second. 
 
 The first idea which naturally suggested itself 
 with reference to the distances of the stars was 
 that the brightest stars were the nearest, and the 
 faintest the farthest from the earth, an idea based, 
 of course, on the assumption that the stars are in 
 general of nearly the same size and intrinsic 
 brilliancy. This hypothesis, although apparently 
 a very reasonable one, has been found by modern 
 researches to have, strange to say, little or no 
 foundation in fact. Although this hypothesis is 
 now proved to be erroneous, it may be interesting 
 to inquire what the relative distances of the stars 
 would be on the assumption of equal size and 
 brightness. To enable us to answer this question, 
 
THE DISTANCES OF THE S^ARS 35 
 
 we must first consider the subject of " star magni- 
 tudes." The stars were divided by the ancient 
 astronomers into " magnitudes," according to their 
 relative brightness ; all the brightest being placed 
 in the 1st magnitude, those considerably fainter 
 being called 2nd magnitude, those fainter still 
 3rd magnitude, and so on to the 6th magnitude, 
 or those just visible to ordinary eyesight. This 
 classification has been practically retained by 
 modern astronomers, but, of course, there are stars 
 of all degrees of brightness, from Sirius down to 
 the faintest visible in the largest telescopes. Sirius 
 is the brightest star in the heavens, and is about 
 equal to eleven average stars of the 1st magnitude, 
 such as Aldebaran. According to the Harvard 
 photometric measures, the following are the 
 brightest stars in the heavens, in order of bright- 
 ness: (1) Sirius, (2) Canopus, (3) a Centauri, 
 (4) Vega, (5) Capella, (6) Arcturus, (7) Rigel, 
 (8) Procyon, (9) Achernar, (10) ft Centauri, 
 (11) Betelgeuse (slightly variable), (12) Altair, 
 (13) a Crucis, and (14) Aldebaran. Of these, 
 Canopus, a and ft Centauri, and Achernar do not 
 rise above the English horizon. Of the stars 
 brighter than the 2nd magnitude, the following 
 are north of the equator : Pollux, a Cygni, Regulus, 
 Castor, c Ursa3 Majoris, y Orionis, ft Tauri, a Persei, 
 7) UrssB Majoris, y Geminorum, and a Ursae Majoris ; 
 and south of the equator: Spica (a Virginis), 
 Antares, Fomalhaut, ft Crucis, y Crucis, e Canis 
 
36 STUDIES IN ASTRONOMY 
 
 Majoris, X Scorpii, e Argus, c Orionis, f3 Argus, a 
 Trianguli Australis, Orionis, Sagittarii, 8 Caiiis 
 Majoris, and /? Canis Majoris. Of those below the 
 2nd magnitude, but brighter than 3*0, there are 
 46 in the northern hemisphere, and 58 in the 
 southern. As the brightness diminishes, the num- 
 bers increase very rapidly. Indeed, the increase 
 is in geometrical progression ; the number of stars 
 in each class of magnitude being from three to 
 four times as many as those in the class one mag- 
 nitude brighter. At least, this is the case down 
 to a certain point, where a " thinning out " seems 
 to begin. 
 
 The difference of one magnitude between any 
 two stars is defined by the "light ratio." This 
 is "the ratio of the intensities of light which 
 shall define the meaning of ' difference of a single 
 magnitude ' between the light of two stars." This 
 ratio is now generally accepted by astronomers 
 as 2*512 ; that is, a star of the 1st magnitude is 
 assumed to be 2*512 times brighter than a star of 
 the 2nd magnitude, a star of the 2nd magnitude 
 2*512 times brighter than a star of the 3rd 
 magnitude, and so on. Hence, as light varies in- 
 versely as the square of the distance, the distance 
 of any star on the assumption of equal size and 
 brightness would be 1*585 (the square root of 
 2*512) times the distance of a star one magnitude 
 brighter; and if we represent the distance of an 
 average star of the 1st magnitude by 1, the 
 
THE DISTANCES OF THE STARS 37 
 
 * 
 
 following would be the relative distances of stars 
 of various magnitudes : 1st magnitude, 1 ; 2nd 
 magnitude, 1*585 ; 3rd magnitude, 2*512 ; 4th mag- 
 nitude, 3*981 ; 5th magnitude, 6*31 ; 6th magnitude, 
 10 ; 7th magnitude, 15*849 ; 8th magnitude, 25*119 ; 
 9th magnitude, 39*811; 10th magnitude, 63*096; 
 llth magnitude, 100; 12th magnitude, 158*489; 
 13th magnitude, 251*189 ; 14th magnitude, 398*107 ; 
 15th magnitude, 630*957 ; and 16th magnitude, 1000. 
 Or if, on a wide level plane, we take a fixed point, 
 and assume the distance of a first magnitude star 
 to be represented by 10 feet from this point, then 
 the distance of a second magnitude star would be 
 approximately 16 feet; 3rd magnitude, 25 feet; 
 4th magnitude, 40 feet; 5th magnitude, 63 feet; 
 6th magnitude, 100 feet; 7th magnitude, 158 feet; 
 8th magnitude, 250 feet ; 9th magnitude, 398 feet ; 
 10th magnitude, 631 feet; llth magnitude, 1000 
 feet; 12th magnitude, 1585 feet; 13th magnitude, 
 2511 feet; 14th magnitude, 3981 feet; 15th magni- 
 tude, 6310 feet, or about 1*2 mile ; and 16th mag- 
 nitude, 10,000 feet, or nearly 2 miles. As, according 
 to recent measures of parallax, light would take 
 about 36 years to reach us from an average star of 
 the first magnitude, it follows that the "light 
 journey" from a star of the 16th magnitude 
 (about the faintest visible in the great Lick tele- 
 scope) would, 011 the above hypothesis, be about 
 36,000 years ! Recent researches, however, have 
 shown that some of the fainter stars are actually 
 
38 STUDIES IN ASTRONOMY 
 
 nearer to us than some of the brighter, and that, 
 therefore, the brightness of a star is no criterion 
 of its distance. 
 
 It was suggested by Galileo that the distance of 
 the nearer stars might possibly be determined 
 by careful measures of double stars, on the 
 assumption that the brighter star of the pair, if 
 the difference of brightness is considerable, is 
 nearer to the earth than the fainter star. Acting 
 on this suggestion, Sir William Herschel, at the 
 close of the eighteenth century, made a careful 
 series of measures of certain double stars. He did 
 not, however, succeed in his attempt, as his 
 instruments were not sufficiently accurate for 
 such a delicate investigation; but his labours 
 were abundantly rewarded by the great discovery 
 of binary or revolving double stars, a most 
 interesting class of objects. Numerous but 
 unsuccessful attempts were also made by Hooke, 
 Flamsteed, Cassini, Molyneux, and Bradley to 
 find the distance of some of the stars. Hooke, 
 in the year 1669, thought he had detected a 
 parallax of 27 to 30 seconds of arc in the star 
 y Draconis, but we now know that 110 star in 
 the heavens has anything like so large a parallax. 
 It should be here explained that the "parallax" 
 of a star is the apparent change in its position 
 caused by the earth's annual motion round the sun. 
 As the earth makes half a revolution in six months, 
 and as its mean distance from the sun, or the 
 
THE DISTANCES OF THE STARS 39 
 
 radius of the orbit, is about 93 millions of miles, 
 the earth is at any given time about 186 millions 
 of miles distant from the point in its orbit which 
 it occupied six months previously. The apparent 
 change of position in a star known as parallax is 
 one-half the total displacement of the star as 
 seen from opposite points of the earth's orbit. In 
 other words, it is the angle subtended at the star 
 by the mean distance between the sun and earth. 
 The measured parallax of a star may be either 
 " absolute " or " relative." An "absolute parallax" 
 is the actual parallax. A "relative parallax" is 
 the parallax with reference to a faint star situated 
 near a brighter star, the fainter star being assumed 
 to lie at a much greater distance from the earth. 
 As, however, the faint star may have a small 
 parallax of its own, the "relative parallax" is the 
 difference between the absolute parallaxes of the 
 two stars. Indeed, in some cases a "negative 
 parallax" has been found, which, if not due to 
 errors of observation, would imply that the faint 
 star is actually the nearer of the two. From the 
 observed parallax the star's distance in miles may 
 be found by simply multiplying the sun's distance 
 from the earth about 93 millions of miles by 
 the number 206265, and dividing the result by the 
 parallax, or about 19 billions of miles, divided by 
 the parallax in seconds. To find the time that 
 light would take to reach us from the star the 
 " light-journey," as it is called it is only necessary 
 
40 STUDIES IN ASTRONOMY 
 
 to divide the number 3*258 by the parallax. Thus, 
 with a parallax of one second the " light- journey " 
 would be 3 years ; for a parallax of one-tenth of 
 a second it would be 32J years, and for a parallax 
 of one-hundredth of a second nearly 326 years. 
 
 In attempting to verify the result found by 
 Hooke for the parallax of y Draconis, Molyiieux 
 and Bradley found an apparent parallax of about 
 20 seconds of arc, thus apparently confirming 
 Hooke' s result. But observations of other stars 
 showing a similar result, Bradley came to the 
 conclusion that the apparent change of position 
 was not really due to parallax, but was caused 
 by a phenomenon now known as the " aberration 
 of light " an apparent displacement in the 
 positions of the stars, due to the effect of the 
 earth's motion round the sun, combined with 
 the progressive motion of light. The result is 
 that "a star is displaced by aberration along a 
 great circle of the star sphere, joining its true 
 place to the point on the celestial sphere towards 
 which the earth is moving." The amount of 
 aberration is a maximum for stars lying in a 
 direction at right angles to that of the earth's 
 motion. The existence of aberration is a positive 
 proof that the earth does revolve round the sun, 
 for were the earth at rest as some paradoxers 
 maintain there would be no aberration of the 
 stars. This effect of aberration must, of course, 
 be carefully allowed for in all measures of stellar 
 
THE DISTANCES OF THE STARS 41 
 
 parallax. To show that aberration could not 
 possibly be due to parallax, it may be stated that 
 aberration shifts the apparent place of a star 
 in one direction, while parallax shifts it in the 
 opposite direction. 
 
 From photometric comparisons, the Rev. John 
 Michell, in the year 1767, concluded that the 
 parallax of Sirius is less than one second of arc 
 a result which has been fully confirmed by modern 
 measures. He considered that stars of the 6th 
 magnitude are probably from 20 to 30 times the 
 distance of Sirius, and, judging from their relative 
 brilliancy, this conclusion would also be nearly 
 correct, but modern measures have shown that 
 the brightness of the stars is no test of their 
 relative distance. 
 
 The stars on which observations were first made 
 with a view to a determination of their distance 
 seem to have been Aldebaran and Sirius. From 
 observations made in the years 1792 to 1804, with 
 a vertical circle and telescope of 3 inches aperture, 
 Piazzi found for Aldebaran an absolute parallax 
 of about one and a half second of arc. In 1857 
 Otto Struve and Shdanow, using a refractor of 
 15 inches aperture, found a "relative" parallax of 
 about half a second. This was further reduced 
 by Professor Hall, with the 26-inch refractor of the 
 Washington Observatory, to about one-tenth of a 
 second, and Dr. Elkin, with a heliometer of 6 inches 
 aperture, finds a relative parallax of 0"'116, or 
 
42 STUDIES IN ASTRONOMY 
 
 about 30 years' journey for light. A parallax of 
 about one-tenth of a second has also been recently 
 found at the Yale University Observatory (U.S.A.). 
 For Sirius, Piazzi found (1792-1804) an absolute 
 parallax of 4 seconds, but this was certainly 
 much too large. All subsequent observers find 
 a much smaller parallax, recent results being a 
 relative parallax of 0"*370 by Dr. Gill, and 0"'407 
 by Dr. Elkin. In the years 1802-1804 Piazzi and 
 Cacciatori found an absolute parallax of 1"*31 for 
 the Pole Star, but this has been much reduced by 
 later observers. The late Professor Pritchard, by 
 means of photography, found a relative parallax 
 of only 0"'073, which agrees closely with some 
 other previous results, and indicates a "light 
 journey " of about 44 years ! For the bright star 
 Procyon, Piazzi found a parallax of about 3 
 seconds ; but this is also much too large, a recent 
 determination by Elkin giving 0"*325, a figure in 
 fair agreement with results found by Auwers and 
 Wagner. For the bright star Vega, Calandrelli, 
 in the years 1805, 1806, found an absolute parallax 
 of nearly 4 seconds; but this has been much reduced 
 by modern measures, Elkin, from measures in the 
 years 1887, 1888, finding a relative parallax of only 
 0"'034. For Arcturus, Brinkley found a parallax 
 of over 1 second, but at Yale Observatory a 
 parallax of 0"'024 was found. If this minute 
 parallax is anything near the truth, Arcturus must 
 be a sun of gigantic size. 
 
THE DISTANCES OF THE STARS 43 
 
 Owing to the large "proper motion" of the star 
 known as 61 Cygni, its comparative proximity to 
 the earth was suspected, and in 1812 Arago and 
 Mathieu found, from measures made with a re- 
 peating circle, a parallax of over one-half a second. 
 Various measures of its parallax have since been 
 made, ranging from about 0"'27 to 0"'566. Sir 
 Robert Ball, when at Dunsink Observatory (Ire- 
 land), found 0"'468; and Prof. Pritchard, by 
 means of photography with a 13-inch refractor, 
 found 0"'437. These values have been somewhat 
 reduced by recent measures, and we may, perhaps, 
 assume that the parallax of 61 Cygni is about 
 0"'4, which gives a "light-journey" of about 
 eight years. The star is only of the 5th magnitude. 
 The small parallax found by Elkin for Arcturus 
 would indicate a distance corresponding to a light- 
 journey of 181 years, although the star is one of 
 the brightest in the heavens. It is usually stated 
 that 61 Cygni is the nearest star in the northern 
 hemisphere ; but for the star known as Lalande 
 21,185, Winnecke found a parallax of 0"'511, and 
 afterwards 0"'501. This has, however, been re- 
 duced by Kapteyu (1885-87) to 0"'434. Recently 
 a parallax of 0"*465 has been found by the photo- 
 graphic method for the binary star 77 Cassiopeiae. 
 
 Nearer to us than 61 Cygni is the bright 
 southern star a Centauri, which, so far as we 
 know at present, is the nearest of all the stars to 
 the earth. The first attempt to find its distance 
 
44 STUDIES IN ASTRONOMY 
 
 was made by Henderson, in the years 1832-1833, 
 using a mural circle of 4 inches aperture and a 
 transit of 5 inches. He found an " absolute paral- 
 lax " of about one second of arc, which, however, 
 subsequent measures have shown to be somewhat 
 too large. Measures in recent years range from 
 0"-512 to 0"-976, but probably the most reliable 
 are those made by Dr. Gill, who found a " relative 
 parallax" of 0"'75. This result would place the 
 star at a distance of 275,000 times the sun's dis- 
 tance from the earth, or about 25 billions of miles, 
 a distance which light would take about 4J years 
 to traverse. 
 
 Dr. Gill's researches on the distance of stars in 
 the southern hemisphere reveal the remarkable 
 fact that some of the brightest stars in the 
 heavens lie at such a vast distance that attempts 
 to measure their parallax have proved futile. 
 Thus Canopus, the second brightest star in the 
 sky, and not much inferior to Sirius in brilliancy, 
 has absolutely no measurable parallax. The same 
 result was found for Rigel, which is about seventh 
 on the list of brightest stars, and also for fi Crucis, 
 one of the stars in the Southern Cross, of which 
 the magnitude is 1*5. These must be suns of 
 enormous size. For Spica, which is very little 
 fainter than a standard star of the 1st magni- 
 tude, Dr. Gill finds a " negative parallax," which 
 implies, if there is no error in the measures, that 
 it is actually further from the earth than some 
 
THE DISTANCES OF THE STARS 45 
 
 faint stars near it. For Achernar (a Eridani), 
 which is about the same brightness as Procyon, 
 Dr. Gill finds a parallax of 0"'043, which implies a 
 journey for light of about 76 years; and for a 
 Gruis, of the 2nd magnitude, a parallax of only 
 0"-015, which indicates a light-journey of 217 
 years. 
 
 From the above results, it will be understood 
 that the parallaxes found for even the nearest 
 stars are so small that their exact determination 
 taxes the powers of the most perfect instruments 
 and the skill of the most experienced observers. 
 One thing seems certain, however, that the 
 brightest stars are not, as a rule, the nearest to 
 the earth, and that even comparatively faint stars 
 may be actually nearer to us than some of the 
 brightest gems that deck our midnight sky. 
 
 When the distances of two stars from the earth 
 have been accurately determined, the actual dis- 
 tance between the stars themselves can be easily 
 computed by simple trigonometry. Take the 
 case of Sirius and Procyon : the parallax of Sirius 
 is 0"-37, and that of Procyon is 0"'325 ; and the 
 apparent angular distance between the two stars 
 is about 25 43'. From these data I find that the 
 actual distance between Sirius and Procyon is 
 almost exactly half the distance which separates 
 Sirius from the earth. From this it follows that 
 the parallax of Procyon, seen from Sirius, or of 
 Sirius, seen from Procyon, is about 0"'74. Hence 
 
46 STUDIES IN ASTRONOMY 
 
 the light of Sirius, seen from Procyon, will be 
 increased four times, or about 1/50 magnitude ; 
 and its magnitude would therefore be 1*58 
 -1-50 = -3-08 magnitude. The light of Procyon, 
 seen from Sirius, would be increased in the pro- 
 
 (0*74 \ 2 
 ^25 j or 5*185 times. This corre- 
 sponds to an increase of 1*78 magnitude. Hence 
 the magnitude of Procyon, as seen from Sirius, 
 would be 0*48 1'78 = 1*30, or nearly as bright 
 as Sirius appears to us. Again, take the case of 
 i) and /x, Cassiopeise, which are at an apparent 
 distance of about 3 50". The parallax of each is 
 about 0"*20. From these data I find that the real 
 distance between the two stars would be only 
 one-fifteenth of the distance which separates them 
 from the earth, and the parallax of each, as seen 
 from the other, would be three seconds of arc. 
 Hence the brilliancy of each would be increased 
 225 times (15 2 ), or about 5*88 magnitudes. As the 
 photometric magnitude of rj Cassiopeise is 3'64, it 
 would therefore shine as a star of 2*24 magni- 
 tude as seen from /,; and the magnitude of /* 
 being 5'22, it would appear as a star of 0'66 
 magnitude as seen from 77. But in this case the 
 parallaxes are not so certain as in the case of 
 Sirius and Procyon. 
 
The Sun's Journey through Space 
 
 IT is now a well-established scientific fact that 
 the sun, together with the earth and all the 
 planets and satellites constituting the solar 
 system, is speeding through space towards the 
 constellation of the Lyre, and some account of the 
 researches which have led to this result may 
 prove of interest to the reader. 
 
 The ancient astronomers, who had no telescopes, 
 and could only observe the heavens with the 
 naked eye, thought that the constellations pre- 
 served through all ages the same forms and 
 dimensions. Hence the term "fixed," which has 
 been applied to the stars from the earliest times. 
 To show this apparent fixity, we may mention 
 the unchanged alignments frequently observed 
 between three stars in various parts of the sky, 
 which were noted by Ptolemy, and which still 
 exist. There are many combinations of three 
 stars nearly in a straight line. Twenty-five of 
 these are noted by Riccioli. Of these may be 
 mentioned the straight line formed by Aldebaran, 
 
48 STUDIES IN ASTRONOMY 
 
 i Aurigae, and Capella. t Aurigse, which is of the 
 3rd magnitude, lies nearly midway between the 
 other two, which are Ist-magnitude stars. The 
 three stars are, however, not exactly in a straight 
 line, the middle star being distant more than half 
 the moon's apparent diameter from the line 
 joining the two brighter stars. But such a small 
 difference would hardly be appreciable to the 
 naked eye. Al-Sufi, the Persian astronomer, who 
 wrote a " Description of the Fixed Stars " in the 
 tenth century, also frequently speaks of three 
 stars being in a straight line. 
 
 Even Copernicus and Kepler believed the stars 
 to be absolutely fixed. Halley was the first who 
 suspected in 1718 that Aldebaran, Sirius, and 
 Arcturus had a " proper motion," as it is termed, 
 on the face of the sky ; but to Cassini is due the 
 credit of having proved beyond doubt the appa- 
 rent motion of certain stars. Observations made 
 by Ptolemy and other ancient astronomers were 
 too rough to rely on for an accurate determination 
 of the motions in question, so Cassini discarded 
 them, and had recourse to more accurate obser- 
 vations made with the telescope. He therefore 
 compared his own observations of Arcturus, made 
 at the Paris Observatory in 1738, with those made 
 by Richer at Cayenne in 1672. From these 
 observations he found that, during the 66 years 
 which had elapsed, this bright star had approached 
 the ecliptic by nearly two minutes of arc, which 
 
THE SUN 71 S JOURNEY THROUGH SPACE 49 
 
 gives an annual motion of about two seconds. 
 Observations made by Flamsteed at Greenwich, in 
 1690, were also in favour of this apparent motion. 
 To test the accuracy of his result, Cassini examined 
 the observations made by Tycho Brahe in 1581 
 observations which, although made with the 
 naked eye, were probably as accurate as they 
 could possibly be without a telescope. He found 
 that in the 154 years which elapsed between 1584 
 and 1738, the latitude of Arcturus, or its distance 
 north of the ecliptic, had diminished by about 
 five minutes of arc. This gives an annual motion 
 of about two seconds of arc, thus agreeing closely 
 with measures made with the telescope. Modern 
 measures give Arcturus a "proper motion" of about 
 2*3 seconds of arc per annum. The neighbouring 
 star, -q Bootis, showed no such change in its appa- 
 rent position on the celestial vault. Cassini also 
 showed that Ptolemy's observations of Sirius, 
 compared with those of Halley, gave a consider- 
 able " proper motion " to that brilliant star. 
 Observations in recent years give a motion of 
 about 1*32 seconds per annum. 
 
 Modern observations have revealed the existence 
 of still larger " proper motions." Thus the small 
 star, No. 1830 of Groombridge's catalogue (the so- 
 called " runaway star "), has an annual proper 
 motion of nearly 7 seconds of arc ; Lacaille 9352, 
 about 6-9 seconds ; Cordoba 32416, 6'1 seconds ; 61 
 Cygni, 5'2 seconds ; Lalande 21,185, 4'7 seconds ; 
 
 E 
 
50 STUDIES IN ASTRONOMY 
 
 c Indi, 4*6 seconds ; ft Cassiopeiae, 3*7 seconds ; 
 a Centauri, 3*7 seconds ; and many others of 
 smaller amount. Quite recently it lias been 
 found by Mr. Innes and Dr. Kapteyn that a 
 star in the southern constellation, Pictor, has a 
 proper motion of no less than 8*7 seconds per 
 annum, a motion which would carry it through a 
 space in the sky equal to the moon's apparent 
 diameter in 214 years. The proper motions of 
 over five thousand stars have now been accurately 
 determined, and further researches may perhaps 
 show that no really " fixed " star exists in the 
 heavens. 
 
 Of twenty-five stars with proper motions ex- 
 ceeding two seconds of arc per annum, there 
 are only two a Centauri and Arcturus whose 
 magnitude exceeds the third. As a large proper 
 motion is considered as a test of proximity 
 to the earth, this result is very significant a 
 significance accentuated by the fact that about 
 half the number have yielded a measurable paral- 
 lax. M. Ludwig Struve found for stars of the 
 6th magnitude an average proper motion of 
 eight seconds per century. As the mean distance 
 of these stars should be on the assumption of 
 uniform size and brightness ten times that of a 
 1st magnitude star, we should find the mean 
 proper motion of Ist-magiiitude stars to be eighty 
 seconds in a hundred years. The twenty brightest 
 stars in the heavens, however, show an average 
 
THE SUN'S JOURNEY THROUGH SPACE 51 
 
 motion of only sixty seconds in the same time. 
 Stars of the 2nd magnitude show a still slower 
 motion. Instead of fifty seconds per century, due 
 to their hypothetical distance, twenty-two stars 
 of this magnitude yielded a mean motion of only 
 seventeen seconds. From these results it is clear 
 that the brightness of a star is not an absolute 
 criterion of its distance ; but, generally speaking, 
 we may assume that the fainter stars are, on the 
 whole, farther from the earth than the brighter 
 ones, and that, as a general rule, faint stars have 
 small proper motions. 
 
 How are these proper motions to be accounted 
 for? They may be due to two causes: either a 
 real motion in the stars themselves, or else a 
 motion of the earth and sun through space, which 
 would produce an apparent motion in the opposite 
 direction. Probably, in most cases of proper 
 motion, both causes combine to produce the 
 observed effect. The sun's motion through space 
 was suggested by the famous Bradley so far back 
 as 1748. He says: "If our own solar system be 
 conceived to Change its Place with respect to 
 Absolute Space, this might, in Process of Time, 
 occasion an apparent Change in the angular Dis- 
 tances of the fixed Stars ; and in such Case, the 
 Places of the nearest Stars being most Affected, 
 than of those that are very remote ; their relative 
 Positions might seem to alter ; tho' the Stars 
 themselves were really immovable. And on the 
 
52 STUDIES IN ASTRONOMY 
 
 other hand, if our System be at Rest, and any of 
 the stars really in Motion, this might likewise 
 vary their apparent Positions ; and the more so, 
 the nearer they are to us, or the swifter their 
 Motions are, or the more proper the Directions of 
 the Motion is, to be rendered perceptible to us. 
 Since then the Relative Places of the stars may 
 be changed from such a Variety of Causes, con- 
 sidering that amazing Distance at which it is 
 certain some of them are placed, it may require 
 the Observations of Many Ages to determine the 
 Laws of the apparent Changes, even of a single 
 Star: much more difficult therefore must it be 
 to settle the Laws relating to all the most remark- 
 able Stars." 1 
 
 In 1760, Tobias Mayer published the proper 
 motion of 80 stars, and from an examination 
 of these Mayer thought them unfavourable to the 
 hypothesis of solar motion. Lambert, in 1761, 
 thought it possible that all the stars, including 
 the sun, had a motion through space, but that 
 the sun's motion of rotation on its axis did not 
 necessarily imply a motion of translation. Lalande, 
 however, considered that a motion of rotation on 
 an axis does necessitate a motion of translation, 
 and this conclusion is now looked upon as highly 
 probable, although we cannot absolutely prove it 
 to be true. 
 
 1 Philosophical Transactions of the Eoyal Society, vol. xlv., for 
 the year 1748, pp. 40, 41. 
 
THE SUN'S JOURNEY THROUGH SPACE 53 
 
 In 1783, Sir William Herschel turned his atten- 
 tion to the question of the sun's motion in space, 
 and found that it was moving towards a point 
 near the star A. Herculis. The investigations of 
 Argelaiider, Peters, and O. Struve led to the follow- 
 ing result, as stated by M. O. Struve, in his Etudes 
 d* Astronomic Stellaire, p. 108 : " Le mouvement 
 du systeme solaire dans 1'espace est dirige vers un 
 point de la voute celeste, situe sur la ligiie droite 
 qui joint les deux etoiles, de troisieme grandeur, 
 TT et ft Herculis, a un quart de la distance apparent 
 de ces etoiles, a partir de IT Herculis. La vitesse 
 de ce mouvement est telle, que le soleil, avec tous 
 les corps qui en dependent, avance annuellement, 
 dans la direction indiquee, de 1*623 fois le rayon 
 de 1'orbite terrestre, ou de 33,550,000 milles geo- 
 graphiques. L'erreur probable de ce dernier chiffre 
 s'eleve a 4,733,000 milles geogr., ou a un septieme 
 de la valeur trouvee. Ou peut done parier 40,000 
 centre un, pour la realite du mouvement propre 
 progressif du soleil, et 1 centre 1 qu'il est compris 
 entre les limites de 38 et de 29 millions de milles 
 geographiques." 
 
 Subsequent researches on this interesting ques- 
 tion have fully confirmed the general accuracy of 
 this conclusion, at least so far as the direction of 
 the motion is concerned. The following are some 
 of the positions found for the solar " apex," as it 
 is termed, or the point towards which the sun 
 is moving. O. Struve placed the apex a little 
 
54 STUDIES IN ASTRONOMY 
 
 following the star p Herculis, and between that star 
 and 6 Herculis; Ubaghs and Airy found a point 
 not far from Sir William Herschel's, near X Her- 
 culis ; L. de Ball between 84 and 106 Herculis 
 Rancken and O. Stumpe, near y Lyrse, and L. Boss 
 a point near e Lyrae, a little north following Vega 
 Subsequent calculations by O. Stumpe place the 
 "apex " at various points in the constellation Lyra, 
 the position of the point found varying with the 
 mean magnitudes, and proper motions of the stars 
 used in the computation, 1 but as Lyra is a com- 
 paratively small constellation, the results may be 
 considered as fairly accordant. Professor New- 
 comb thinks (1902) that the most probable posi- 
 tion of the apex is in R.A. 18 h 40 m , Declination 
 N, 35. This is a point about 4 degrees South of 
 Vega, and between that star and p Lyrse, a little 
 nearer to /?. 
 
 As to the actual velocity with which the sun is 
 speeding through space, O. Struve has found, from 
 a consideration of the proper motions of 392 stars, 
 that the distance travelled by the sun in one year 
 is equal to the mean distance of stars of the 1st 
 magnitude divided by 600,000. Now, the mean 
 parallax of stars of the 1st magnitude has been 
 found by Dr. Elkin to be 0*089 of a second of arc, 
 which corresponds to a distance of about 2,317,500 
 times the sun's distance from the earth. Hence 
 the distance traversed by the sun in one year 
 
 1 Tlie Observatory, November, 1896. 
 
THE SUN'S JOURNEY THROUGH SPACE 55 
 
 would be about four times the sun's distance from 
 the earth, or about two-thirds of the earth's 
 velocity in its orbit round the sun. Now as the 
 latter velocity is about 18 miles per second, we 
 have the sun's velocity in space about 12 miles a 
 second. Following Struve's method, other astrono- 
 mers have found a velocity ranging from about 6 
 to 30 miles a second. The discordance in these 
 results is chiefly due to our imperfect knowledge 
 of the distances of stars of different magnitudes. 
 
 By means of the spectroscope we can obtain a 
 probably more accurate determination of the sun's 
 velocity through space. As is well known, the 
 velocity of a star in the line of sight can be found 
 by measuring the displacement of the lines visible 
 in the star's spectrum. Now, the stars near the 
 position of the solar " apex " should be approach- 
 ing the earth on account of the solar motion, and 
 those at the opposite point of the sky, called the 
 " ant-apex," should be receding. This method has 
 been employed by several astronomers, especially 
 by Vogel at the Potsdam Observatory. This able 
 astronomer has found, from an examination 
 of 40 stars, that the sun's velocity throtigh space 
 is about 7 miles a second, but an examination of 
 a larger number of stars would be necessary before 
 we could consider this result as thoroughly estab- 
 lished. From an examination of the spectra of 
 14 nebulae, the late Professor Keeler, of the Lick 
 Observatory, found velocities in the line of sight, 
 
56 STUDIES IN ASTRONOMY 
 
 and from these the French astronomer, Tisserand, 
 has deduced a velocity of about 9J miles for the solar 
 motion, a result which does not differ very widely 
 from that found by Vogel. More recent estimates 
 vary from about 11 '4 to 12'3 miles a second. The 
 latter velocity would represent an annual motion 
 of about four times the sun's distance from the 
 earth. This agrees with O. Struve's result, and 
 would carry the sun to the distance of Neptune's 
 orbit in about 7J years. To reach a star at the 
 distance of a Centauri would take nearly 69,000 
 years ! 
 
 An interesting question is suggested with refer- 
 ence to the sun's motion through space. Does 
 this motion take place in a straight line, or in a 
 gigantic orbit round some unknown centre ? In 
 The Observatory for January, 1896, Mr. G. C. 
 Bompas considers that the various determinations 
 of the " solar apex " show a tendency to a drift 
 along the edge of the Milky Way, and that this 
 drift " seems to point to a plane of motion of the 
 sun, nearly coinciding with the plane of the Milky 
 Way, or perhaps more nearly with the plane of 
 that great circle of bright stars, first described by 
 Sir William Herschel as inclined about 20 to the 
 Galaxy, and which passes through Lyra, in or 
 near which constellation the solar apex lies." 
 
 Recent researches seem to show that the centre 
 of the Milky Way probably lies in a direction 
 south of Cassiopeia's Chair and a little south of 
 
THE SUN 7 'S JOURNEY THROUGH SPACE 57 
 
 the Milky Way (about R.A. 24 h ), the sun and 
 solar system being probably situated a little to the 
 south of the Galactic centre, and a little to the 
 north of the plane of the Milky Way. Now, 
 the " apex " of the solar motion lies, roughly, 90 
 from this point, and, judging from the position 
 of the " apex " found by Sir William Herschel, 
 Argelander, and Airy (about R.A. 17 h 30 m ), 
 and that indicated by recent researches (about 
 R.A. 19 h ), there may perhaps be a shift of the 
 "apex" towards the centre of the Milky Way, 
 which would be the case were the sun revolving 
 round that centre. This supposed shift in the 
 position of the "apex" may of course be more 
 apparent than real, and may possibly be partly, 
 or altogether, due to errors of calculation. The 
 various positions, however, found for the "apex" 
 show a tendency at least to shift in position 
 towards the supposed centre of the Milky Way. 
 However this may be, it seems not improbable 
 that the sun may be revolving round the centre 
 of gravity of the Milky Way, which may also be 
 the centre of gravity of the whole system of stars 
 composing our visible stellar universe. 
 
 The existence of dark bodies in the universe has 
 been suspected by astronomers. Should the sun, 
 in its journey through space, come into collision 
 with one of these dark bodies, the result would 
 be were the body a large one most disastrous 
 to the earth. The sun's heat would be increased 
 
58 STUDIES IN ASTRONOMY 
 
 to an enormous extent, and, as foretold by St. 
 Peter, " the heavens being 011 fire " would " be 
 dissolved," and " the elements " would " melt with 
 fervent heat, the earth also and the works that 
 are therein" w'ould "be burned up." As, how- 
 ever, the approaching dark body would at a 
 certain distance begin to shine by reflected light 
 from the sun, it would if a large body like the 
 sun be visible for some years before the final 
 catastrophe. It would first appear as a small star, 
 and then becoming brighter and brighter as it 
 approached the sun, would form a veritable " sign 
 of the Son of man in heaven." 1 
 
 1 St, Matthew, chap. 24, v. 30. 
 
VI 
 
 The Story of Gamma Virginia 
 
 THE famous binary or revolving double 
 star, known to astronomers as Gamma (y) 
 Virginis, lies close to the celestial equator 
 about 1 to the south and about 15 to the 
 north- west of the bright star Spica (Alpha of the 
 same constellation) with which and the brightest 
 stars of the constellation Virgo, or the Virgin, it 
 forms a V-shaped figure, Gamma being at the 
 junction of the two upper branches. The bright- 
 ness of y Virginis is about that of an average star 
 of the 3rd magnitude (2*91 Harvard). Variation 
 of light has, however, been suspected in one or 
 both components, and this question of light vari- 
 ation will be considered further on. The Persian 
 astronomer, Al-Sufi, in his "Description of the 
 Fixed Stars," written in the tenth century, rates 
 the star of the 3rd magnitude, and describes it 
 as " the third of the stars of al-anva, which is a 
 mansion of the moon," the first and second of 
 these "mansions" being (3 and 77 Virginis, the 
 fourth 8, and the fifth e Virginis, these five stars 
 
60 STUDIES IN ASTRONOMY 
 
 forming the two upper branches of the V-shaped 
 figure referred to above, y was called zawiyah-al- 
 anvd, " the corner of the barkers," from its posi- 
 tion in the figure which formed the thirteenth 
 lunar mansion of the old astrologers. It was also 
 called Porrima and Postvarta in the old calendars. 
 These ancient names for the stars are curious, and 
 their origin is doubtful. 
 
 The fact that y Virginis really consists of two 
 stars close together seems to have been discovered 
 by the famous astronomer Bradley in 1718. He 
 recorded the position of the components by 
 stating that the line joining them was then 
 exactly parallel to the line joining the stars a and 
 8 Virginis. This was, of course, only a rough 
 method of measurement, and the position thus 
 found by Bradley being probably more or less 
 erroneous, has given much trouble to computers 
 of the orbit described by the two stars round 
 each other, or rather round their common centre 
 of gravity. Bradley does not give the apparent 
 distance between the component stars in his time, 
 but we may conclude from the orbit which is 
 now w r ell determined that they were then nearly 
 at their greatest possible distance apart. The 
 pair was again measured by Cassini in 1720, by 
 Tobias Mayer in 1756, and by Sir William Herscliel 
 in the years 1780 to 1803. These measures showed 
 clearly that the distance was steadily diminishing, 
 and that the "position angle" of the two stars 
 
THE STORY OF GAMMA VIRGIJ4IS 61 
 
 was also decreasing. This decrease in position 
 angle measured from north round by east, south, 
 and west, from to 360 shows that the apparent 
 motion is what is called retrograde, or in the 
 direction of the hands of a clock, direct, or 
 " planetary motion," being in the opposite direc- 
 tion. The star was again measured by Sir John 
 Herschel and South in the years 1822 to 1838, by 
 W. Struve in the same years, and by Dawes and 
 other observers from 1831 to the present time. 
 The recorded measures are very numerous, and 
 have enabled computers to determine the orbit 
 with considerable accuracy. The rapid decrease 
 in the distance between the components from 
 1780 to 1834 indicated that the apparent orbit is 
 very elongated, and that possibly the two stars 
 might "close up" altogether, and appear as a 
 single star even in telescopes of considerable 
 power. This actually occurred in the year 1836, 
 or at least the stars were then so close together 
 that the best telescope of those days failed to 
 show y Virginis as anything but a single star. 
 Of course, it would not have been beyond the 
 reach of the giant telescopes of our day. From 
 the year 1836 the pair began to open out again, 
 and at present the distance is again approaching 
 a maximum. It can now be seen with a small 
 telescope, and forms a fine telescopic object with 
 an instrument of moderate power. A good tele- 
 scope of 2 inches aperture should show it well. 
 
62 STUDIES IN ASTRONOMY 
 
 The general character of the orbital motion 
 may be described as follows: In 1718, at the 
 time of Bradley's observation, the companion star 
 was to the north-west of the primary star. It 
 then gradually moved towards the west and 
 south, and in 1836, when at its minimum distance, 
 it was to the south-east. From that date it again 
 turned towards the north, and at present it is 
 north-west of the primary star, and not far from 
 the position found by Bradley in 1718. 
 
 The first to attempt a calculation of the orbit 
 described by this remarkable pair of revolving 
 suns was Sir John Herschel, who in the year 1831 
 found a period of 513 years. In 1833 he recom- 
 puted the orbit, and found a period of nearly 
 629 years. Both these periods were much too 
 long, but the data then available were not suffi- 
 cient for the calculation of an accurate orbit. 
 From these results, however, Sir John Herschel 
 predicted that " the latter end of the year 1833, 
 or the beginning of the year 1834, will witness 
 one of the most striking phenomena which sidereal 
 astronomy has yet afforded, viz. the perihelion 
 passage of one star round another, Avith the 
 immense angular velocity of between 60 and 70 
 per annum that is to say, of 1 in five days. 
 As the two stars will then, however, be within 
 a little more than half a second of each other, 
 and as they are both large and nearly equal, none 
 but the very finest telescopes will have any chance 
 
THE STORY OF GAMMA VIRGINIS 63 
 
 of showing this magnificent phenomenon. The 
 prospect, however, of witnessing a visible measur- 
 able change in the state of an object so remote, 
 in a time so short, may reasonably be expected 
 to call into action the most powerful instrumental 
 means which can be brought to bear on it." This 
 prediction was not fulfilled until the year 1836, 
 when the pair " closed up out of all telescopic 
 reach," except at the Dorpat Observatory, where 
 a magnifying power of 848 still showed an 
 " elongation " in the apparent disc of the star. 
 
 The orbit found by Sir John Herschel was a 
 tolerably elongated ellipse, with its longer axis 
 lying north-east and south-west. This was not 
 quite correct, for w~e now know that the axis lies 
 north-west and south-east, and that the apparent 
 orbit is much more elongated than Sir John 
 Herschel at first supposed. This was soon recog- 
 nized by Herschel himself, and he came to the 
 conclusion that he and other computers had been 
 misled by Bradley's observation in 1718. He then 
 rejected this early and evidently rough observation, 
 and using measures up to 1845 he found a period 
 of about 182 years, which we now know to be not 
 far from the truth. The orbit was also computed 
 by the German astronomer Madler, w r ho found 
 periods of 145, 157, and 169 years ; by Hind, 141 
 years ; by Henderson, 143 years ; by Jacob, 133, 
 157 J, and 171 years; by Adams, 174 years; by 
 Flammarion, 175 years ; and by Admiral Smyth, 
 
64 STUDIES IN ASTRONOMY 
 
 who found 148 and 178 years. All these periods 
 we now know are too small, and they show the 
 difficulty and uncertainty of calculating a binary 
 star orbit when the data are insufficient. Two 
 orbits were computed by Dr. Doberck in recent 
 years, one with a period of 180*54 years and the 
 other 179*65 years. These orbits represent the 
 measures well, but the period has been further 
 extended by Dr. See, who finds a period of 194*0 
 years. This orbit represents recent measures 
 closely, and seems to be very satisfactory. The 
 angular motion is now very slow, and the star will 
 remain an easy object for small telescopes for 
 many years to come. The apparent orbit of the 
 pair is a very elongated ellipse, and, as Admiral 
 Smyth says, " more like a comet's than a planet's." 
 The real ellipse has a very high eccentricity, nearly 
 0*9, indeed the highest of all known binary stars, 
 and not much less than that of Halley's comet ! 
 
 As I said above, the variability of the light of 
 one or both components of y Virginis has been 
 strongly suspected. So far back as 1851 and 1852 
 O. Struve paid particular attention to this point. 
 His observations in those years show that some- 
 times the two stars were exactly equal in bright- 
 ness, and sometimes the southern star, the one 
 generally taken as the primary star, was from 0*2 
 to 0*7 of a magnitude brighter than the other. 
 There seems to be little doubt that some variation 
 really takes place in the relative brightness of the 
 
THE STORY OF GAMMA VIRGINIS 65 
 
 pair. This is clearly indicated by the measures of 
 the position angle. For example, in the year 1886 
 Professor Hall recorded the position angle as 154*9, 
 e T idently measuring from the northern star as the 
 brighter of the two, while in 1887 Schiaparelli 
 gives 334*2 (or about 180 more), thus indicating 
 that he considered the southern star as the 
 primary, or brighter of the pair. Burnham found 
 153-4 in 1889, and Dr. See 332'5 in 1891. This is 
 also shown by earlier measures, for Dembowski 
 found 353'6 in 1854, and 171'2 in 1855. The period 
 of variation would seem to be short, for O. Struve 
 found the southern star half a magnitude brighter 
 than the other on April 3, 1852, while on April 
 29 of the same year he found them "perfectly 
 equal." He thought that the variation was about 
 0*7 of a magnitude, but that the climate of 
 Poulkova, where he observed, was not suitable for 
 such observations. This variation is very inte- 
 resting, and the question should be thoroughly 
 investigated with a good telescope. 
 
 The distance of y Virginis from the earth has 
 not been directly measured, but from spectroscopic 
 measures of motion in the line of sight, Belopolsky 
 has recently found a parallax of 0"'051, and a com- 
 bined mass equal to 15 times the mass of the sun. 
 Taking this mass, we have the mass of each com- 
 ponent equal to 7J times the sun's mass. Hence, 
 if of the same density and surface luminosity as 
 the sun, each component would be 3*83 times 
 
 F 
 
66 STUDIES IN ASTRONOMY 
 
 brighter than the sun, and therefore both com- 
 ponents 7'66 times brighter. Now, the sun placed 
 at the distance indicated by Belopolsky's parallax 
 would, I find, be reduced to the brightness of a 
 star of 6'53 magnitude, or just below the range of 
 naked-eye vision, and as the photometric magni- 
 tude of y Virginis, as measured at the Harvard 
 Observatory, is 2*91, it follows that the star is 
 really 3' 62 magnitudes, or about 28 times brighter 
 than the sun. And as it should be only 7 '66 times 
 brighter, as shown above, it follows that the 
 intrinsic brightness of the star is nearly four times 
 greater than that of the sun, or else the density is 
 less and the surface greater. Possibly both causes 
 may combine to produce the above result. The 
 spectrum of the star is of the type F (Pickering), 
 that of the sun being G, so the two bodies are not 
 exactly comparable. Stars with the F type of 
 spectrum are probably somewhat brighter than 
 the sun. It seems, therefore, that the mass and 
 distance of y Virginis, as found by Belopolsky, are 
 probably not far from the truth. 
 
VII 
 
 The Pleiades 
 
 THE Pleiades form perhaps the most remark- 
 able group of stars in the heavens, and are 
 probably familiar to most people, even to 
 those whose knowledge of the sky is limited to a 
 few of the brightest stars. The cluster is a very 
 interesting and beautiful one, and forms a striking 
 object in a clear sky. There is no other group in 
 the heavens similar to it in the brightness and 
 closeness of the component stars, and it seems to 
 have attracted attention from the earliest ages. 
 Job says, " Canst thou bind the sweet influences of 
 the Pleiades, or loose the bonds of Orion ? " And 
 Hesiod, writing nearly 1000 years B.C., speaks of 
 the Pleiades in words thus translated by Cooke 
 
 " There is a time when forty days they lie, 
 And forty nights conceal'd from human eye, 
 But in the course of the revolving year, 
 When the swain sharps the scythe, again appear." 
 
 This passage refers to the disappearance of the 
 group in the sun's rays in summer, and their re- 
 appearance in the evening sky in the east at harvest- 
 time. Hesiod also speaks of them as the Seven 
 
68 STUDIES IN ASTRONOMY 
 
 Virgins, daughters of Atlas and Hesperus, and in 
 Cicero's Aratus they are represented as female 
 heads, bearing the names Alcyone, Celaeno, Electra, 
 Taygeta, Asterope, and Maia, names by which they 
 are still known to astronomers. The origin of the 
 name Pleiades is somewhat doubtful. Some think 
 that it is derived from the Greek word plein, " to 
 sail," as their appearance before sunrise in May 
 announced the arrival of the season for navigation. 
 Others derive the name from the word pleios, 
 " full," a name perhaps suggested by the appear- 
 ance of the cluster. 
 
 Although seven stars are almost universally 
 referred to by the ancients, Homer only speaks of 
 six ; and this is the number now visible to average 
 eyesight. A larger number has, however, been 
 seen with the naked eye by those gifted with 
 exceptionally keen vision. Mostlin, a contempo- 
 rary of Kepler, is said to have seen fourteen, and 
 he actually measured and recorded the position 
 of eleven with wonderful accuracy, without the 
 aid of a telescope. In recent years Carringtoii 
 and Denning have seen fourteen, and Miss Airy, 
 daughter of the late Astronomer Royal, could see 
 twelve. But to most eyes, probably only six are 
 visible with any certainty. There is a tradition 
 that, although seven stars were originally visible 
 in the group, one disappeared at the taking of 
 Troy. Professor Pickering has recently discovered 
 that the spectrum of Pleione, which forms a wide 
 
THE PLEIADES , 69 
 
 pair with Atlas, bears a striking resemblance to 
 that of P Cygni, the so-called " temporary star" 
 of 1600. The similarity of the spectra suggests* 
 that Pleione may possibly, like the star in Cygnus, 
 be subject to occasional fluctuations of light, 
 which might perhaps account for its visibility to 
 the naked eye in ancient times. It is a curious 
 fact that both Ptolemy and Al-Sufi give the posi- 
 tions of only four stars in the Pleiades, and it is 
 very difficult or impossible to identify these stars 
 with stars in the group as they are at present. 
 The brightest of all, Alcyone, now about 3rd mag- 
 nitude, does not seem to be mentioned at all by 
 Al-Sufi, as he says distinctly that the brightest 
 (No. 32 of Taurus) is outside the Pleiades, " on 
 their northern side." This 32nd star seems to 
 have disappeared or, at least, diminished greatly 
 in brightness since the days of Ptolemy and 
 Al-Sufi. More than four stars were, however, 
 seen by Al-Sufi, for he adds, " It is true that the 
 stars of the Pleiades much exceed the four men- 
 tioned, but I limit myself to these four because 
 they are very near each other and the largest; 
 this is why I have mentioned them, neglecting 
 the others." It seems therefore probable that 
 Alcyone has increased considerably in brightness 
 since Al-Sufi observed the group in the tenth 
 century. 
 
 The grouping of even six stars visible to the 
 naked eye in so small a space is very remarkable. 
 
70 STUDIES IN ASTRONOMY 
 
 Considering the number of stars visible in the 
 whole sky without optical aid, Michell, writing in 
 1767, calculated by the mathematical theory of 
 probabilities that the chances are 500,000 to 1 
 against the close arrangement of the six stars in 
 the Pleiades being merely the result of accident. 
 He therefore concluded " that this distribution 
 was the result of design, or that there is reason or 
 cause for such an assemblage." Modern observa- 
 tions show that his conclusion was sound. The 
 common "proper motion" of a large number of 
 the stars composing the Pleiades show that they 
 are in some way physically connected. 
 
 Although to a casual observer the component 
 stars may appear of nearly equal brightness, 
 there is in reality a considerable difference in their 
 relative brilliancy. Photometric measures show 
 that Alcyone, the brightest of the group, is of the 
 3rd magnitude; Maia, Electra, and Atlas of the 
 4th; Merope, 4|; Taygeta, 4J; Celseno about 5J; 
 and Asterope about the 6th. Pleione is about 5, 
 but it lies so close to Atlas that to most eyes the 
 two will probably appear as one star. About 
 thirty more range from the 6th to the 8th magni- 
 tude, and this is about the number visible Avith a 
 good opera-glass or binocular. Galileo counted 
 thirty-six with his small telescope, but with 
 modern telescopes the number is largely increased. 
 Some years ago M. Wolf published a chart show- 
 ing about 600 stars, made from his own observa- 
 
THE PLEIADES , 71 
 
 tions. Photography has further added to this 
 number. On a photograph taken at the Paris 
 Observatory, in 1887, with an exposure of three 
 hours, no less than 2326 stars have been counted 
 on a space of about 3 square degrees. The faintest 
 stars on this photograph are supposed to be of the 
 17th magnitude; and as Alcyone is of the 3rd, 
 there is a difference of 14 magnitudes between the 
 brightest and faintest star of the group. This 
 would seem to indicate that there must be an 
 enormous difference in size between the compo- 
 nents ; but from photographs taken by Professor 
 Bailey of the Pleiades and the surrounding regions, 
 it appears that the latter are quite as rich in 
 faint stars as the cluster itself. We may there- 
 fore conclude that many of the faint stars, appa- 
 rently mixed up with the brighter stars, do not 
 really belong to the cluster, and probably lie at a 
 great distance behind it. 
 
 The late Mr. Webb noticed the remarkable 
 "absence of colour" hi the Pleiades, most of the 
 stars being white, with the exception of "one 
 minute ruby star and an orange outlier;" and 
 this has been confirmed by other observers. 
 Professor Pickering finds that most of the brighter 
 components show a spectrum of the first or Sirian 
 type, and he says, "It is very improbable that 
 chance alone has brought together so many bright 
 stars in the same portion of the heavens. Most of 
 them probably had a common origin." 
 
72 STUDIES IN ASTRONOMY 
 
 The brilliancy of the Pleiades cluster would 
 naturally suggest a comparative proximity to the 
 earth. Attempts to determine their distance 
 have, however, hitherto proved unsuccessful. 
 This would indicate that the distance is very 
 great, and would, of course, lead to the conclusion 
 that the group is of vast dimensions. An idea of 
 the approximate distance may, however, be 
 arrived at indirectly by a consideration of the 
 " proper motion " of the principal stars. Professor 
 Newcomb finds a proper motion for Alcyone of 
 about 5*8 seconds of arc per century. This motion 
 is in a direction nearly opposite to that of the 
 sun's motion in space, and may possibly be due to 
 that cause. If we assume that this proper motion 
 is wholly due to the effect of the sun's real motion 
 at the rate of, say, 10 miles a second, the distance 
 of Alcyone would, I find, correspond to a " light- 
 journey " of about 192 years. Placed at this dis- 
 tance, the sun would be reduced to a star of about 
 the 9th magnitude, or 6 magnitudes fainter than 
 Alcyone. This would imply that Alcyone is 
 about 250 times brighter than the sun. If of the 
 same density, its volume would therefore be 
 nearly 4000 times the sun's volume. But as its 
 spectrum is of the Sirian type, it cannot be 
 properly compared with our sun. 
 
 In the year 1859 the well-known astronomer 
 Tempel announced his discovery of a faint nebu- 
 losity extending in a southerly direction from 
 
THE PLEIADES 73 
 
 
 
 Merope, the nearest bright star to Alcyone. This 
 interesting discovery was partially confirmed by 
 other astronomers ; but from its visibility to some 
 observers with small telescopes, and the failure of 
 others to see it with much larger instruments, the 
 variability of its light was strongly suspected. 
 The question remained in doubt for many years, 
 but has now been finally set at rest by photo- 
 graphy, which shows not only a mass of nebulous 
 light surrounding Merope, but other nebulous 
 spots, involving Alcyone, Maia, and Electra. 
 Indeed, photographs taken by Dr. Isaac Roberts 
 show that all the brighter stars of the group are 
 more or less immersed in nebulosity, the remains, 
 perhaps, of the nebulous matter from which the 
 cluster has been evolved. Tennyson's simile of 
 "tangled in a silver braid" is now shown to be a 
 physical reality. On a photograph taken at the 
 Paris Observatory a remarkable, narrow, nebulous 
 ray runs nearly east and west from the Maia nebula, 
 north of Alcyone, and apparently connects some 
 stars of the 8th to llth magnitude. The nebula 
 surrounding Mala is of a somewhat spiral form, 
 and probably represents the spiral nebula from 
 which the star has been evolved. The existence 
 of this nebula was not even suspected until it was 
 revealed by photography. It was afterwards seen 
 with the great 30-inch refractor of the Russian 
 Observatory at Pnlkowa. Were its existence un- 
 known, however, it would probably have escaped 
 
74 STUDIES IN ASTRONOMY 
 
 detection, even with this large telescope, as it is 
 one thing to see a faint object known to exist, 
 and another to discover it independently. Maia is 
 surrounded by several faint stars of the 12th to 
 14th magnitude, and the Russian observers believe 
 that one of these is variable in light, as it was 
 distinctly seen on February 5, 1886, when its magni- 
 tude was carefully determined with reference to 
 the neighbouring stars ; but on February 24 of the 
 same year it could not be seen with a telescope of 
 15 inches aperture. Some other stars in the group 
 have also been suspected of variation. 
 
 Photographs by Barnard, Wolf, and others 
 show that the cluster is surrounded by patches of 
 nebulous light, "covering at least a hundred 
 square degrees of the sky." All the principal 
 stars of the Pleiades show a spectrum interme- 
 diate between the " Orion " and the Sirian type, 
 showing that they are in an early stage of their 
 life-history, and have only recently, compara- 
 tively speaking, emerged from the nebulosity 
 which surrounds them. 
 
 \\ 
 
VIII 
 
 Globular Star Clusters 
 
 THE term " globular cluster" has been applied 
 to those clusters of stars which evidently 
 occupy a space of more or less spherical 
 form. Some of these "balls of stars," as they 
 have been called, are truly wonderful, and are 
 among the most interesting objects visible in the 
 stellar heavens. Good specimens of the class are, 
 however, rare objects, and there are not very 
 many in the northern hemisphere. The most 
 remarkable is that called " the Hercules cluster," 
 but known to astronomers as 13 Messier, it being 
 No. 13 in the first catalogue of remarkable 
 "nebulae" formed by Messier, the famous dis- 
 coverer of comets. It was discovered by Halley 
 in 1714. This wonderful object lies between the 
 stars and 17 Herculis, nearer to the latter star. 
 It may be seen with a binocular or good opera- 
 glass as a hazy star of about the 6th magnitude. 
 When examined with a good telescope, it is at 
 once resolved into a multitude of small stars, 
 which can be individually seen and even counted 
 
76 STUDIES IN ASTRONOMY 
 
 with large telescopes. The number of stars in- 
 cluded in the cluster was estimated at 14,000 by 
 Sir William Herschel, but the real number is 
 probably much smaller. Were the number so 
 great as Herschel supposed, I find that the cluster 
 would form a much brighter object than it does. 
 From a photograph taken in America by Mr. H. 
 K. Palmer, with an exposure of two hours, he 
 finds the number of stars in the cluster to be 5482, 
 of which 1016 are " bright" and 4466 "faint." It 
 has been also well photographed at the Paris, 
 Harvard, and Lick Observatories, and by Dr. 
 Roberts and Dr. W. E. Wilson. Its globular 
 shape is evident at a glance, and we cannot doubt 
 that the stars composing it form a gigantic 
 system, probably isolated in space. Most people 
 might think that this cluster was a mass of double 
 and multiple stars, but this is not so ; the com- 
 ponents, close as they are, are too far apart to be 
 considered as true double stars. Mr. Burnham, 
 the famous double-star observer, finds one close 
 double star near the centre, and notes the remark- 
 able absence of close double stars in bright and 
 apparently compressed clusters. 
 
 In the same constellation, Hercules, between 
 the stars 17 and i, but nearer the latter, will be 
 found another object of the globular class, but 
 not so bright or so easily resolvable into stars as 
 the cluster described above. This is known as 
 92 Messier. Buffhani, examining it with a 9-inch 
 
GLOBULAR STAR CLUSTERS 77 
 
 mirror, thought the component stars brighter but 
 more compressed than in 13 Messier. A photo- 
 graph by Dr. Roberts, taken in May, 1891, shows 
 the cluster involved in nebulosity ; but Professor 
 Barnard finds that there is no trace of any real 
 nebulosity in any of the great globular clusters 
 when seen with the great Yerkes telescope. 
 
 Another fine globular cluster is that known as 
 5 Messier. It lies closely north of the 5th magni- 
 tude star 5 Serpentis. It was discovered by Kirch 
 in 1702, and was observed in 1764 by Messier, who 
 found he could see it with a telescope of one foot 
 in length, but could not resolve it into stars. Sir 
 William Herschel, with his 40-foot telescope, 
 counted about 200 stars, but could not distinguish 
 the stars near the central blaze. Sir John Her- 
 schel describes it as an excessively compressed 
 cluster of a globular form, with stars of the llth 
 to the 15th magnitude, condensed into a blaze at 
 the centre. Lord Rosse found it more than seven 
 or eight minutes of arc in diameter, with a 
 nebulous appearance in the centre. A photograph 
 taken by Dr. Roberts in April, 1892, with a 20-inch 
 reflector, shows the stars to the 15th magnitude. 
 No less than 85 variable stars have been detected 
 among the outliers of this cluster 
 
 Another fine object of this class is that known 
 as 15 Messier, discovered by Maraldi in 1745. Sir 
 John Herschel describes it as a remarkable globu- 
 lar cluster, very bright and large, and blazing in 
 
78 STUDIES IN ASTRONOMY 
 
 the centre, and he estimated the component stars 
 at about the 15th magnitude. A photograph by 
 Dr. Roberts, in November, 1899, "confirms the 
 general description." 
 
 The cluster 3 Messier in Canes Venatici is 
 another fine object of the globular class. Sir John 
 Herschel describes it as a remarkable object, 
 exceedingly bright and very large, with stars 
 from the llth to the 15th magnitude. Admiral 
 Smyth thought it contained at least 1000 stars. 
 Buffham found it resolved even in the centre with 
 a 9-inch mirror. A photograph by Dr. Roberts, 
 taken in May, 1891, with an exposure of two 
 hours, confirms the general descriptions of the 
 cluster. No less than 132 variable stars have been 
 detected among the outliers of this cluster. 
 
 We may also mention the globular cluster 
 known as 2 Messier, situated a little north of the 
 star ft Aquarii. It was discovered by Maraldi in 
 1746. Sir John Herschel compared it to a mass of 
 luminous sand, and estimated the stars to be of 
 the 15th magnitude. Sir William Herschel, with 
 his 40-foot telescope, could actually " see and dis- 
 tinguish the stars even in the central mass." Seen 
 as a single star, it was measured 7*6 magnitude at 
 Harvard Observatory, and taking the component 
 stars at 15th magnitude, I have computed that 
 the cluster contains about 800 stars. 
 
 In the southern hemisphere there are some 
 magnificent examples of globular clusters ; and, 
 
GLOBULAR STAR CLUSTERS 79 
 
 indeed, this hemisphere seems to be richer in 
 these objects than the northern sky. Among the 
 southern clusters is the truly marvellous object 
 known as o> Centauri. Its apparent size is very 
 large about two-thirds of the moon's diameter 
 and it is distinctly visible to the naked eye as a 
 hazy star of the 4th magnitude, and I have often 
 so seen it in the Punjab sky. It is mentioned, as 
 a star, by the Persian astronomer Al-Sufi, who 
 wrote a description of the heavens in the tenth 
 century. Sir John Herschel, observing it with a 
 large telescope at the Cape of Good Hope, de- 
 scribes it as " beyond all comparison the richest 
 and largest object of its kind in the heavens. . . . 
 All clearly resolved into stars of two sizes, viz. 
 thirteen and fifteen . . . the larger lying in lines 
 and ridges over the smaller ; . . . the larger form 
 rings like lace- work on it." This wonderful 
 object has recently been photographed by Sir 
 David Gill at the Royal Observatory, Cape of 
 Good Hope, and also at Arequipa, Peru, by Pro- 
 fessor Bailey, with a telescope of 13 inches aperture. 
 On the latter photograph the individual stars can 
 be distinctly seen and counted. The enumeration 
 has been made by Professor and Mrs. Bailey, and 
 a mean of their counts gives 6387 ; but Professor 
 Pickering thinks that the actual number of stars 
 contained in the cluster is about 5050, some of 
 those counted being really outside the cluster 
 itself. 
 
80 STUDIES IN ASTRONOMY 
 
 Another wonderful object is that known as 
 47 Toucani, which lies near the smaller Magellanic 
 cloud. Sir John Herschel describes it as " a most 
 magnificent globular cluster. It fills the field with 
 its outskirts, but within its more compressed part 
 I can insulate a tolerably defined circular space of 
 90 inches diameter, wherein the compression is 
 much more decided, and the stars seem to run 
 together, and this part has, I think, a pale pinkish 
 or rose colour . . . which contrasts, evidently, 
 with the white light of the rest. The stars are 
 equal, fourteen magnitude, immensely numerous 
 and compressed. . . . Condensation in three 
 stages. ... A stupendous object." There are a 
 number of other globular clusters of smaller size 
 in the southern hemisphere, but the above are the 
 most remarkable. 
 
 The actual dimensions of these globular clusters 
 is an interesting question. Are they composed of 
 stars comparable in size with our sun, or are the 
 component stars really small and comparatively 
 close together? This is a difficult question to 
 answer, as the distance of these objects from the 
 earth has not yet been determined. They may, 
 on the one hand, be collections of suns similar in 
 size to ours, and situated at a vast distance from 
 the earth; or, on the other hand, the stars com- 
 posing them may be comparatively small objects 
 lying at a distance from the earth, not exceeding 
 that of some stars visible to the naked eye. 
 
GLOBULAR STAR CLUSTERS 81 
 
 
 
 Perhaps the latter hypothesis might, at first sight, 
 be considered the more probable of the two. But 
 really there is no reason to suppose that these 
 swarms of suns are comparatively near our system. 
 The probability seems to be in favour of their 
 great distance, for in all these clusters the com- 
 ponent stars are very faint. The question of the 
 probable size of the component stars is one which 
 has not hitherto been sufficiently considered. Let 
 us examine both alternatives, and let us take the 
 cluster CD Centauri, as one in which the number of 
 the component stars has been actually counted. 
 Assuming that the number of stars producing the 
 light of the cluster, as seen with the naked eye, is 
 6387, and that they are individually equal, on an 
 average, to our sun in size, we may estimate the 
 distance and dimensions of the cluster. Taking 
 the stellar magnitude of to Centauri as 4 (as 
 estimated at the Cordoba Observatory), I find 
 that with the number 6387, the average magnitude 
 of the components would be 13J. This agrees 
 fairly well with Sir John Herschel's estimate of 
 13th to 15th magnitude. Now, to reduce the sun 
 to a star of the 13 J magnitude, I find that, 
 assuming the sun to be 27J magnitudes brighter 
 than an average star of the 1st magnitude, it 
 would be necessary to remove it to a distance 
 equal to 100 million times the sun's distance 
 from the earth a distance so great that light 
 would take over 1500 years to reach us from 
 
 G 
 
82 STUDIES IN ASTRONOMY 
 
 the cluster. Taking the apparent diameter of 
 the cluster at 20 minutes of arc, I find that its 
 real diameter, if placed at the above distance, 
 would be 581,760 times the sun's distance from the 
 earth a diameter so great that light would take 
 over 9 years to cross it. 
 
 The distance found above for w Centauri is cer- 
 tainly enormous, but judging from the average 
 distance recently found for stars of the 1st and 
 2nd magnitude, the distance of stars of magni- 
 tude 13^ on the assumption that they are of the 
 same size and brightness, and that their light is 
 merely reduced by distance would be about five 
 times greater than that found above for o> Centauri. 
 If, then, we increase the distance of the cluster 
 five times, it will be necessary to also increase the 
 diameters of the component stars to five times 
 that of the sun. This would give them a volume 
 125 times (the cube of 5) greater than that of our 
 sun a result which seems highly improbable. 
 
 If, on the other hand, we do not like to admit 
 that each of the faint points of light composing 
 the cluster is equal in volume to our sun, let us 
 reduce the distance five times. If we do so we 
 must also diminish the diameters of the component 
 stars five times. This would make them about 
 173,000 miles in diameter. Even this reduction of 
 the distance to one-fifth of the value found above 
 would still leave the cluster at an immense distance 
 from the earth a distance represented by over 
 
GLOBULAR STAR CLUSTERS 83 
 
 
 
 300 years of light-travel. Portions of the Milky 
 Way are, however, probably farther from us than 
 this. 
 
 If we reduce the distance to one-fifth, we must 
 also reduce the diameter of the cluster to one-fifth. 
 This gives a diameter of 116,350 times the sun's 
 distance from the earth. Now, assuming that the 
 6000 stars included in the cluster are equally dis- 
 tributed through the spherical space containing 
 the cluster, I find that the distance from each star 
 to its nearest neighbour would be over 6000 times 
 the sun's distance from the earth. 
 
 There is, however, another point to be considered 
 with reference to the size of the bodies composing 
 a globular cluster. This is the character of their 
 light. If the components of to Centauri give a 
 spectrum of the first or Sirian type, the above 
 conclusions would be modified to some extent. 1 I 
 have shown in another chapter that Sirius is much 
 brighter than our sun would be if placed at the 
 same distance, although the mass of Sirius is but 
 little more than twice the sun's mass. The com- 
 ponents of a star cluster might therefore be if of 
 the Sirian type of spectrum as bright as the sun, 
 and at the same time have a smaller mass and 
 volume than their apparent brightness might 
 suggest. 
 
 The most probable conclusion seems to be that 
 
 1 Professor Pickering finds that the majority of the stars in 
 globular clusters have spectra of the first or Siriau type (A). 
 
84 STUDIES IN ASTRONOMY 
 
 these globular clusters are composed of stars 
 smaller than our sun in absolute size, and dimin- 
 ished in brightness by their great distance from 
 the earth. They are, however, probably well 
 within the boundary of our visible universe, and 
 must not be looked upon as external galaxies. 
 
IX 
 
 The Sun's Stellar Magnitude 
 
 THE stellar magnitude of the sun is the num- 
 ber which represents its brightness on the 
 same scale in which the " magnitudes " or 
 brightness of the stars are represented. In this 
 scale the "light-ratio," as it is termed, is now 
 generally taken at 2*512 (of which the logarithm is 
 0-4). This " light-ratio " denotes that a star of the 
 1st magnitude is 2*512 times brighter than a star 
 of the 2nd magnitude, a star of the 2nd mag- 
 nitude 2*512 times brighter than one of the 3rd 
 magnitude, and so on. 
 
 In ancient times all the very bright stars were 
 classed together as of the 1st magnitude, but as 
 many of the so-called 1st magnitude stars, such 
 as Sirius, Arcturus, Vega, Capella, etc., are con- 
 siderably brighter than other 1st magnitude 
 stars, like Aldebaran, Altalr, Spica, etc., this 
 classification is not sufficiently accurate for the 
 requirements of modern science. These very 
 bright stars are therefore now considered as 
 brighter than the 1st magnitude, and their 
 
86 STUDIES IN ASTRONOMY 
 
 brightness is represented by a decimal fraction, 
 the scale thus beginning from 0, or zero. But 
 Sirius, the brightest star in the heavens, has been 
 found by photometric measures to be brighter than 
 the zero magnitude, and its stellar magnitude is 
 therefore represented by a minus quantity. The 
 Harvard measures make it 1*58, or 1*58 magni- 
 tudes brighter than "zero magnitude." Now, 
 what figure would represent the brightness of the 
 sun on this scale ? The sun's brightness is so 
 vastly greater than even a star like Sirius that it 
 might be supposed that a very large number would 
 be required to represent its brightness in the stellar 
 scale of magnitudes. This, however, is not the 
 case. As will be seen, the relative brightness of 
 the stars in the assumed scale form a geometrical 
 series, and increases very rapidly. Thus an 
 average star of the 1st magnitude is 100 times 
 brighter than one of the 6th, and 10,000 times 
 brighter than a star of the llth magnitude, and 
 so on. 
 
 Various attempts have been made to determine 
 the sun's stellar magnitude, but owing to its 
 intense brilliancy, its accurate determination is a 
 matter of no small difficulty. Comparing its light 
 with that of the moon, Wollaston, in 1829, found it 
 801,072 times brighter; Bond,in 1861, found 470,000, 
 and (by another method) 340,000; and Zollner 
 found 618,000. These results are rather discordant, 
 but Zollner's estimate is the one usually accepted 
 
THE SUN'S STELLAR MAGNITUDE 87 
 
 as the most reliable. Wollaston found that the 
 sun is 25*75 magnitudes brighter than Sirius ; 
 Steinheil, 23*96 ; Bond, 24*44, and Clark, 23*89. The 
 arithmetical mean of these determinations is 24*51. 
 Taking the magnitude of Sirius at 1*58, we have 
 the sun's stellar magnitude -26*09. Professor 
 Simon Newcomb adopts the value 26*4, and Pro- 
 fessor C. A. Young -26*3. 
 
 As there seems to be some uncertainty as to the 
 accuracy of this value, the following method of 
 computing it has been suggested to me by my 
 friend, Mr. W. H. S. Monck. Taking one of the 
 larger planets when in "opposition," we can 
 determine with the photometer its exact stellar 
 magnitude. We can also compute the apparent 
 diameter of the planet as seen from the sun, and 
 thus ascertain the fraction representing the area 
 of its disc compared with the area of the 
 hemisphere illuminated by the sun. If the surface 
 of the planet were a perfect reflector of light, we 
 could in this way knowing the distance of the 
 sun and planet from the earth compute the 
 brightness of the sun in terms of the apparent 
 brightness of the planet, and thus find the sun's 
 stellar magnitude. But as no surface is a perfect 
 reflector, a correction must be made for the 
 "albedo" or reflecting power of the planet in 
 question. Let us see what result this method 
 will give in the case of Mars, Jupiter, and Saturn. 
 
 From recent measures of the diameter of Mars, 
 
88 STUDIES IN ASTRONOMY 
 
 I find that its apparent diameter in opposition 
 
 as seen from the sun is 6"-188. This gives the 
 
 area of its disc 30'0 square seconds. Now, in a 
 
 hemisphere of the star sphere there are 20626*5 X 
 
 12,960,000 = 267,319,440,000 square seconds, and 
 
 hence area of hemisphere is 8,910,648,000 the area 
 
 of the disc of Mars as seen from the sun. Hence, 
 
 if the surface of Mars were a perfect reflector, 
 
 the sun as seen from Mars would be 8,910,648,000 
 
 times brighter than Mars appears to us when in 
 
 opposition. But Mars is not a perfect reflector. 
 
 Its " albedo " or reflecting power is, according to 
 
 Zollner, only 0*2672 (that of a perfect reflector 
 
 being 1). Hence, we must divide the above 
 
 number by 0'2672, which gives 33,348,233,500 for 
 
 the ratio of the light of the sun to the reflected 
 
 light of Mars. Now, as the mean distance of Mars 
 
 from the sun is 1'5237 (that of the earth being 1), 
 
 we must multiply this result by the square of 
 
 0*5237, or 0'2742, to obtain the light of the sun as 
 
 seen from the earth. This gives the light of the 
 
 sun 9,144,085,626 times the light of Mars when 
 
 in opposition, a number which corresponds to 
 
 24*9 stellar magnitudes. Now, Professor Pickering 
 
 found the stellar magnitude of Mars at mean 
 
 opposition to be 2*25, that is 2i magnitudes 
 
 brighter than a star of the zero magnitude. 
 
 Hence we have the sun's stellar magnitude equal 
 
 to -(24-9 + 2'25) = -27-15, a somewhat brighter 
 
 value than that generally assumed. 
 
THE SUN'S STELLAR MAGNITUDE 89 
 
 Let us now see what value can be derived from 
 the planet Jupiter. The mean diameter of Jupiter 
 as seen from the sun may be taken as the result 
 of recent measures at 37"'38. This gives an area 
 of disc = 1097*4 square seconds, and a ratio of the 
 area of the hemisphere to that of Jupiter's 
 disc = 243,600,000. Dividing this by Jupiter's 
 " albedo," as found by Zollner, viz. 0*62, we obtain 
 392,903,100 for the ratio of the light of the sun 
 to the reflected light of Jupiter. Now, as the 
 mean distance of Jupiter from the sun is 5*2028 
 (that of the earth being 1), its distance from the 
 earth when in opposition is 4*2028, and we must 
 multiply the above number by the square of this, 
 or 17*063, which gives 6,939,845,689, a number 
 which corresponds to 24*60 stellar magnitudes. 
 Professor Pickering finds the stellar magnitude 
 of Jupiter in opposition to be 2*52, and adding 
 this to -24*60 we obtain 27*12 for the sun's stellar 
 magnitude, a result in close agreement with that 
 found from Mars. 
 
 In the case of Saturn, we may take its mean 
 apparent magnitude as seen from the sun at 17". 
 This gives an area of 226*98 square seconds, and a 
 ratio of the area of the hemisphere to that of 
 Saturn's disc of 1,177,722,200. Dividing this 
 number by Saturn's albedo, 0*52, as found by 
 Zollner, we obtain 2,264,940,000 for the ratio of 
 the sun's light to the reflected light of Saturn. 
 Now, as the mean distance of Saturn from the 
 
90 STUDIES IN ASTRONOMY 
 
 sun is 9*5388, its distance from the earth when 
 in opposition is 8 '5388 ; we must multiply by 
 the square of this, or 72*9111, which gives 
 165,139,266,834, a number which corresponds to 
 28*04 stellar magnitude. Now, it has been found 
 by photometric measures that Saturn, when in 
 opposition and the rings invisible, is about equal 
 to a star of 0*88 magnitude. Hence we have the 
 sun's stellar magnitude = -28*04 -f 0*88 = -27*16, 
 a result in close agreement with those found from 
 Mars and Jupiter. 
 
 From the above calculations it would seem that 
 the sun's stellar magnitude is about 27, but as 
 there may be some doubt with reference to the 
 accuracy of the "albedo" assumed for each planet, 
 this result must still be considered as open to 
 some uncertainty. We may, however, conclude 
 with great probability that the true value of the 
 sun's stellar magnitude lies between 26*3 and 
 27*1. From some recent calculations I have 
 made with reference to the sun's brightness com- 
 pared with the brightness of binary stars having 
 a similar spectrum, I have found from Ursae 
 Majoris 26*34; from rj Cassiopeiae 26*65, and 
 from a Centauri 26-70. 1 The mean of these is 
 about 26*5, and this is the value I have adopted 
 in the present volume. 
 
 1 Monthly Notices, Royal Astronomical Society, January, 1908. 
 
The Suns of Space 
 
 THE fact that the stars are suns like our own 
 sun has long been known to astronomers. 
 So far back as 1750, Thomas Wright of 
 Durham, in his work on the " Construction of the 
 Milky Way," said, "The sun is a star, and the 
 stars are suns ; " and the poet Young, in his " Night 
 Thoughts," says 
 
 " One sun by day, by night ten thousand shine/* 
 
 The truth of this theory, which must have 
 always seemed a most probable one to a thinking 
 mind, has been fully proved in recent years by the 
 spectroscope, which shows that the stars are in- 
 candescent bodies shining by their own light, and 
 that many of them are almost identical in physical 
 constitution with our own sun. All the stars, 
 however, do not show exactly the same character 
 of spectrum, and they have, therefore, been divided 
 into classes or types, according to the nature of 
 the light which they emit. Stars of the first type, 
 like Sirius, Vega, Regulus, Altair, etc., show a 
 
92 STUDIES IN ASTRONOMY 
 
 spectrum with strong dark lines of hydrogen, and 
 are believed by astronomers to be intrinsically 
 hotter and brighter than stars with a solar 
 spectrum, which constitute the second type of 
 stellar spectra. The third and fourth types are 
 essentially different from the other two, and 
 include the red stars, many of which are variable 
 in light. Although all the types probably repre- 
 sent suns of various kinds, and in various stages 
 of their life-history, those of the second type only 
 are strictly comparable with our sun in their 
 physical constitution. But how are we to compare 
 the sun with any star ? The first thing necessary 
 to know is, of course, the distance of the star from 
 the earth, for without this knowledge the star 
 might be of any size. It might be comparatively 
 near the earth and of small size compared with the 
 sun, or it might be at a great distance from us and 
 have a large diameter. The next thing to ascer- 
 tain is the relative brightness of the star compared 
 with that of the sun. This is also most important, 
 for the apparent brightness of any self-luminous 
 sphere varies directly as the square of its diameter, 
 so that if we can find the relative brightness of the 
 sun and a star, we can compute their relative 
 diameters, if their relative distances are known, 
 provided that their intrinsic brilliancy of surface 
 is the same. This latter condition we may assume 
 to be practically true if the star's spectrum is 
 similar to that of the sun. These two factors of 
 
THE SUNS OF SPACE 93 
 
 distance and relative brightness being known, it 
 becomes possible to compare directly the diameter 
 of the sun (and hence its volume) with that of a 
 star having the same type of spectrum. Now, it 
 has been computed l that the brightness of the sun 
 may be represented by stating that it is 26J 
 magnitudes above the zero of stellar magnitudes, 
 or 27| magnitudes brighter than an average star 
 of the 1st magnitude, such as Aldebaran or 
 a Crucis. The meaning of " stellar magnitude " is 
 that a star of the 1st magnitude is 2*512 times 
 brighter than a star of the 2nd magnitude ; a 
 star of the 2nd magnitude 2*512 times as bright 
 as a star of the 3rd magnitude, and so on. Or, 
 generally, if n be the difference in magnitude, then 
 (2*512)" will represent the difference in brightness. 
 Hence a star of the 1st magnitude will be 100 
 times brighter than a star of the 6th magnitude, 
 and the sun will be (2'512) 27 ' 5 times brighter than 
 an average star of the 1st magnitude, that is, the 
 sun's light is equal to 100,000 million stars of the 
 1st magnitude. 
 
 In comparing the sun with the stars, I will first 
 consider those stars of which the distance has been 
 determined with some approach to accuracy, and 
 of which the spectrum is, according to the Harvard 
 observations, of the solar type, and therefore fairly 
 comparable with that of the sun. 
 
 The first star I will consider is /? Cassiopeiae, 
 1 See chapter on " The Sun's Stellar Magnitude." 
 
94 STUDIES IN ASTRONOMY 
 
 one of the stars forming the well-known " Chair of 
 Cassiopeia." For this star the late Professor 
 Pritchard found by means of photography a 
 parallax of 0'154 of a second of arc, and Herr 
 Kostinsky has recently found about 0*1 of a 
 second. This latter value would place the star at 
 a distance of 2,062,650 times the sun's distance 
 from the earth. Were the sun placed at this vast 
 distance about 32 J years' journey for light its 
 brightness would, I find, be reduced to that of a 
 star of 5*07 magnitude (light varying inversely as 
 the square of the distance). Now, the photometric 
 magnitude of /? Cassiopeise, as measured at 
 Harvard, being 2*42, we have the star 2*65 magni- 
 tudes, or about 11 \ times brighter than the sun 
 would be at the same distance. Hence, if strictly 
 comparable with the sun in physical constitution, 
 the diameter of the star would be 3*39 (= VH'5) 
 times that of the sun, and its mass nearly 39 times 
 the sun's mass. The spectrum of /? Cassiopeise is, 
 according to the Harvard observations, F 5 G, or 
 intermediate between the sub-types F and G, that 
 of the sun being G. It may therefore be slightly 
 hotter and brighter than the sun, and its mass 
 somewhat smaller than that found above. 
 
 For the bright star Procyon, which has the same 
 spectrum as ft Cassiopeise, Auwers found a parallax 
 of 0-240 of a second, Wagner 0"'229, and Elkin 
 0"-325. Elkin's value would place the star at a 
 distance of 634,660 times the sun's distance from 
 
THE SUNS OF SPACE 95 
 
 the earth. This would reduce the sun's light to a 
 star of 2*51 magnitude, and as the photometric 
 magnitude of Procyon is 0*48, it follows that the 
 star is about 6 J times brighter than the sun. This 
 would make its diameter 2J times that of the sun, 
 and its volume about 16J times the sun's volume. 
 Dr. See finds from the orbit of the faint satellite 
 that the mass of Procyon is about 5 times the sun's 
 mass, so that the above result would indicate a 
 rather hotter and brighter sun than ours, and this 
 agrees with the star's spectrum (F Pickering). 
 
 Another star with a spectrum of nearly the solar 
 type (F 8 G) is Ursa3 Majoris. A small parallax 
 of 0*046 of a second was found by Kapteyn. 
 Placed at the distance indicated, the sun would 
 shine as a star of only 6*75 magnitude, or invisible 
 to the naked eye. The star's photometric magni- 
 tude being 3*22, it follows that the star is 3*71 
 magnitudes, or 30*48 times brighter than the sun. 
 Its volume would therefore be about 168 times the 
 sun's volume, so that if the parallax is at all 
 reliable we have here a sun of large size. It has a 
 proper motion of about 1"*1 per annum, which 
 indicates an actual velocity of about 70 miles a 
 second. 
 
 For the star 85 Pegasi, Brtinnow found a small 
 parallax of 0"*054. This would reduce the sun to 
 a star of 6*6 magnitude, and as the star's photo- 
 metric magnitude is 5*83, we have the star about 
 twice the brightness of the sun. The star is a 
 
96 STUDIES IN ASTRONOMY 
 
 well-known binary, or revolving double star, and 
 from an orbit recently computed by Dr. See, and 
 the above parallax, I find that the mass of the 
 system would be about eight times the mass of the 
 sun. From recent measures by Gomstock, he finds 
 a combined mass of 11*3 times the sun's mass, and 
 he arrives at the "almost incredible conclusion" 
 that the companion star, which is only of the 
 llth magnitude has a greater mass than the 
 primary star. 1 The star's spectrum is E, indicating 
 probably a hotter and brighter sun than ours. If 
 this be so, Brilnnow's parallax is probably too 
 large. 
 
 The southern star j3 Hydri of magnitude 2'90, 
 has a spectrum of the solar type and a parallax of 
 about 0"'13, found by Sir David Gill. This would 
 reduce the sun to a star of 4| magnitude. The 
 star is therefore 1*6 magnitude, or about 4*37 times 
 brighter than our sun, and its mass about 9 times 
 the sun's mass. It has a large proper motion of 
 2"* 28 per annum. 
 
 Tucanae, another southern star, has a spectrum 
 nearly the same as that of the sun. Its photo- 
 metric magnitude is 4*34, and its parallax about 
 0"'15. From these data I find that its mass is 
 nearly equal to the sun's mass. 
 
 The bright stars Canopus and Procyon have 
 spectra of the second type, that of Canopus being 
 F, and that of Procyon F 5 G. The parallax of 
 
 1 Aatrophysical Journal, April, 1903. 
 
THE SUNS OF SPACE 97 
 
 Procyon is about 0"*325, while that of Canopus 
 does not exceed 0"'01, according to Gill. This 
 would make the distance of Canopus 32 times that 
 of Procyon. Still Canopus is a brighter star than 
 Procyon, its photometric magnitude being, accord- 
 to the Haward measures, 0'86, while that of 
 Procyon is +0*48. From these data I find that 
 Canopus is 3518 times brighter than Procyon, and 
 it would follow that its volume is over 200,000 
 times the volume of Procyon. If the densities 
 were the same the masses would be hi the same 
 ratio, and as the mass of Procyon as found from 
 the orbit of its satellite is about 5 tunes the 
 sun's mass, we have the mass of Canopus more 
 than that of a million of suns ! This is probably 
 the largest sun of which we know anything. 
 
 Although stars with spectra of the Sirian type 
 are not directly comparable with our sun in 
 brightness, being probably much hotter and 
 brighter, it will be interesting to consider some 
 of the stars having this type of spectrum. In 
 the case of Sirius itself, I find that the sun, placed 
 at the distance indicated by a parallax of 0"*38 
 found by Gill, would shine as a star of 2*17 magni- 
 tude, and as the photometric magnitude of Sirius 
 is 1*58, or 1*58 magnitudes brighter than the "zero 
 magnitude," it follows that the star is 3*75 magni- 
 tudes, or 31*6 times brighter than the sun. Now, 
 Dr. See finds from the orbit of the satellite that 
 the mass of the bright star is 2*36 times the sun's 
 
 H 
 
98 STUDIES IN ASTRONOMY 
 
 mass. From this it follows that if Sirius had the 
 same density and surface luminosity as the sun, 
 it would be only 1*77 times brighter. Hence its 
 intrinsic brightness is nearly 18 times greater than 
 it would be if its physical constitution were the 
 same as that of our sun. It would therefore seem 
 that the great apparent brilliancy of Sirius is due 
 to its comparative nearness to the earth combined 
 with a great luminosity of surface, and not, as was 
 formerly supposed, to its being a sun of enormous 
 size. As Dr. See says, " there is reason to suppose 
 that Sirius is very much expanded, more nearly 
 resembling a nebula than the sun." 
 
 For the bright star Vega (a Lyrse) Dr. Elkin 
 found a parallax of 0"'082. This would reduce the 
 sun to 5*5 magnitude if placed at the distance of 
 the star, and the star's magnitude being 0'14, we 
 have the star 5'36 magnitudes, or 139 times brighter 
 than the sun. This would make Vega a consider- 
 ably larger body than Sirius, but situated at a 
 much greater distance from the earth. 
 
 I have elsewhere considered the mass and 
 relative brightness of those binary stars for which 
 a parallax has been found. Let us now consider 
 the probable distance and mass of some binary 
 stars for which a parallax has not yet been found. 
 The method of procedure I propose to adopt is 
 to compute what is called the " hypothetical 
 parallax " of the binary star, that is, its parallax 
 on the assumption that the mass of the system is 
 
THE SUNS OF SPACE 99 
 
 equal to the mass of the sun, and then to find the 
 magnitude to which the sun would be reduced if 
 removed to the distance indicated by this hypo- 
 thetical parallax, assuming that the sun's stellar 
 magnitude at its present distance is 26|_ Com- 
 paring, then, the star's magnitude, as measured 
 with the photometer, with the sun's reduced 
 magnitude, it will at once appear whether the 
 binary star is brighter or fainter than it should 
 be if placed at the distance indicated by the 
 "hypothetical parallax." I have computed the 
 " hypothetical parallax " and the corresponding 
 magnitude of the sun for all the binary stars for 
 which an orbit has hitherto been computed, and I 
 find that in most cases the star is brighter than 
 the sun would be if placed at the distance indicated 
 by the hypothetical parallax. This fact would 
 suggest that most of the binary star systems at 
 least, those with spectra of the solar type have a 
 smaller mass than that of the sun, and are at 
 a less distance than that indicated by the hypo- 
 thetical parallax. This reasoning, of course, only 
 applies to those binaries which have spectra of the 
 solar type, for stars of the first type are, as already 
 shown, much brighter than the sun in proportion 
 to their mass. 
 
 In a valuable and interesting volume recently 
 published by Dr. See, he gives a re-computation of 
 the orbits of 40 of the best-known binary systems. 
 Some of his results, all of which are based on a 
 
100 STUDIES IN ASTRONOMY 
 
 careful consideration of the best recorded measures, 
 do not differ widely from those of other computers. 
 In other cases, however, his orbits differ consider- 
 ably from those previously published, and as he 
 has included very recent measures in his discus- 
 sions, his results are probably more accurate than 
 most of those hitherto published. In the Appendix 
 I give the period (P), and the semi-axis major (a) 
 of the orbits found by Dr. See. From these I have 
 computed the hypothetical parallax, or the 
 parallax of the system, on the assumption that the 
 mass of the system is equal to the sun's mass. To 
 these I have added the magnitudes of the stars 
 which have been photometrically determined at 
 Harvard,[and the character of the stars' spectrum, 
 I. being the Sirian and II. the solar type. 
 
 Let us now consider some of the most remarkable 
 cases in the list which have spectra of the solar 
 type. I omit those in which the difference of 
 magnitude between the sun and the star does not 
 exceed 1J magnitude, or about 4 times the light. 
 
 Scorpii. Here the sun would be reduced to 
 magnitude 6*14, giving a difference of 1*98 magni- 
 tude in favour of the star. Hence the mass of the 
 system would be -^ of the sun's mass. The 
 spectrum is nearly of the solar type (F 8 G). 
 
 T Ophiuchi. In this case the sun would be 
 reduced to magnitude 7*48 if placed at the distance 
 indicated by the hypothetical parallax, and the 
 star's magnitude being 4'88, there is a difference of 
 
THE SUNS OF SPACE . 101 
 
 2*6 magnitudes in favour of the star. Hence the 
 mass of the system would be ^ * tne sun's mass. 
 The spectrum is F, probably indicating a hotter 
 and brighter star than the sun. 
 
 In the case of 99 Herculis, the sun would be 
 reduced to 5'84 magnitude, or only 0'48 magnitude 
 fainter than the star, and the spectrum being of 
 the solar type, the mass of the system is probably 
 about equal to that of the sun. The companion 
 is very faint and of a purple colour, and it may 
 possibly be approaching the planetary stage of its 
 life-history. 
 
 There are two remarkable cases in which the 
 sun, if placed at the distance indicated by the 
 hypothetical parallax, would be considerably 
 brighter than the binary star. One of these is 
 /M! Herculis. Here the sun would be reduced to 
 4*84 magnitude, and, taking the star's magnitude 
 at 9-4, we have a difference of about 4| magnitudes 
 in favour of the sun. This would reduce the 
 star's parallax to 0"'013, and would make its mass 
 about 545 times the mass of the sun I The star 
 being so faint, its spectrum has not been deter- 
 mined, but it forms a distant companion to /^ 
 Herculis, the magnitude of which is 3' 48 or 1*36 
 magnitude brighter than the sun would be if 
 placed at the hypothetical distance. According 
 to the Harvard observations, the brighter star has 
 a spectrum of the type G 5 K. It does not follow, 
 of course, that the fainter star (the binary) has a 
 
102 STUDIES IN ASTRONOMY 
 
 spectrum of the same type ; but as both stars 
 have a common proper motion through space, 
 they probably lie at practically the same distance 
 from the earth, and the only explanation of the 
 startling result found above seems to be that the 
 binary star has like the companion to Sirius 
 cooled down, and is therefore not comparable in 
 its physical constitution with our sun. 
 
 Another remarkable case is that of Burnham 
 883, a binary of very short period, whose rapidity 
 of motion has recently been discovered by Dr. See. 
 Here the difference of brightness is 4*24 magni- 
 tudes in favour of the sun, which would make the 
 mass of the system about 350 times the sun's 
 mass! But here again we do not know the 
 character of the star's spectrum, so cannot say 
 whether it is really comparable with the sun in 
 brightness. 
 
 From the above results we may conclude that 
 many of the stars are larger than our sun. On 
 the other hand, many are probably much smaller. 
 In fact, the visible universe probably contains 
 suns of various sizes, a result which might reason- 
 ably be expected. That they are all, however, 
 incandescent bodies shining by their own light is 
 a fact which admits of no reasonable doubt. 
 
XI 
 
 Stellar Satellites 
 
 THE term "satellite" is usually applied to 
 the moons which revolve round the planets 
 of the solar system, like our own moon 
 and the satellites of Jupiter and Saturn. But the 
 term is also sometimes used with reference to the 
 faint companions of bright stars. In most of 
 the known binary, or revolving double stars, the 
 component stars which form these interesting 
 stellar systems are usually of nearly equal bright- 
 ness, or at least do not differ very much in 
 relative brilliancy. These may be called pairs of 
 suns, or " twin suns." There are, however, some 
 notable exceptions to this rule. Among those for 
 which orbits have been computed, the following 
 are the most remarkable : Procyon, magnitudes 
 0-5 and 13 (or 12^ magnitudes difference in bright- 
 ness) ; Sirius, 1'6 and 10 (about 11| magnitudes 
 difference) ; 8 Cygni, 3 and 8 ; 99 Herculis, 6, 11 ; 
 85 Pegasi, 6, 10 ; and 77 Cassiopeise, 4 and 7'6. Of 
 these double stars which are known to be binary, 
 but in which the motion hitherto has not been 
 
104 STUDIES IN ASTRONOMY 
 
 sufficient to enable an orbit to be computed, the 
 following may be mentioned: 6 Ursae Majoris, 3 
 and 14 ; a Ursae Majoris, 2 and 11 ; J3 Leporis, 3, 
 11 ; t Ursse Majoris, 3, 10 ; 77 Geminomm, 3, 10 ; 
 34 Pegasi, 6, 12 ; and 26 Draconis, 5| and 11. 
 
 According to an orbit computed by Dr. See, and 
 a parallax of 0"'38, the mean distance of the 
 companion of Sirius from the bright star is about 
 21 times the sun's distance from the earth, or a 
 little more than the distance of Uranus from the 
 sun. The mass of the system is 3'47 times the 
 sun's mass ; the bright star being 2'36 times, and 
 the companion I'll that of the sun. With the 
 above distance I find that the companion, as seen 
 from Sirius, would shine with about the bright- 
 ness of full moonlight. As the mass of the satel- 
 lite is about the same as that of our sun, its 
 inherent light must be very small. If the sun 
 were placed at the same distance from the earth, 
 its light would still be more than 1300 times that 
 of the full moon. 
 
 The bright star Procyon forms a very similar 
 system to that of Sirius. A close companion was 
 strongly suspected by Otto Struve in 1873, and 
 this was discovered in November, 1896, by Schae- 
 berle with the great 36-inch telescope of the Lick 
 Observatory. It is about the 13th magnitude. 
 Dr. See finds a period of about forty years, or about 
 the same as that found by Auwers in 1861 from a 
 consideration of some irregularities in the proper 
 
STELLAR SATELLITES 105 
 
 motion of Procyon. Dr. See finds the mean dis- 
 tance to be 21 times the sun's distance from 
 the earth, and the mass of the satellite equal to 
 that of the sun, or the same as in the Sirian 
 system. The proper motion of Procyon is the 
 same as that of Sirius 1*3 seconds of arc per 
 annum and its parallax about 0"*32 only 
 slightly less. The similarity of the two systems 
 of Sirius and Procyon, in almost every particular, 
 is very curious. The spectrum of Procyon is, 
 however, of the second or solar type, and its mass 
 about double that of Sirius. As Sirius is con- 
 siderably brighter than Procyon, we have here 
 another proof that stars with the solar type of 
 spectrum have a larger mass in proportion to 
 their brilliancy than stars of the Sirian type ; or, 
 in other words, of two stars having the same 
 mass, but one with a Sirian and the other a solar 
 spectrum, the Sirian star would be much brighter 
 than the solar one. 
 
 The 6th magnitude star, 85 Pegasi, has a com- 
 panion of the 10th magnitude. It is a binary 
 star, and See finds a period of 24 years, with a 
 mean distance of 0"*89. Burnham finds 25*7 years 
 and 0"'78. The proper motion of the system is 
 about l"-3, and a small parallax of 0"'054 was 
 found by Brunnow. From these data I find that 
 the mean distance is about 15 times the earth's 
 distance from the sun. The orbits referred to 
 above give the mass of the system from 4 to 8 
 
106 STUDIES IN ASTRONOMY 
 
 times the sun's mass. 1 The companion as seen 
 from the primary would shine as a small sun, so 
 that it must be considered rather as a twin sun 
 than a satellite. The accuracy of the small 
 parallax is, of course, somewhat doubtful ; but 
 if nearly correct, the large proper motion would 
 indicate a real velocity of about 70 miles a second ! 
 In addition to the above, there are some stars 
 which have faint companions or " satellites," the 
 measures of which do not directly show orbital 
 motion, but which are known to be physically 
 connected from the fact that the bright star and 
 its faint satellite have the same " common proper 
 motion." In other words, they are moving to- 
 gether through space with the same velocity and 
 in the same direction, and are therefore near 
 enough to be bound together by the laws of 
 gravitation. In such cases the "satellite" prob- 
 ably revolves round its primary, but owing to the 
 great distance from its central sun, the period of 
 revolution would be very long, and the angular 
 motion would not be perceptible for many years 
 at the great distance at which the system usually 
 lies from the earth. Let us consider some of these 
 stellar systems. From the comparatively great 
 apparent distance which separates these satellites 
 from the primary star, they seem to be constituted 
 on a much vaster scale than those binary stars, in 
 
 1 Professor Comstock has, however, recently found a mass of 
 11 '3 times the sun's mass (see last chapter). 
 
STELLAR SATELLITES , 107 
 
 which the motion is so rapid that an orbit can be 
 computed. We will consider those stars for which 
 a measurable parallax has been found, and for 
 which, therefore, the distance from the earth is 
 approximately known. 
 
 The bright reddish star Aldebaran has a faint 
 companion of about the llth magnitude at a 
 distance of about 117 seconds of arc, which was 
 originally discovered by Sir William Herschel. 
 In the year 1888 this faint star was found to be 
 a close double star by the famous American 
 astronomer Buriiham, with the 36-inch refractor of 
 the Lick Observatory. He also found a closer and 
 fainter companion about the 14th magnitude 
 while using the 18J-inch refractor of the Chicago 
 Observatory in the year 1877. The distance of 
 this faint satellite from the bright star is about 
 31". Measures in subsequent years have shown 
 that the distant double companion is not moving 
 with Aldebaran, which has a proper motion of 
 about ^ 0"'190 per annum ; but, curious to say, 
 Burnham's faint satellite has notwithstanding 
 its comparatively great distance from its primary 
 exactly the same proper motion as the bright 
 star, and is therefore most probably physically 
 connected with it. The result of this is that 
 Herschel's distant companion is being gradually 
 left behind at least for the present while 
 Burnham's companion is accompanying Aldebaran 
 in its flight through space. A parallax of 0"*107 
 
108 STUDIES IN ASTRONOMY 
 
 was recently found for Aldebaran at the Yale 
 University Observatory (U.S.A.). Assuming this 
 parallax and the above proper motion, the velocity 
 of Aldebaran at right angles to the line of sight 
 comes out about 5 miles a second. 1 The double 
 companion has a proper motion of its own in a 
 slightly different direction, amounting to about 
 half that of Aldebaran. As this proper motion 
 small as it is is an unusually large one for so 
 faint a star, it has been suggested by Professor 
 Barnard that possibly its apparent motion may be 
 really due to orbital motion round Aldebaran. 
 However this may be and time alone can decide 
 the question there can be no doubt that Burn- 
 ham's faint companion is physically connected 
 with Aldebaran, and that the double companion 
 also forms a physical system of its own. These 
 facts render Aldebaran and its companions an 
 interesting object of study. 
 
 Assuming that the line joining Aldebaran and 
 Burnhain's faint companion is at right angles to 
 the line of sight an assumption which would 
 give the minimum distance between them I 
 find that the distance of the satellite from Alde- 
 baran is about 300 times the sun's distance from 
 the earth. Placed at this great distance from its 
 central sun (10 times the distance of Neptune from 
 our sun), the period of revolution round Aldebaran 
 
 1 The motion in the line of sight seems to be much greater 
 about 30 miles a second away from the earth. 
 
STELLAR SATELLITES ^ 109 
 
 would be very long, and it is not a matter for 
 surprise that no relative motion has been detected 
 in the twenty -five years which have elapsed since 
 its discovery. It will probably be many years 
 more before its motion round its brilliant primary 
 will become perceptible. Were our sun placed at 
 the distance of Aldebaran, I find that it would be 
 reduced in brightness to a star of about the 5th 
 magnitude, or about 40 times fainter than Alde- 
 baran appears to us. This indicates that Aldebaran 
 is a more massive sun than ours, and by its greater 
 attractive power it is able to control the motion 
 of its distant satellite. As light varies inversely 
 as the square of the distance, if we know the 
 distance of Aldebaran from the earth, and the 
 distance of the satellite from Aldebaran, we can 
 easily compute the brightness of the satellite as 
 seen from Aldebaran, or from some planet revolving 
 close to the bright star. Making the necessary 
 calculation, I find that the light of the satellite 
 would be increased by 19 magnitudes if seen at 
 the distance of 300 times the sun's distance from 
 the earth. Hence, as seen from Aldebaran, the 
 satellite would shine as a star of 5 magnitude, 
 that is 5 magnitudes or 100 times brighter than a 
 star of zero magnitude, like Arcturus, or somewhat 
 brighter than Venus is seen by us at her maximum 
 brilliancy. 
 
 A somewhat similar case is that of Regulus 
 (a Leonis). This bright star has a companion of 
 
110 STUDIES IN ASTRONOMY 
 
 about 8J magnitude at a distance of about 177". 
 This small attendant was discovered by Sir 
 William Herschel in 1781. It was found to be 
 double by Winlock, the companion being of the 
 13th magnitude, and distant about 3" from the 
 8J magnitude star. Burnham's measures show 
 that this double companion is moving through 
 space with Regulus, the common proper motion 
 being 0"'267 per annum. A small parallax of 0"'022 
 was recently found for Regulus at the Yale Ob- 
 servatory. With these data I find that the dis- 
 tance of Regulus from the earth is 9,375,700 times 
 the sun's distance from the earth, and the 8J mag- 
 nitude star is at a distance from Regulus of about 
 8000 times the same unit. From this it follows 
 that the 8 magnitude star, as seen from Regulus, 
 would shine as a star of 6 J magnitude, or about 
 8 times brighter than Venus at her brightest. The 
 13th magnitude star would appear as a star of 
 2 magnitude, or somewhat brighter than Sirius 
 as seen by us. The combination of a star 8 times 
 brighter than Venus with one as Jbright as Sirius, 
 and about one degree apart, would form a fine 
 spectacle in the sky of Regulus. Our sun placed 
 at the distance of Regulus would, I find, shine as 
 a star of about 8*3 magnitude, or about the same 
 brightness as the satellite appears to us. The 
 satellite is therefore probably as large as our sun. 
 The difference of about 7 magnitudes between 
 Regulus and the sun at equal distances indicates 
 
STELLAR SATELLITES 111 
 
 that Regulus is over 600 times brighter than the 
 sun. It must therefore be a very massive body, 
 probably much larger than Sirius, 1 and may 
 therefore be able to control the motions of a 
 satellite even at the great distance of 8000 times 
 the earth's distance from the sun. The parallax 
 and proper motion of Regulus indicate that its 
 velocity at right angles to the line of sight is 
 about 36 miles a second. 
 
 The bright star Rigel (ft Orionis) has a com- 
 panion of the 8th magnitude at a distance of 9J", 
 discovered by Sir William Herschel. This small 
 star was found to be an excessively close double 
 star by Burnham in 1871. The measures are not yet 
 sufficient to enable an orbit to be computed, but 
 Burnham thinks that the period of the close pair 
 may possibly be very short. The measures of the 
 8th magnitude star, with reference to Rigel, do 
 not yet indicate any well-defined motion, but as it 
 has the same proper motion as Rigel, it is certain 
 that there is a physical connection between them. 
 The proper motion is small about 0"*018 per 
 annum. According to Sir David Gill, the parallax 
 of Rigel does not exceed the hundredth of a second, 
 or 0"*01. Assuming this parallax, the distance of 
 Rigel would be at least twenty million times the 
 sun's distance from the earth, and considering its 
 great apparent brilliancy (0'28 magnitude), it is 
 probably a sun of enormous size. Placed at the 
 1 Regains has a spectrum of the Sirian type. 
 
STUDIES IN ASTRONOMY 
 
 distance indicated by the above parallax, the sun 
 would, I find, be reduced to a star of about the 
 10th magnitude ! A parallax of 0"*01 would place 
 the satellite at a distance from Rigel of 950 times 
 the sun's distance from the earth. At this dis- 
 tance its magnitude, as seen from Rigel, would be 
 about 13, or somewhat brighter than our moon 
 appears to us. 
 
 The 4 \ magnitude star, o 2 (40) Eridani, has a 
 small companion of about the 9th magnitude at 
 a distance of about 82". This satellite was found 
 to be double by Sir William Herschel in 1783. It 
 is a binary pair, and Burnham finds a period of 
 about 180 years. It has the same large proper 
 motion as the bright star about 4"'l per annum 
 and a parallax, found by Hall, of 0"'22. This 
 gives a distance from the earth of 937,570 times 
 the sun's distance, and a distance between the 
 bright star and its binary companion of 372 times 
 the distance of the earth from the sun. The 
 measures of position show evident signs of orbital 
 motion, but the period is probably very long, 
 perhaps several thousand years. Placed at the 
 distance of o 2 Eridani, the sun would, I find, be 
 reduced to a star of about 3J magnitude, or about 
 1 magnitude brighter than the star. I find that 
 the binary satellite seen from its primary would 
 shine as a star of about 8 magnitude, or, in 
 other words, it would give the light of a small 
 moon. The parallax and proper motion indicate 
 
STELLAR SATELLITES % 113 
 
 a velocity across the line of sight of about 54 
 miles a second. 
 
 It was suggested by Sir John Herschel, with 
 reference to the faint companion of t Ursae 
 Majoris, that it might possibly shine by light 
 reflected from the bright star; and Admiral 
 Smyth says, with reference to the double star 
 Struve 946, "the possibility of the conies being 
 variable awakens considerations of peculiar in- 
 terest ; it having been surmised that certain small 
 acolyte stars shine by reflected light." 1 But it 
 may be easily shown that this is highly improb- 
 able, if not impossible. Let us take the system 
 of Sirius. In this case the satellite, although very 
 faint for its computed mass, certainly does not 
 shine by reflected light from Sirius. This will 
 appear from the following considerations, which 
 I have carefully worked out: Assuming for a 
 moment that ^the satellite shines merely by re- 
 flected light, let us see what its brightness would 
 be as seen from the earth. According to the 
 computed orbit and parallax of Sirius which are 
 probably as reliable as those of any binary star 
 hitherto computed the mean distance of the 
 satellite from the bright star is a little more than 
 the distance of Uranus from the sun. Let us 
 assume this distance. (A greater distance would 
 strengthen my argument.) As the computed mass 
 of the satellite is about the same as that of the 
 1 Bedford Catalogue, p. 155. 
 
 I 
 
114 STUDIES IN ASTRONOMY 
 
 sun, let us assume that it lias the same diameter, 
 or 866,000 miles (a smaller diameter would, of 
 course, strengthen my argument), and let us take 
 the diameter of Uranus at 33,000 miles which is 
 very near the truth. Now, assuming the same 
 " albedo," or reflective power, for Uranus and the 
 satellite of Sirius (the albedo of Uranus is very 
 high), we have the satellite, as seen from Sirius, 
 shining with a greater brightness than Uranus, as 
 seen from the sun, in the proportion of 866,000 
 squared to 33,000 squared, or as 688 to 1. This is 
 on the supposition that Sirius and the sun are of 
 the same brightness. But from the photometric 
 measures of Sirius and its known distance from 
 the earth, I find that Sirius is at least 20 times 
 brighter than our sun. We must therefore in- 
 crease the above ratio 20 times to obtain the 
 illumination of the satellite by the light of Sirius. 
 This gives 688 X 20 = 13,760. That is, the satellite 
 as seen from Sirius would be about 13,760 times 
 brighter than Uranus as seen from the sun. This 
 number corresponds to 10'3 stellar magnitude. 
 Now, taking the magnitude of Uranus as seen 
 from the sun at 5*8 (which must be very near the 
 truth), we have the brightness of the Sirian 
 satellite, as seen from Sirius, equal to 5*8 10*3, 
 or 4*5 magnitude, that is 4-| magnitudes brighter 
 than a star of zero magnitude, like Arcturus, or 
 slightly brighter than Venus appears at her 
 greatest brilliancy as seen from the earth. Now, 
 
STELLAR SATELLITES . 115 
 
 the simple problem is this : If a body shines with 
 a stellar magnitude of 4*5, as seen at the 
 distance of Uranus, what would be its magnitude 
 if placed at the distance of Sirius? Taking the 
 parallax of Sirius at 0"'38, we have the distance of 
 Sirius from the earth equal to 542,800 times the 
 sun's distance from the earth. Hence the light 
 of a body at the distance of Uranus would, if 
 removed to the distance of Sirius, be reduced in 
 the proportion of the square of 542,800 to the 
 square of 19, or as 816,244,900 to 1. This corre- 
 sponds to 22*3 stellar magnitudes. Hence the 
 magnitude of the satellite of Sirius, as seen from 
 the earth, if shining only by reflected light from 
 Sirius, would be 22*3 4*5, or 17'8 magnitude, and 
 it would be quite invisible in the great 40-inch 
 telescope of the Yerkes Observatory. As its 
 actual magnitude is about 10, it follows that it 
 is about 1300 times brighter than if it shone 
 merely by reflected light, and it is evident that 
 it must have some inherent light of its own. I 
 have shown in the beginning of this paper that 
 the actual brightness of the satellite, as seen from 
 Sirius, is equal to that of full moonlight on the 
 earth. We should obtain a similar result if we 
 assumed that Sirius is very much brighter than 
 20 times the brightness of the sun. If we assume it 
 to be 10 times brighter than this, or 200 times the 
 sun's brightness a very improbable supposition 
 we should still have the satellite reduced to about 
 
116 STUDIES IN ASTRONOMY 
 
 the 15th magnitude, and, placed as it is so close to 
 such a brilliant star as Sirius, it would probably 
 still remain invisible in our largest telescopes. 
 The assumption I have just made is, however, 
 quite inadmissible. For if we increase the light 
 of Sirius, we must increase its distance ; and this 
 would further diminish the computed light of the 
 satellite. We may therefore dismiss the idea that 
 the satellite of Sirius could possibly shine merely 
 by reflected light from its primary. The same 
 considerations will apply to the case of Procyoii 
 and its satellite, and with greater force, as the 
 satellite of Procyon is about 3 magnitudes fainter 
 that the satellite of Sirius, and Procyoii is a less 
 luminous sun than " the monarch of the skies." 
 
XII 
 
 Spectroscopic Binaries 
 
 ANEW class of binary stars has been dis- 
 covered in recent years by the aid of the 
 spectroscope. These are called " spectro- 
 scopic binaries," to distinguish them from those 
 binaries or revolving double stars in which the 
 component stars are visible in a telescope. These 
 spectroscopic binaries consist of two (or more) 
 components so close together that the highest 
 powers of the largest telescopes fail to show them 
 as anything but single stars ! Indeed, the veloci- 
 ties shown by the spectroscope indicate that, in 
 most cases at least, they must be so close that the 
 components will probably for ever remain invisible 
 in the most powerful telescopes which man could 
 ever construct. In some of these remarkable 
 objects the doubling of the spectral lines indicates 
 that the components are both bright bodies ; but 
 in other cases the lines are merely shifted from 
 their normal position, not doubled, which shows 
 that one of the components is a dark body, or at 
 least gives so little light that its spectrum is not 
 
118 STUDIES IN ASTRONOMY 
 
 visible. In either case the motion in the line of 
 sight can be measured with the spectroscope, and 
 we can then calculate the actual dimensions of 
 the system in miles, and thence its mass in terms 
 of the sun's mass, although the star's distance 
 from the earth may remain wholly unknown. 
 Judging, however, from the apparent brightness 
 of the star and the character of its spectrum, we 
 can make an estimate of its probable distance 
 from the earth. 
 
 Let us first consider the case of the famous 
 variable star, Algol, which the spectroscope shows 
 to be a binary star with one component a dark 
 body, or at least very much fainter than Algol 
 itself. The variation in the light of Algol is due 
 to a partial eclipse by the companion. According 
 to the Harvard observations, the spectrum is 
 nearly similar to that of Sirius. It may, therefore, 
 be comparable with that brilliant star in intrinsic 
 brightness and density. Assuming the mass of 
 Sirius to be 2'36 times the mass of the sun, as 
 determined by Dr. See, from the orbit of its satel- 
 lite, and that of the bright component of Algol at 
 | of the sun's mass, as computed by Vogel from 
 the spectroscopic measures, I find that for the 
 same distance Sirius should be about 3 times 
 brighter than Algol. But the photometric 
 measures of relative brightness made at Har- 
 vard show that Sirius is about 31 times brighter 
 than Algol. From this it follows since light 
 
SPECTROSCOPIC BINARIES . 119 
 
 varies inversely as the square of the distance 
 that Algol is 3- 16 times farther from the earth 
 than Sirius. Assuming the parallax of Sirius at 
 0"-37, as found by Sir David Gill, this would give 
 for the parallax of Algol 0"*117, or a journey 
 for light of about 29 years. From the 
 dimensions of the system, as found by Vogel 
 about 3,269,000 miles from centre to centre of the 
 components this parallax would give an apparent 
 distance of less than ^ of a second of arc, a 
 quantity too small to be visible in the largest 
 telescopes, or probably in any telescope which 
 could ever be constructed by man. It is, therefore, 
 no matter for surprise that Burnham, the famous 
 observer of double stars, failed to see any trace of 
 duplicity in Algol with the highest powers of the 
 great Lick telescope. From a consideration of 
 irregularities in the proper motion of Algol, and 
 in the period of its light-changes, Dr. Chandler 
 infers the existence of a second dark body, and he 
 finds a parallax of 0"'07. This would indicate 
 that Algol is about 2*8 times brighter than Sirius. 
 This greater brilliancy would suggest greater 
 heat, and would agree with its small density, 
 which from its diameter, as given by Vogel 
 1,074,100 miles is only about one-third that of 
 water. 
 
 Let us next consider the case of ft Aurigse, which 
 spectroscopic observations show to be a close 
 binary star, with a period of about 4 days, and 
 
120 STUDIES IN ASTRONOMY 
 
 a distance between the components of about 8 
 millions of miles. This period and distance imply 
 that the mass of the system is about 5 times 
 the sun's mass. As in this case the spectral lines 
 are doubled at intervals, and not merely shifted 
 from their normal position (as in the case of Al- 
 gol), we may conclude that both the components 
 are bright bodies ; and we may not be far wrong 
 in supposing that both are of equal mass, each 
 having 2-| times the mass of the sun. 1 As the 
 spectrum of ft Aurigse is of the same type as 
 Sirius, we may compare it with that star, as we 
 did in the case of Algol, and with more confidence, 
 as the mass of each component of ft Aurigse differs 
 but little from the mass of Sirius. Assuming the 
 same density and the same surface luminosity for 
 both ft Aurigse and Sirius, I find that ft Aurigae 
 should be about twice as bright as Sirius at equal 
 distances. Now, according to the Harvard 
 measures, Sirius is about 28*8 times brighter than 
 ft Aurigse. Hence it follows that the distance of 
 ft Aurigse from the earth should be about 7'6 
 times greater than that of Sirius, and assuming 
 the parallax of Sirius at 0"'37, as before, that of 
 ft Aurigse would be about 0"'05. From actual 
 measures of the parallax made by Professor 
 Pritchard at Oxford, he found from one comparison 
 star a parallax of 0"'065, and from another star 
 
 1 Pickering says, " Both components are nearly equal in bright- 
 ness, and have similar spectra." 
 
SPECTROSCOPIC BINARIES % 121 
 
 0"'059, results in good agreement with that found 
 above from a consideration of the star's mass and 
 brightness compared with those of Sirius. We 
 may therefore conclude with some confidence that 
 the parallax of (3 Aurigse is about ^ of a second 
 of arc, or a " light-journey " of about 65 years. 
 Recent observations by M. Tickhoff, of the Poul- 
 kowa Observatory, indicate that the star is really 
 quadruple, each of the components being itself 
 double, and each pair revolving round their centre 
 of gravity in a period of about 19 hours. 1 If 
 this result is confirmed, the star will be a most 
 interesting object a spectroscopic quaternary ! 
 
 The bright star Spica is also a spectroscopic 
 binary. Vogel found a period of 4 days, with a 
 distance between the components of 6| millions 
 of miles. He finds that the mass of the system is 
 about 2'6 times the mass of the sun. Assuming 
 that each of the components has 1*3 times the 
 sun's mass, it follows that the light of Sirius 
 should, for equal distances, be 1*48 times the light 
 of Spica (one of the components being nearly 
 dark). Now, the Harvard measures make Sirius 
 about 13 times brighter than Spica. Hence 
 it follows that the distance of Spica should 
 be about 3 times the distance of Sirius. This 
 would make the parallax of Spica about 0"'12 if 
 it had the same density and surface luminosity as 
 
 1 Nature, December 24, 1903. TickhofPs results have, however, 
 since been disputed by Vogel. 
 
122 STUDIES IN ASTRONOMY 
 
 Sirius. But the spectrum of Spica is of the 
 " Orion type " (B 2 A, Pickering), and it is, there- 
 fore, probably an intrinsically brighter body 
 than Sirius. The parallax may, therefore, be 
 smaller than that found above. So far as I know, 
 a measurable parallax has not yet been found for 
 Spica. Brioschi, in the years 1819-20, found a 
 negative parallax, which would imply either that 
 the parallax is too small to be measurable or that 
 the small comparison stars in the vicinity, used in 
 measuring the parallax, are actually nearer to us 
 than the bright star. A negative parallax was also 
 found by Sir David Gill. In addition to its orbital 
 motion, Vogel found that Spica is approaching 
 the earth at the rate of 9 miles a second; but, 
 owing to its great distance, this would have no 
 effect 011 its brightness in historical times. 
 
 Proceeding in the same way, I find for Ursse 
 Majoris (Mizar), which is also a spectroscopic 
 binary, a parallax of 0"*057. Klinkerfues found 
 0"-0429 to 0"-0477. 
 
 Belopolsky has found that the brighter com- 
 ponent of the well-known double star, Castor, 
 which has a spectrum of the Sirian type, is a 
 spectroscopic binary with a " dark " companion, 
 like Algol. The period is about 2'98 days, and 
 the orbital velocity 20*7 miles a second. With 
 these data, and assuming that the bright com- 
 ponent has double the mass of its dark companion, 
 and that this component of the visual pair has 
 
SPECTROSCOPIC BINARIES 
 
 x 
 
 4 times the mass of the visual companion (as 
 its brightness would indicate), I find that the 
 total mass of the system would be about 0*36 of 
 the sun's mass. This would give a parallax of 
 about 0"'165. From heliometer measures made 
 in the years 1854-55, Johnson found a parallax 
 of 0"*198, which does not differ widely from the 
 above result. 
 
 The Pole star has also been found to be a spec- 
 troscopic double, for which Professor Campbell 
 finds a period of 3 d 23 h 14 m> 3. 1 The presence of a 
 third component is suspected, "the visible star, 
 with invisible companion, describing an orbit 
 round a third body." 
 
 Another spectroscopic binary is r) Pegasi, for 
 which Professor Campbell finds a period of 818 
 days, with an orbital velocity of about 8*8 miles 
 a second. The spectrum is of the solar type (G, 
 Pickering), and the mean distance between the 
 components about 200 millions of miles. This 
 would indicate a minimum mass of about twice 
 the sun's mass, and a parallax of about 0"'18. 
 
 The star 77 Orionis is also a spectroscopic binary, 
 for which Walter S. Adams finds a period of 7-9876 
 days. The mean distance between the components 
 is about 20 millions of miles, and " one component 
 is relatively dark." The above data indicate a 
 minimum mass of about 20 times the sun's mass. 
 The orbital velocity is very great about 89 
 
 1 Astrophysical Journal, vol. xiv. p. 2. 
 
124 STUDIES IN ASTRONOMY 
 
 miles a second. The star is receding from the 
 earth at the rate of 22 miles a second, but this 
 may be partly due to the sun's motion in the 
 opposite direction. 17 Orionis is also a visual 
 double star, the components being of the 4th and 
 5th magnitude, and distant 1", but they seem to 
 be relatively fixed, as no motion has been detected. 
 The brighter of the two is the spectroscopic binary. 
 Comparing it with Spica, which has a somewhat 
 similar spectrum, I find that 17 Orionis would be 
 about 7 times farther from the earth than Spica. 
 Its distance is therefore probably very great. 
 
 The star o Persei is a spectroscopic binary with 
 a period of 4*39 days, and Vogel finds a mass equal 
 to 0*6 of that of the sun. This is on the assump- 
 tion that the plane of the orbit coincides with the 
 line of sight. As the spectrum is the same as that 
 of 17 Orionis (B 1 A, Pickering), we can compare the 
 two stars. Supposing them to have the same 
 density and surface luminosity, I find that 
 f] Orionis is 2' 5 5 times farther from the earth 
 than o Persei. All these "Orion type" stars 
 seem to lie at a great distance from our system. 
 
 The bright star Capella is also a spectroscopic 
 binary, and forms rather an astronomical enigma. 
 It consists of two components of nearly equal 
 mass, but one about twice as bright as the other. 
 The period of revolution is about 104 days, and 
 the spectroscopic observations would imply a 
 mass of about 2*3 times that of the sun, on the 
 
SPECTROSCOPIC BINARIES ^ 125 
 
 assumption that the plane of the orbit is in the 
 line of sight. But visual observations with 
 the 28-inch refractor at Greenwich have shown 
 the star " elongated," and indicate that the orbit 
 plane is inclined about 60 to the line of sight. 
 This would make the mass 8 times greater, or 
 about 18*4 times the sun's mass. This is, however, 
 a comparatively small mass if we consider the 
 great brilliancy of the star. A parallax of 0"-081, 
 found by Dr. Elkin and this is confirmed by the 
 Greenwich observations would reduce the sun to 
 a star of only 5'5 magnitude, and as the photo- 
 metric magnitude of the star is 0*21, we have the 
 star 133 times brighter than the sun. From the 
 Greenwich observations we may assume that one 
 of the components is about twice as bright as the 
 other. This would make them about 89 and 44 
 times, respectively, brighter than the sun. Now, 
 as the spectrum of Capella closely resembles the 
 solar spectrum, we may perhaps assume that the 
 surface luminosity of the components is the same 
 as that of the sun. On this assumption I find 
 that their diameters would be 9'4 and 6'6 times 
 the sun's diameter, their combined volume about 
 1140 tunes the sun's volume, and their density 
 about 0-016 that of the sun, or 0'0224 that of water. 
 This result seems improbable, considering the 
 character of the spectrum, and it would, perhaps, 
 be safer to assume that their diameters are 
 smaller and their densities greater than the 
 
126 STUDIES IN ASTRONOMY 
 
 results found above. On this assumption, how- 
 ever, their surface luminosity would necessarily 
 be greater than that of the sun, and as greater 
 surface luminosity would probably be indicated 
 by a spectrum of the Sirian or " Orion " type, the 
 enigma remains without a satisfactory solution. 
 
 One of the components of the short period 
 binary, K Pegasi, has been found to be a spectro- 
 scopic binary with a period of about six days, and 
 an orbital velocity of about 25 miles a second. 
 From these data I find a mass of 0'32 of the sun's 
 mass, and if we suppose all three components of 
 the system to be equal in mass, we have a total 
 mass of 0*48 of the sun's mass. Combining this 
 result with the elements of the orbit found for 
 the visual pair, 1 I find a parallax of about ^ of 
 a second. Placed at this distance, the sun would 
 shine as a star of about the 5th magnitude, and 
 as the star's photometric magnitude is 4*24, it 
 would follow that the star is somewhat brighter 
 than the sun. This is, perhaps, indicated by its 
 spectrum, which, according to Pickering, is F 5 G. 
 
 The star X Andromedse is an interesting case. 
 This is a spectroscopic binary with a period of 
 about 19'2 days, and an orbital velocity of about 
 5'6 miles a second. From this I find, supposing 
 the orbit plane to be in the line of sight, a mass 
 of only O f 012 of the sun's mass. As the star's 
 spectrum is K, it is not exactly comparable with 
 1 Period = 11-42 years, and a = 0"-4216 (See). 
 
SPECTROSCOPIC BINARIES 127 
 
 the sun, but as its photometric magnitude is 4*14, 
 the very small mass would suggest that the star 
 is comparatively near the earth. If we suppose 
 the inclination of the orbit to be 30 (or 60 to the 
 line of sight), the mass would be increased 8 times ; 
 but even then the mass would be less than ^ of 
 the sun's mass. 
 
 There are many other known spectroscopic 
 binaries, 1 but the above are some of the most 
 interesting cases. 
 
 It should be mentioned that in the case of 
 (3 Aurigae, Spica, Castor, and others, as there is 
 no variation of light, as in Algol, the plane of the 
 orbit is probably inclined to the line of sight. This 
 would have the effect of increasing the computed 
 mass of the system, and thus diminish the calcu 
 lated parallax. As the calculations given above 
 have been made on the assumption that the plane 
 of the orbit passes through the earth, it follows 
 that the computed parallaxes are a maximum, and 
 that these remarkable objects may be really farther 
 from the earth than even the small parallaxes 
 found above would indicate. But as a com- 
 paratively small inclination of the orbit plane to 
 the line of sight would prevent an eclipse, the 
 parallaxes may not be far from the truth. 
 
 By the aid of these parallaxes we can easily 
 compute the relative brightness of the sun com- 
 pared with that of the spectroscopic binaries. 
 1 See chapter on " Some Recent Advances in Stellar Astronomy." 
 
128 STUDIES IN ASTRONOMY 
 
 Assuming that the sun is 26 magnitudes brighter 
 than a star of zero magnitude a value now pretty 
 generally adopted and taking the parallax of 
 Algol at 0"-07, I find that the sun placed at the 
 distance of Algol would be reduced in brightness 
 to a star of 5*84 magnitude, or about 3^ magni- 
 tudes fainter than Algol. This implies that Algol 
 is over 25 times brighter than the sun, although 
 its mass is smaller. In the case of ft Aurigae, if 
 the sun were placed at the distance indicated by 
 a parallax of 0"'05, it would be reduced to a star 
 of 6'57 magnitude, or 4J magnitudes fainter than 
 ft Aurigae, which would imply that the star is 
 about 63 times brighter than the sun! In the 
 case of Castor we have the sun reduced to about 
 the 4th magnitude, and as the photometric mag- 
 nitude of Castor is 1*58, the star would be over 9 
 times brighter than the sun, although its mass is 
 considerably less. These results show the great 
 relative brilliancy of stars with a Sirian type of 
 spectrum when compared with that of the sun, a 
 conclusion which has been already arrived at from 
 other considerations. 
 
 The " spectroscopic binaries " are probably very 
 numerous. Professor Campbell estimates that one 
 star in every five or six is a spectroscopic binary, 
 and that, if so, there should be at least 800 Algol 
 variables brighter than the 9th magnitude. At 
 present the number of known variables of this 
 type is not much over 30. Professor H. N. Russell 
 
SPECTROSCOPIC BINARIES 129 
 
 and Dr. A. W. Roberts have shown, independently, 
 that the density of the Algol variables (and there- 
 fore presumably other spectroscopic binaries) is 
 very small. Professor Russell finds the mean 
 density of 17 Algol variables to be only 0*19 
 that of water, and Dr. Roberts finds a mean 
 density of 0*!87 for 4 southern Algol variables. 1 
 This would suggest that these systems are in 
 an early stage of their evolutional history, and 
 this evidence is strengthened by the character of 
 their spectra, which are all of the "Orion" or 
 Sirian type. 
 
 1 Astrophysical Journal, vol. x. p. 314. 
 
XIII 
 
 "The Darkness behind the Stars" 
 
 THOSE who have not given the matter 
 sufficient consideration seem to think that 
 the number of the stars is practically 
 infinite. But this idea is quite erroneous, and due 
 to complete ignorance of astronomical investiga- 
 tions. The number of stars visible to the naked 
 eye, even with very good eyesight, is not only 
 comparatively but absolutely small, not much 
 exceeding 7000 for the whole heavens, and pro- 
 bably double this number would exhaust those 
 which can be seen by persons gifted with excep- 
 tionally keen vision. An attempt to count those 
 seen with certainty in any selected portion of the 
 sky will convince any intelligent person that the 
 number visible to ordinary eyesight, instead of 
 being large, is really very small, and that the idea 
 of a countless multitude is simply a popular 
 fallacy based on an optical illusion. 
 
 Of course, the number of stars visible is largely 
 increased when we use a telescope even a small 
 one and it is true that the larger the telescope 
 
< THE DARKNESS BEHIND THE STARS" 131 
 
 the more the number of stars seems to increase. 
 But we now know that there is a limit to this 
 increase of telescopic vision, and that the number v 
 of stars visible even in the largest telescopes, or 
 disclosed by photography, is certainly limited. 
 Let us consider some of the evidence derived from 
 telescopic observation which leads to this con- 
 clusion. In the Philosophical Transactions of 
 the Royal Society for the year 1784, Sir William 
 Herschel says that from his " gauges " of stars in 
 the Milky Way near Orion with a reflecting tele- 
 scope of 18'7 inches aperture, he found an average 
 of 79 stars for each field of view of 15 minutes of 
 arc. From this he concludes that "a belt of 
 15 degrees long and 2 broad " would contain about 
 50,000 stars, and he " suspected at least twice as 
 many more," or a total of 150,000 stars on an 
 area of 30 square degrees. Taking the smaller 
 number, we have about 1700 stars to the square 
 degree. This would give for the whole sky 
 which contains 41,253 square degrees a total of 
 69 millions ; and if we take the larger estimate, 
 we have a total of about 207 millions. But this 
 counting of stars was made in the Milky Way, 
 which is, of course, exceptionally rich in stars, 
 and cannot therefore be taken as representing the 
 whole heavens. In a recent investigation on star 
 distribution, Professor Pickering, of the Harvard 
 Observatory, finds that the richer portions of the 
 Milky Way cover 10,999 square degrees, and the 
 
132 STUDIES IN ASTRONOMY 
 
 fainter parts 4613 square degrees, leaving 25,641 
 square degrees in which there is no Milky Way 
 light. 1 Now, if we take the richer parts as con- 
 taining 5000 stars to the square degree, the fainter 
 parts 2000 to the square degree, and the rest of 
 the sky at 1000 stars to the square degree, we 
 obtain a total of about 64 millions in the Milky 
 Way and 26 millions outside, or a grand total of 
 90 millions for the whole sky. Professor Picker- 
 ing says, "As estimates are frequently given 
 which are still more uncertain ... it may be 
 stated that the number of stars corresponding to 
 the magnitude 15, or which would be visible in a 
 telescope of 15 inches aperture, would be about 
 18 millions, and the increase for larger apertures 
 would be surprisingly small." 
 
 Let us now consider some other results of 
 modern observations and photographs. And let 
 us first take an exceptional case of stellar richness. 
 On a photograph of the great globular cluster 
 CD Centauri, taken in Peru by Professor Bailey, 
 with a telescope of 13 inches aperture, the indi- 
 vidual stars can be distinctly seen and counted, 
 although to the eye it seems to be a mass of 
 " innumerable " stars. The enumeration has been 
 carefully made by Professor and Mrs. Bailey, and 
 gives a total of about 6389 on an area of about 
 30 minutes square. This gives 25,556 stars to the 
 square degree, and a total of about 1054 millions 
 
 1 Annals of Harvard College Observatory, vol. xlviii. No. V. 
 
"THE DARKNESS BEHIND THE STARS" 133 
 
 for the whole sky. But clusters like <o Centauri 
 are of course remarkable, and rare exceptions to 
 the general rule of stellar distribution, and the 
 heavens as a whole are not even hi the very 
 richest portions of the Milky Way nearly so rich 
 in stars as the globular clusters. The fact of 
 these clusters being " remarkable " objects proves 
 that they are unusually rich in stars, and there is 
 strong evidence evidence amounting to absolute 
 proof that the stars in these clusters are really 
 and not apparently close, and that they are 
 actually systems of stars which fill a compara- 
 tively limited volume in space. We cannot, then, 
 estimate the probable number of the visible stars 
 by counting those visible hi one of the globular 
 clusters. We must therefore draw our conclusions 
 from other portions of the sky. 
 
 On a photograph of a rich spot in Cygnus, taken 
 by Dr. Isaac Roberts in September, 1898, in that 
 luminous region of the Milky Way between y and 
 ft Cygni, about 30,000 stars have been counted on 
 a space of about 3^ square degrees. This would 
 give a total of about 360 millions for the whole 
 sky. But as the region in question is a very rich 
 one, this number is evidently too large. On this 
 beautiful photograph the stars, although thickly 
 strewn, have numerous and comparatively large 
 black spaces between them, and " the dark back- 
 ground of the heavens " is very conspicuous even 
 in this " rich " region. A glance at the photograph 
 
134 STUDIES IN ASTRONOMY 
 
 shows that there would be ample room, and 
 to spare, for at least ten times the number^of 
 stars actually visible. With reference to this 
 photograph, Dr. Roberts says 
 
 " A photograph of this region was also taken on 
 the 17th August, 1895, with an exposure of sixty 
 minutes only, and on comparing the original 
 negative with that of the plate annexed, all the 
 star-images, down to the faintest, are found to be 
 visible on both, notwithstanding the fact that one 
 had sixty minutes' exposure and the other two 
 hours and thirty-five minutes. The sensitiveness 
 of the films and the quality of the sky during 
 both exposures may be considered equal, and the 
 only noticeable difference in the star-images on 
 the two negatives is greater density on that with 
 the longer exposure. This is an illustration (one 
 of several which could be adduced) pointing to 
 the probability that all the stars existent upon 
 this area of the sky are charted upon these 
 negatives. Of course the faintest star-images are 
 lost on the photo-enlargement on paper. The 
 inferences we may draw from these results are 
 that this, apparently one of the most densely 
 crowded star areas in the Milky Way, can be seen 
 through, and that nothing visible within the limit 
 of our powers lies beyond ; and, further, that the 
 limit in space of the Galactic System is now 
 probably revealed to us." 
 
 In comparison with the exceptionally rich region 
 considered above, there are many poor regions in 
 the sky in which the visible stars are compara- 
 tively few in number. A photograph taken by 
 Dr. Roberts near the pole of the Milky Way 
 showed only 178 stars to the square degree. This 
 
"THE DARKNESS BEHIND THE STARS" 135 
 
 would give a total of only 7,343,000 stars for the 
 whole sky ! 
 
 With reference to a photograph of the cluster 
 Messier 35, and the small cluster near it, Dr. 
 Roberts says l 
 
 "Both the clusters and the stars surrounding 
 them present to view in a striking manner 
 numerous curves and lines of stars with void 
 spaces between them, which enable us to view 
 the darkness of space beyond the Galactic 
 stellar universe, of which the solar system forms 
 a relatively insignificant point. Many astronomers 
 have tacitly adopted the assumption that the 
 stars extend indefinitely into the expanse of space, 
 but that the reason they cannot be seen is the 
 absence of sufficient telescopic power to reveal 
 the very feeble light of stars that are beyond the 
 range-limit of all existing telescopes. But the 
 evidence obtained by the aid of photography 
 during the past twelve years strongly indicates, 
 if it does not demonstrate, that those vacant 
 spaces which are visible on photographs that have 
 been exposed to the sky during intervals of seven 
 to twelve hours are really void of stars. This 
 inference is based upon the fact that photographs 
 have been taken of identically the same areas 
 in the sky, but with exposures of only ninety 
 minutes show the same stars, including those of the 
 faintest magnitudes, that were shown on the plates 
 exposed up to twelve hours. Therefore we are 
 justified (by our present knowledge) in adopting 
 the inference that no fainter stars exist, and that 
 the universe which includes all the stars and the 
 nebulosity of the Milky Way is limited in extent, 
 and that it may be considered as a separate and 
 
 1 Knowledge, January, 1901. 
 
136 STUDIES IN ASTRONOMY 
 
 distinct aggregation of stars and of material of 
 which stars are made, independently of other 
 similar stellar aggregations which may exist in 
 the inconceivable expanse of space beyond the 
 Milky Way. This view, based as it is 011 credible 
 evidence, would reduce the whole of the solar 
 system, including the planets and satellites, to a 
 mere speck relatively with the Galactic universe 
 alone, and relatively with the others that may be 
 beyond, inconceivably small a microscopic speck. 
 What, then, about the earth, which we naturally 
 look upon as a world of great importance ? 
 Important, of course, it is to the million forms of 
 life that exist upon it, ranging between the monad 
 and the elephant, or the whale, or man, but 
 very small relatively with the solar system, 
 and insignificant relatively with the Galactic 
 
 I quote this long extract to show that my views 
 respecting the limited number of the visible stars, 
 and which I have maintained for many years, are 
 supported by those of the greatest living authority 
 on astronomical photography. 
 
 Taking into consideration the rich and poor 
 regions of the heavens, it is now generally admitted 
 by astronomers who have studied this particular 
 question and who alone are qualified to express 
 an opinion on the subject that the total number 
 of stars visible in our largest telescopes does not 
 probably exceed 100 millions a number which, 
 large as it absolutely is, is comparatively small 
 when compared with even the human popula- 
 tion of the earth estimated at 1500 millions 
 
PHOTOGRAPH OF THE MILKY WAY NEAR MESSIER 11. 
 
 By E. E. Barnard, Lick Observatory. 
 
"THE DARKNESS BEHIND THE STARS' 1 137 
 
 and may be considered as a vanishing quantity 
 when compared with an infinite number. 
 
 Taking this total number ofj 100 millions, and 
 supposing the stars equally distributed over the 
 whole sky which, of course, they are not I find 
 that the apparent distance between them would 
 be about 73 seconds of arc, so that the stars would 
 be widely separated even in a small telescope. 
 For Dr. Roberts' photograph in Cygnus the dis- 
 tance would be about 38 seconds, and even for 
 the closely "compressed" cluster w Centauri the 
 distance between the stars would be 22 seconds. 
 
 In some remarkable photographs of the Milky 
 Way in Sagittarius and Aquila, taken by Professor 
 Barnard at the Lick Observatory, there are, in 
 addition to clouds of small stars and apparent 
 nebulosity, numerous dark- spots and "lanes" 
 which seem quite black in comparison with the 
 luminous portions adjoining them. These photo- 
 graphs give us the impression that we are looking 
 through a comparatively thin stratum of stars, 
 that is, thin in comparison with its distance from 
 the earth. 
 
 The fact of the visible stars being limited in 
 number seems to show beyond a doubt that we 
 live in a limited universe, which is isolated by a 
 dark and starless void from any other universes 
 which may exist in the infinity of space beyond. 
 
XIV 
 
 The Nebular Hypothesis 
 
 THE origin of the solar system has always 
 formed a subject of interest to the human 
 mind from the earliest ages to the present 
 time. The question which naturally suggests 
 itself is, whether the system was originally created 
 in its present form, or whether it has been evolved 
 and developed in the course of ages from some 
 pre-existent form of matter. The account of the 
 creation of the world given in the first chapter of 
 the Book of Genesis throws no light on the point 
 at issue. The opening words of that remarkable 
 and graphic narrative are, " In the beginning God 
 created the heaven and the earth. And the earth 
 was without form, and void." This clearly refers 
 to the state of the earth before the appearance 
 of life on its surface, but gives us no information 
 with reference to its condition previous to that 
 epoch, except that it was "without form, and 
 void." With exception of the sun and moon, no 
 other members of the solar system are referred 
 to. The account, therefore, leaves it an open 
 
THE NEBULAR HYPOTHESIS 139 
 
 question as to how the system acquired its present 
 form and constitution, and we seem justified in 
 considering any theory admissible, or at least 
 provisionally acceptable, which will explain 
 satisfactorily in what manner the sun, planets, 
 and satellites which compose the solar system 
 came to exist in their present relative positions. 
 Of course, on the theory of creation by the direct 
 will of the Almighty, we might assume at once 
 that the system was created as it stands (or rather 
 moves) ; but such a theory is highly improbable, 
 and in view of recent discoveries, a hypothesis 
 of this kind would be repugnant to the scientific 
 mind, and indeed, we might say, opposed to 
 observed facts. 
 
 The nebular hypothesis of the origin of the 
 solar system has been supposed by some shallow- 
 minded and ignorant people to be in opposition 
 to revealed religion. But this is not the case. 
 On the contrary, such a hypothesis should tend to 
 exalt our ideas of the great Creator. As Herbert 
 Spencer has well said, " Creation by manufacture 
 is a much lower thing than creation by evolution. 
 A man can put together a machine; but he cannot 
 make a machine develop itself." This is evidently 
 true, and the hypothesis of evolution from matter 
 previously created should increase our wonder 
 and admiration for the power of the Almighty 
 Creator. 
 
 The hypothesis of the formation of the solar 
 
140 STUDIES IN ASTRONOMY 
 
 system from a mass of gaseous matter usually 
 ascribed to the famous French mathematician 
 Laplace seems to have been first suggested by 
 the great German philosopher Immaiiuel Kant. 
 In the year 1755 this great thinker published a 
 work on the construction of the heavens, in the 
 second part of which he deals with the origin of 
 the solar system, and] suggests that it might have 
 been formed by the condensation of gaseous matter 
 scattered through space. He supposed that these 
 scattered portions of gas were drawn together 
 by the force of gravitation, and that, slowly con- 
 solidating, they eventually became solid bodies, 
 which now form the sun and planets. But his 
 views are sometimes rather vague and un- 
 satisfactory, and lack the unity we find in 
 Laplace's hypothesis. To Kant, however, seems 
 certainly due the credit of having first advanced 
 the bold hypothesis of the evolution of worlds 
 from a primitive mass of gas. But his primitive 
 mass differs essentially from Laplace's nebula, 
 both in its properties and in the character 
 of its motion, and his views are frequently in 
 direct opposition to those of Laplace. In dealing, 
 therefore, with the nebular hypothesis, we will 
 only consider the views propounded by Laplace 
 with reference to this remarkable and interesting 
 theory. 
 
 Laplace's hypothesis was first published towards 
 the close of the eighteenth century, in a work 
 
THE NEBULAR HYPOTHESIS 141 
 
 entitled " Exposition du Systerae du Monde." In 
 putting it forward he says, "I present this 
 hypothesis with the distrust which everything 
 ought to inspire that is not the result of 
 observation and calculation." 
 
 The fundamental idea of Laplace's hypothesis 
 is very simple and easily understood. He supposed 
 that the matter which now forms the sun, planets, 
 and satellites originally existed in the state of 
 gas, and that this gaseous mass formed a vast 
 globe, which extended from the sun's present 
 position [as a centre out to, and perhaps beyond, 
 the orbit of Neptune. 1 Laplace does not attempt 
 to explain how this gaseous mass originated. He 
 merely assumed its existence, and uses it as a 
 starting-point from which the solar system was 
 eventually evolved by condensation and solidifica- 
 tion. We might conjecture that this enormous 
 mass of gas of nearly globular shape was possibly 
 formed by the collision of two dark bodies in 
 space, but at present we will assume, as Laplace 
 did, that the gaseous mass existed, and examine 
 the consequences which will follow. To suit his 
 hypothesis, Laplace was obliged to make another 
 assumption, and this was that the gaseous mass 
 was endowed with a motion of rotation on itself, 
 
 1 Laplace's words are : " L'atmosphere du Soleil s'est primitive- 
 ment etendue an dela des orbes de toutes les planetes, et qu'elle 
 s'est reservee successivement jusqu'a ses limites actnelleB." The 
 existence of Neptune was, of course, unknown to Laplace. 
 
STUDIES IN ASTRONOMY 
 
 in the same way that the earth rotates on its 
 axis. This assumption was clearly necessary, for 
 if we suppose the gaseous mass to have had no 
 rotation, it would, when its particles began to 
 fall towards the centre, eventually consolidate 
 into a single body or sun, without planets or 
 satellites. To account, therefore, for the exist- 
 ence of planets and satellites in the solar system, 
 we must suppose that the original gaseous globe 
 had a motion of rotation on an axis. For what 
 will be the result of such a rotation ? Rotating and 
 revolving bodies develop what is called centrifugal 
 force. A stone whirled round in a sling is an 
 example. The stone constantly tends to escape 
 from the sling by the centrifugal force produced 
 by the rapid revolution of the stone round the 
 head. A similar tendency is produced in any 
 rotating 'Jbody. The fly-wheel of a steam engine 
 is an example. It exists on the earth's surface, but 
 is restrained by the force of the earth's attraction. 
 If the earth rotated 17 times faster than it does 
 (or in 1 hour 25 minutes), bodies at the equator 
 would have no weight. The attraction of the 
 earth would just be counterbalanced by the 
 centrifugal force. With a slightly more rapid 
 velocity of rotation they would be shot into space, 
 never to return. That is, of course, loose bodies 
 lying on its surface. The solid rock would not, of 
 course, move, as the force of cohesion would 
 prevent it from being disturbed. In the gaseous 
 
THE NEBULAR HYPOTHESIS 143 
 
 mass supposed by Laplace the power of cohesion 
 would be, of course, very weak, so that a slight 
 motion of rotation would be sufficient to detach 
 portions from its surface. The tenuity of such 
 a mass is almost inconceivable. It~ has been' com- 
 puted that if the total mass of matter contained 
 in the solar system were reduced to a gas of such 
 rarity that it would fill a sphere of a diameter 
 equal to that of the orbit of Neptune, its density 
 would be over 16 million times less than that of 
 hydrogen gas ! 
 
 It may be easily imagined that the force of 
 cohesion in such an attenuated gas would be very 
 small indeed, and that a very small motion of 
 rotation would be necessary to produce disruption 
 at the external surface. Further, the gaseous 
 mass would soon begin to consolidate, owing to 
 the gravitation of its particles towards the centre 
 of the sphere, and this condensation and reduction 
 of volume would according to a well-known law 
 increase the velocity of rotation. A point would 
 then be reached when, according to Laplace's 
 hypothesis, some of the gaseous matter would be 
 detached from the parent mass in the form of a 
 ring. This ring would probably break up into 
 separate globular masses, and if one of these 
 masses was very much larger than the others, it 
 would gradually gather them in by the force of 
 its attraction, and eventually form a spherical 
 gaseous mass, which would afterwards consolidate 
 
144 STUDIES IN ASTRONOMY 
 
 into a planet revolving in an orbit round the 
 original mass. Before consolidating, however, 
 these smaller masses might, in their turn, detach 
 rings, which would subsequently form satellites 
 revolving round the planet. The parent mass 
 would go on condensing and throwing off rings, 
 one for each planet, until at last it had con- 
 solidated into a central nucleus, forming the sun 
 as we see it now. 
 
 Such is an outline of Laplace's famous nebular 
 hypothesis, which has been attacked and defended 
 for nearly a hundred years, and which still forms 
 a subject of discussion among astronomers and 
 physicists. 
 
 Laplace's hypothesis satisfactorily explains the 
 following facts connected with the solar system : 
 (1) the approximate coincidence of the planetary 
 orbits with the plane of the sun's equator; (2) the 
 small eccentricity of the orbits, which originally 
 were probably circular ; (3) the direction of revo- 
 lution of the planets round the sun, and their 
 rotation on their axes; also the motion of the 
 satellites round their primaries all in the same 
 direction (with exception of the satellites of 
 Uranus and Neptune). This accordance in the 
 motions is very remarkable, and the probability 
 against such an arrangement being the result of 
 mere chance is enormous. Indeed, the agreement 
 of the above facts with Laplace's hypothesis has 
 always been justly considered as strong evidence 
 
THE NEBULAR HYPOTHESIS 145 
 
 in its favour. When Laplace's views were 
 published, only five small planets were known 
 between Mars and Jupiter. The number of these 
 small bodies has now risen to over 500, and they 
 all revolve round the sun in the same direction as 
 the other planets, a fact which further strengthens 
 the hypothesis. The theory has, however, been 
 assailed by numerous writers, and the following 
 are some of the principal objections which have 
 been advanced against it. 
 
 1. The objection has been raised that in a nebu- 
 lous mass of such tenuity as the original nebula 
 must have had, the formation of rings would be 
 impossible. But those who advanced this objec- 
 tion seem to have overlooked the fact that Laplace 
 supposed that, previous to the formation of the 
 rings, a nucleus had been formed of considerably 
 greater density, and that the rings were separated 
 from this nucleus, and not from a homogeneous 
 mass of gas. This nucleus had probably a very 
 flattened, disc-like shape. 
 
 2. Admitting the formation of rings from the 
 parent mass of gas, it has been argued that the 
 formation of planets by condensation of these 
 rings would be impossible. But M. Roche has 
 shown that the formation of planets from rings 
 of nebulous matter would be possible under 
 certain conditions. 1 
 
 1 Professor Stockwell has, however, recently shown that the 
 rings would not consolidate (Astronomical Journal, No. 557). 
 
 L 
 
146 STUDIES IN ASTRONOMY 
 
 3. Another objection which has been raised 
 against the nebular hypothesis is that many of 
 the satellites are at distances from their primary 
 which are inconsistent with Laplace's theory. 
 The moon is a case in point. Its distance from 
 the earth is greater than the radius which the 
 earth in its gaseous state would probably have 
 had at the time of the moon's formation from 
 a nebulous ring. The inner satellite of Mars, 
 Phobos, forms an exception in the opposite direc- 
 tion, its period of revolution being less than the 
 planet's period of rotation on its axis. This point 
 was briefly considered by Laplace with reference 
 to the satellites of Jupiter. A more careful in- 
 vestigation of the subject has, however, been 
 undertaken by M. Roche. He considers that the 
 satellites were not formed during the early 
 existence of the planetary nebula, and would not 
 be formed until the nebula had like the original 
 solar nebula considerably condensed at the centre, 
 the mass being influenced in this case also by 
 strong tidal solar action. It follows from M. 
 Roche's investigations that the planets nearest to 
 the sun, being acted on by a stronger tide, would 
 produce satellites more slowly and at a smaller 
 distance from their primary. The moon, being an 
 exception to this rule, must have been formed 
 under peculiar conditions. The moon's com- 
 paratively great distance of 60 times the earth's 
 radius is considered as an objection to Laplace's 
 
THE NEBULAR HYPOTHESIS 147 
 
 hypothesis. Calculation shows that when the 
 gaseous mass, which afterwards consolidated and 
 formed the earth, rotated in a period of 27'3 days 
 (the moon's period of revolution), the nebulous 
 mass would have extended to only three-fourths 
 of the distance which now separates the moon 
 from the earth. M. Roche, however, points out 
 that in considering the effects of tidal action on 
 the nebulous mass, we should as in the case of 
 oceanic tides on the earth take into account, not 
 the absolute attraction of the sun, but the differ- 
 ence between the solar attraction exercised on a 
 molecule of the atmosphere and that exerted on 
 the centre of the earth. On .this view of the 
 matter he finds that the longer axis of the nebu- 
 lous spheroid would, at the epoch referred to, be 
 exactly 60 radii of the earth as it exists at present. 
 This axis would, however, be always directed to- 
 wards the sun, the other axis at right angles to 
 the major axis being shorter. M. Roche then 
 concludes that the moon had its origin, not in a 
 ring, but in matter thrown off at the extremity of 
 the longer axis at a time when the nucleus had 
 sufficiently consolidated. Researches by M. Simon 
 agree with those of M. Roche. The formation of 
 internal rings in the nebulous mass which after- 
 wards formed the planet Mars would account, on 
 this hypothesis, for the formation of Phobos, which 
 seems to have been of relatively recent origin 
 compared with the age of our moon. 
 
148 STUDIES IN ASTRONOMY 
 
 4. Another objection to the nebular hypothesis 
 is that the satellites of Uranus and Neptune revolve 
 round these planets in a retrograde direction. 
 The question of the rotation of these planets 011 
 their axes has not yet been finally decided, but it 
 seems highly probable from analogy that they 
 rotate in the same direction as the satellites 
 revolve round their primary. It seems probable, 
 according to the nebular hypothesis, that the 
 planets, when first formed, had their axes of 
 rotation at right angles to the planes of their 
 orbits, or the general plane of rotation of the 
 original nebula. How, then, was the axis of 
 Uranus brought so nearly into coincidence with 
 the plane of the planet's orbit? The answer to 
 this question would involve the general one, Why 
 are the equators of all the planets more or less 
 inclined to their orbit planes? As far as is 
 accurately known, the planet Jupiter is the only 
 one whose equator plane nearly "' Incides with the 
 plane of the orbit, the angle between the two 
 being about 3 degrees. This question has been 
 considered by Professor G. H. Darwin and M. Simon. 
 The latter has shown that if we suppose the earth 
 to have been formed by the consolidation of a 
 series of rings, the inclination of these rings, acted 
 on by the attraction of the sun or central nucleus, 
 would have increased in time. Professor Darwin, 
 supposing the planet to be in the state of a viscous 
 spheroid which, according to Lord Kelvin, is 
 
THE NEBULAR HYPOTHESIS 149 
 
 subject to the same laws as a nebulous mass 
 concludes that every increase in the equatorial 
 protuberance would tend to increase the inclination 
 of the equator to the plane of the planet's orbit. 
 In the case of the more distant planets he is obliged 
 to invoke the aid of the satellites. 
 
 M. Wolf, reviewing all the objections which have 
 been raised against the nebular hypothesis, con- 
 siders that most of them have been satisfactorily 
 answered. He thinks that only two doubtful 
 points remain : (1) how the gaseous matter of the 
 ring left behind by the original nebulous mass was 
 consolidated into a planet of large size ; and (2) 
 how the inclinations of the planetary equators 
 and the orbits of the satellites on the planes of the 
 planets' orbits have been produced. These diffi- 
 culties are, however, not peculiar to Laplace's views, 
 but are common to all theories which suppose the 
 planetary system to have been evolved from a 
 rotating nebulous mass. 
 
 Let us now consider the evidence which modern 
 discoveries afford in support of the nebular 
 hypothesis. The existence of spiral nebulae was 
 unknown to Laplace. Had he known them he 
 would probably have considerably modified his 
 theory, and we should probably have heard less 
 of ring formation in nebulous masses. These 
 wonderful objects were discovered by Lord Rosse 
 about the middle of the nineteenth century, and 
 his discovery at one time doubted has been 
 
150 STUDIES IN ASTRONOMY 
 
 fully confirmed by photographs taken by Dr. Isaac 
 Roberts and others. The Crossley reflector at the 
 Lick Observatory (U.S.A.) has revealed by photo- 
 graphy thousands of new nebulae, and the late 
 Professor Keeler estimated that it would show in 
 the whole sky at least 120,000 nebulae. Of these 
 he considered that at least one-half would be 
 spiral. According to Scheiner, the spectra of 
 spiral nebulae are generally continuous ; in other 
 words, " a spiral nebula is not gaseous." They 
 have probably sufficiently condensed from their 
 original gaseous state, and formed liquid or solid 
 particles. These would give a continuous spectrum. 
 He finds that the spectrum of the great nebula 
 in Andromeda which Dr. Roberts finds to be 
 spiral shows no trace of bright lines (as gaseous 
 nebula show), and he considers that the component 
 particles, although too small to be visible in the 
 largest telescopes, may still constitute small stars. 
 The spectrum is somewhat similar to that of the 
 sun. Dr. Roberts' photographs show that the 
 spiral nebulae, when seen edgewise, are compara- 
 tively thin in proportion to their diameter, and 
 approximate to a disc-like form. This tendency 
 to formation in a plane is also shown by the solar 
 system, the rings of Saturn, and even by the 
 Milky Way. It may be shown by the principles of 
 dynamics that this tendency to motion in a plane 
 is due to a law known as " the conservation of the 
 moment of momentum." The original amount of 
 
THE NEBULAR HYPOTHESIS 151 
 
 energy with which the system was endowed may 
 be slowly dissipated by conversion of motion into 
 heat, and radiation of this heat into space, but the 
 moment of momentum must be preserved, and it 
 may be shown that motion in a plane fulfils 
 this requirement with the minimum amount of 
 energy. 
 
 The great nebula in Orion, and other similar 
 gaseous nebulae, probably represent the original 
 form from which spiral nebulae are evolved. The 
 transformation is probably caused by loss of 
 energy and reduction of volume. The diminution 
 of volume would, of course, produce .consolidation, 
 and hence the fact is explained that spiral nebulae 
 show a continuous spectrum, indicating that the 
 original gaseous mass has partially condensed, and 
 will eventually, in the course of ages, assume the 
 solid form. The spiral nebulae at least the larger 
 members of the class are, of course, on a much 
 vaster scale than our solar system. They probably 
 represent stellar systems and globular clusters in 
 process of formation. It is probable, however, 
 that the solar system was formed from a small 
 spiral nebula. 
 
 These recent discoveries seem to show that it is 
 now necessary to modify Laplace's original hypo- 
 thesis to a considerable extent. Instead of the 
 formation of the solar system from a globe of 
 gaseous matter, we must now assume that the sun, 
 planets, and satellites were evolved from a spiral 
 
152 STUDIES IN ASTRONOMY 
 
 nebula by portions of partially condensed nebulous 
 matter being detached, or left behind, in the form 
 of masses, and not in the shape of rings, as Laplace 
 supposed. This gets rid of the difficulty of ex- 
 plaining the condensation of the rings into planets 
 which has always been one of the chief objections 
 to Laplace's hypothesis. 
 
 Other objections are also satisfactorily met by 
 modern discoveries, and we may say that the 
 nebular hypothesis of the evolution of the solar 
 system from a gaseous mass now stands on a firmer 
 foundation than it ever did before. In the great 
 spiral nebula in Canes Venatici (Messier 51), in 
 the great nebula in Andromeda, and in other 
 beautiful and perfect specimens of the spiral 
 nebulae, we seem to see stellar and solar systems 
 in the actual process of formation before our 
 eyes. 
 
 The phenomena of " new " or " temporary " stars, 
 which, in most cases like the recent new star in 
 Perseus have turned into gaseous nebulae, seem 
 to suggest that the original nebula from which 
 suns and systems are formed may possibly have 
 been produced by the collision of two dark bodies 
 in space, as suggested by the late Dr. Croll, a 
 collision which would have the effect of converting 
 the solid bodies into the gaseous state by the 
 transformation of motion into heat. As no 
 "moment of momentum" could be produced by 
 a perfectly direct collision, it was probably 
 
THE SPIRAL NEBULA, 51 3JESSIER. 
 
 From a Photograph by W. E. Wilson, F.R.S. 
 
THE NEBULAR HYPOTHESIS 153 
 
 a " grazing " one. This would give rise to a 
 motion of rotation in the gaseous mass, a motion 
 which still survives in the revolutions of the, 
 planets and satellites and the rotations on their 
 axis. 
 
XV 
 
 Stellar Evolution 
 
 A CORDING to the nebular hypothesis of 
 Kant and Laplace, the sun and solar system 
 were in the course of ages gradually 
 evolved by condensation from a primitive mass of 
 nebulous matter. To account for the existence of 
 this original nebula, the late Dr. Croll, the well- 
 known geologist, imagined the nebulous mass to 
 have been formed by the collision of two dark 
 bodies in space, a collision which would have had 
 the effect of converting the solid bodies into the 
 gaseous state, owing to the heat produced by the 
 collision. This hypothesis would evidently be 
 applicable to all the stars as well as to the sun, 
 which is merely the nearest of the stars to the 
 earth ; and whether all nebulae had their origin 
 in such collisions or not, it seems probable that we 
 now see in the heavens many nebulae which are 
 evidently going through the process of conversion 
 into suns and planets. The wonderful spiral 
 nebulas, which have been disclosed in recent years 
 
STELLAR EVOLUTION 155 
 
 by telescopic and photographic research, suggest 
 strongly the idea that we see before our eyes the 
 evolution of nebulous matter into suns and planets. 
 Laplace's nebular hypothesis supposed that the 
 planets were formed from the original solar nebula 
 by the condensation of rings detached from the 
 parent mass by the force of the rotation, a rotation 
 for which Laplace assigned no reason, but which, 
 on Croll's hypothesis, might be accounted for by 
 supposing the dark bodies to have collided, not in 
 a direct line, but in an oblique or " grazing " 
 collision. However this may be, the spiral nebulae 
 are evidently endowed with rotation. Their aspect 
 clearly implies this, and the photographs of these 
 wonderful objects show that the portions in pro- 
 cess of formation into stars or planets are detached 
 from the parent nebula, not in the form of rings, 
 but in separate masses. And this process seems 
 much easier to understand, and appears much more 
 probable than the separation of rings supposed by 
 Laplace. The hypothesis of ring formation was 
 probably suggested by the existence of Saturn's 
 rings. But in this case the formation of a ring 
 was probably due to an abortive attempt at the 
 formation of a planet too close to Saturn's globe. 
 According to Roche's law, a satellite could not 
 have been formed in this position, as it would have 
 been torn to pieces by tidal action. This has 
 actually happened in Saturn's rings, which are 
 composed of a multitude of small bodies. 
 
156 STUDIES IN ASTRONOMY 
 
 Photographs of the great nebula in Andromeda 
 seemed at first sight to show a good example of 
 ring formation in a nebulous mass. But Dr. 
 Roberts' photographs now show that this wonder- 
 ful object is not annular but spiral, and even in 
 the annular nebula in Lyra, Schaeberle finds 
 evidence of a spiral structure. As there was 
 always considerable difficulty in explaining satis- 
 factorily how Laplace's rings could have con- 
 solidated into planets, the evidence derived from 
 the spiral nebulae should tend to simplify the 
 nebular hypothesis, and make it, in its general 
 form, more acceptable and probable. 
 
 From an inquiry into the structure of nebulae, 
 the late Professor Keeler found that spiral nebulae 
 are much more numerous in the heavens than was 
 formerly supposed, and that " any small, compact 
 nebula not showing evidence of spiral structure 
 appears exceptional." Even Herschel's " spindle- 
 shaped nebulae " probably belongs to the spiral 
 class. 
 
 These marvellous creations are more magnificent 
 and sublime objects than all the art that Ruskin 
 wrote of. One is the work of the Almighty 
 Architect, the other the feeble efforts of weak 
 and fallible man. 
 
 From the probably great distance of these spiral 
 nebulae from the earth, and their comparatively 
 large apparent size, we may conclude that they 
 are in reality of vast dimensions. The apparent 
 
STELLAR EVOLUTION 157 
 
 diameter of some of them shows that they must 
 be much larger than our solar system. Seen from 
 the nearest fixed star a Centauri the diameter 
 of the solar system would subtend an angle of 
 about 45 seconds of arc, while the apparent 
 diameter of the spiral nebula in Canes Venatici 
 (51 Messier) and that of 74 Messier is about 300 
 seconds, and these nebulae are probably much 
 farther from the earth than a Centauri. The 
 great nebula in Andromeda is of still larger 
 dimensions. What the origin of spiral nebulae 
 was we do not, of course, know ; but possibly they 
 may have been formed, as in Croll's hypothesis, by 
 a " grazing " collision between two dark bodies of 
 large size. Dr. Roberts thinks that the globular 
 clusters of stars have probably been evolved from 
 spiral nebulae, and Schaeberle has recently found 
 evidence of spiral structure in the great globular 
 cluster in Hercules (13 Messier). 1 
 
 Admitting that suns and stars have been evolved 
 in some way from nebulous masses, let us now try 
 and follow their life history from the time that 
 they have sufficiently consolidated to present the 
 appearance of a star down to the distant time 
 when they shall have lost all their heat and light 
 by radiation, and " roll through space a cold and 
 dark ball." It has been known for ages that 
 the stars are of different colours white, yellow, 
 orange, and red and this fact suggested some 
 1 The Astronomical Journal, No. 552, December 31, 1903. 
 
158 STUDIES IN ASTRONOMY 
 
 essential physical difference between them ; but 
 until the discovery of the principles of spectrum 
 analysis, it was impossible to determine their 
 chemical composition. The application of spec- 
 trum analysis to the observation of stars and 
 nebula? now forms an important and interesting 
 branch of astronomy, known as Astrophysics. 
 The pioneer in this department of astronomical 
 research was Dr. (now Sir William) Huggins, Avho, 
 in 1856, erected an observatory in connection 
 with his private house at Upper Tulse Hill, 
 London. His first instrument was a telescope of 
 5 inches aperture by Dollond ; but in 1858 this 
 was replaced by one of 8 inches in diameter, the 
 work of the famous American optician Alvan 
 Clark. For the first few years he worked in 
 conjunction with Dr. Miller, but afterwards by 
 himself. In 1870 he obtained a loan from the 
 Royal Society of a larger instrument, the work 
 of Sir Howard Grubb. This instrument consisted 
 of a 15-inch refractor and a 15-inch Cassegrain 
 reflector mounted on the same stand. Having 
 designed a suitable star spectroscope for this 
 instrument, he first directed his attention to the 
 brighter stars Sirius, Vega, Aldebaran, etc. and 
 succeeded] in comparing their spectra with those 
 of terrestrial substances, such as hydrogen, iron, 
 sodium, etc., and proved the existence of these 
 elements in the stars referred to. About the 
 same time similar observations were made 
 
STELLAR EVOLUTION 159 
 
 independently by Rutherford in America, Secchi 
 at Rome, and Vogel in Germany. 
 
 Remembering Sir William Herschel's views as 
 to the probably gaseous nature of some of the 
 nebulae, Dr. Huggins determined to test the 
 question by a spectroscopic examination. On 
 the evening of August 29, 1864, he turned his 
 spectroscope for the first tune on the planetary 
 nebula in Draco, which lies near the pole of the 
 ecliptic. To his surprise, he found that the 
 spectrum consisted of only one bright line instead 
 of the continuous spectrum crossed by dark lines, 
 which he had found in the spectra of the stars. 
 On closer examination, he detected two other 
 bright lines towards the blue end of the nebular 
 spectrum. This decided the question, and proved 
 beyond all doubt that the light emitted by this 
 nebula came from glowing gas. The question 
 then arose as to the chemical character of these 
 bright lines, and later observations have shown 
 that the two fainter lines are due to hydrogen ; 
 but the origin of the brightest line the "chief 
 nebular line," as it is called still remains un- 
 determined. It is probably due to some hitherto 
 undiscovered chemical substance, and to this 
 substance the name " nebulium " has been given. 
 There is another line in the spectrum, also 
 apparently due to the unknown substance, and 
 photographs have disclosed the existence of some 
 forty lines or more in various nebulae, showing the 
 
160 STUDIES IN ASTRONOMY 
 
 probable presence of helium, carbon, iron, calcium, 
 and probably magnesium. Of sixty of the 
 brightest nebulae and clusters, Dr. Huggins found 
 that one-third showed the bright-line spectrum. 
 Among these were the so-called planetary nebulas 
 and the great nebula in Orion. He found the 
 great nebula in Andromeda to show a faint con- 
 tinuous spectrum, so that, nebulous looking as 
 this wonderful object is, it is probably not truly 
 gaseous. All the spiral nebulas seem to show a 
 continuous spectrum. Gaseous nebulas are usually 
 of a bluish or greenish colour, while those with 
 a continuous spectrum are dull white. 
 
 Stars differ in the character of their spectra, 
 and these spectra have been divided into several 
 types. The 1st, or Sirian type, has the hydrogen 
 lines very strong, and the lines of the metallic 
 elements very faint, or invisible. In the 2nd, or 
 solar type, the metallic lines are numerous and 
 very visible. The 3rd type shows spectra in 
 which, besides the metallic lines, there are nume- 
 rous dark bands in all parts of the spectrum, and 
 the blue and violet portions are very faint. This 
 type has been sub-divided into two types one in 
 which the dark bands are fainter towards the red 
 end of the spectrum, and the other in which these 
 bands are fainter towards the violet. This latter 
 type is now known as type IV. Stars of the 3rd 
 and 4th types are much less numerous than those 
 of the 1st and 2nd types ; but there are about 
 
STELLAR EVOLUTION 161 
 
 1000 stars of the 3rd type now known, and 250 
 of the 4th type. Stars of the 5th type, which 
 are also known as Wolf-Rayet stars, from their 
 discoverers, are comparatively rare. They have 
 a spectrum which, according to Professor Picker- 
 ing, consists of " wide, bright bands superposed 
 on a faint continuous spectrum, the strongest one 
 of them probably coincident with a bright band 
 in the spectrum of the gaseous nebulae, and most 
 of the others probably coincident with hydrogen 
 lines and prominent Orion lines." The stars of 
 types I. and V. are usually of a white colour ; 
 those of type II., yellow ; type III., orange red ; 
 and type IV., all red. Most of the long-period 
 variables have 3rd and 4th type spectra. There 
 is a variety of type I. known as the " Orion type," 
 many of the stars in Orion showing this type of 
 spectrum. Between all these types there are 
 many transitional types. 
 
 The question now arises, which of these various 
 types represent the oldest and which the youngest 
 stars? That is, which are nearest to the nebular 
 stage, and which are farthest advanced in their 
 " life history " ? From an examination of a large 
 number of stellar spectra, Professor Pickering is 
 disposed to think that stars showing the "Orion 
 type " of spectrum are probably " in an early stage 
 of development," and that stars with spectra of 
 the 5th type may possibly form a connecting link 
 between the Orion stars and those of the nebulae." 
 
162 STUDIES IN ASTRONOMY 
 
 After the Orion stars come the stars of the 1st 
 type (the Sirian), then those of the 2nd (the 
 solar), and lastly the 3rd type, which is the 
 oldest, and probably belongs to stars which are 
 approaching the total extinction of their light. 
 The 4th type may also ' represent stars far ad- 
 vanced in their "life history," but their relation 
 to the 3rd type stars is not very obvious. In this 
 view of the evolutional order Sir William Huggins 
 concurs. In his address to the British Association 
 at Cardiff in 1891, he said, "This order is essentially 
 the same as Vogel had previously proposed in his 
 classification of the stars in 1874, in which the 
 white stars, which are the most numerous, repre- 
 sent the early adult and most persistent stage of 
 stellar life, the solar condition that of maturity 
 and of commencing age ; while in the orange and 
 red stars we see the setting in and advance of old 
 age." At that time he considered that the order 
 of evolution was represented by the following 
 stars : Sirius and Vega, a Ursse Majoris, a Virginis, 
 a Aquilse, Bigel, a Cygni, Capella, and the sun, 
 Arcturus, Aldebaran, and Betelgeuse, the first 
 named being the youngest, and the last the oldest. 
 That stars of the " Orion " and the Sirian types 
 are of less density than those of the solar type has 
 been recently shown by Roberts and Russell, who 
 have made calculations respecting the densities of 
 the Algol type variables, which all show spectra of 
 the " Orion " or Sirian type. These investigations 
 
STELLAR EVOLUTION 163 
 
 show that the average density of these stars is 
 much less than that of the sun, 1 and that, there- 
 fore, they are in an earlier stage of condensation. 
 The same remark applies to Sirius itself, which 
 has a small mass in proportion to its brightness, 
 indicating that it is probably a body with a large 
 volume, high temperature, and small density. 
 
 Stars in the earlier stages of evolution have 
 probably no well-developed photosphere, and 
 being greatly expanded by heat, and of small 
 density, a large proportion of their light comes to 
 us from a greater depth below their surface than 
 in stars of the solar type. 
 
 It has been shown by Homer Lane, an American 
 physicist, that so long as a star " remains subject 
 to the laws of a purely gaseous body, its tempera- 
 ture will increase as condensation advances." 
 When, however, owing to the continual radiation 
 of heat the gaseous state has been passed, the star 
 will begin to cool, its light will diminish, and 
 changes will take place in its spectrum. Sir 
 William Huggins is now disposed to think that 
 the hottest stars must be looked for among those 
 of the Solar type. The " evolutional order " now 
 adopted by him seems to be (omitting the so-called 
 Wolf-Rayet stars) Bellatrix, Rigel, a Cygni, Regu- 
 lus Vega, Sirius, Castor (fainter component), 
 Altair, Procyon, y Cygni, Capella (hottest star), 
 Arcturus, and Betelgeuse; the youngest being 
 * The Astronomical Journal, vol. 10, p. 308. 
 
164 STUDIES IN ASTRONOMY 
 
 Bellatrix, and the oldest Betelgeuse. The order 
 would then be : Nebulae, " Orion type," type I., 
 type II., and type III. The exact position of type 
 IV. has been considered doubtful, but from a 
 recent elaborate discussion of their spectra, Pro- 
 fessor Hale thinks that " 4th type stars probably 
 develop from stars like the sun through loss of 
 heat by radiation." Types III. and IV. are there- 
 fore collateral branches of development from the 
 sun, as Vogel regarded them. Hale finds a close 
 agreement between the spectra of the 4th type 
 stars and those of sun-spots, and he suggests that 
 these stars may be covered with numerous sun- 
 spots. 1 
 
 There are many long-period variables with 
 spectra of the 3rd and 4th types. These are 
 probably suns which have far advanced in the 
 process of condensation and cooling, and are sub- 
 ject to periodical outbursts of light, owing to the 
 escape of imprisoned gases. This is suggested by 
 the appearance of bright lines in the spectra of 
 many of them near the time of maximum light. 
 This process might go on for centuries, or even 
 thousands of years, until at last the whole mass 
 of the star would become so cooled down that 
 there would be no further outbursts of light. The 
 star would then cease to rise to a maximum, and 
 it would slowly diminish in brightness, until its 
 light became wholly extinguished. One stage of 
 1 Publications of the Yerkes Observatory, vol. 3, part 5. 
 
STELLAR EVOLUTION 165 
 
 this process would seein to have been actually 
 reached in the case of the long-period variable 
 T. Ophiuchi. Discovered by Pogson in 1860, it was 
 found to be variable from the 10th to the 12th 
 magnitude in a period of about 361 days between 
 the maxima, but for the last sixteen years it has 
 not risen to a maximum, and remains at a perma- 
 nent minimum of light. In the course of time this 
 star will probably slowly diminish until it becomes 
 wholly extinguished, and it will then " roll through 
 space a cold and dark ball." Possibly this may 
 be the ultimate fate of our own sun, and of the 
 thousands of stars which now sparkle in our mid- 
 night sky. 
 
XVI 
 
 The Construction of the Visible Universe 
 
 A examination of the evidence we] have at 
 present with reference to the distribution 
 of the visible stars in space has recently 
 been undertaken by Professor Kapteyn, of Groii- 
 iiigen, and a popular account of the conclusions he 
 has arrived at may prove of interest to the general 
 reader. 
 
 It must first be explained that, in order to obtain 
 a clear view of the construction of the visible 
 heavens, it would be necessary to know the relative 
 distances of a large number of stars ; but as the 
 distances of only a few stars have yet been 
 determined, and the results hitherto obtained are 
 open to much uncertainty, we must have recourse 
 to some other method of estimating these distances. 
 In travelling in a railway carriage, if we fix our 
 attention on the trees, buildings, and other objects 
 we pass on our journey, it will be noticed that all 
 objects apparently move past us in the opposite 
 direction to that in which we are travelling, and 
 that the nearer the object is, the faster it seems to 
 
CONSTRUCTION OF VISIBLE UNIVERSE 167 
 
 move, with reference to~distant objects near the 
 horizon. The telegraph-poles and mile-posts fly 
 past rapidly, while trees and houses at some dis- 
 tance have a much slower apparent motion. So 
 it is with the stars. The sun is moving through 
 space, carrying along with it the earth and all 
 the planets, satellites, and comets forming the 
 solar system. The effect of this motion is to 
 cause an apparent small motion of the stars in the 
 opposite direction, and the nearer the star is to 
 the earth the greater will be this apparent motion 
 as in the case of a railway train. In addition to 
 this apparent motion, the stars are themselves 
 like the sun moving through space, and this real 
 motion is also visible ; that is, it can be measured 
 by accurate astronomical observations. If this 
 real motion takes place in the opposite direction 
 to that in which the earth is moving, it will adtf 
 to the apparent motion, and will increase the 
 " proper motion," as it is termed. If, on the other 
 hand, the real motion is in the same direction as 
 the earth's motion, it will tend to diminish the 
 proper motion. In either case, the nearer the star 
 is to the earth the greater will be its apparent 
 annual displacement on the background of the 
 heavens. The amount of this " proper motion " is, 
 therefore, considered by astronomers to form a 
 reliable criterion of the stars' distance from the 
 earth, and the actual measures of distance which 
 have been made show that this assumption is 
 
168 STUDIES IN ASTRONOMY 
 
 approximately true. Of fourteen stars which 
 have a proper motion of over three seconds of arc 
 per annum, eleven have yielded a measurable 
 parallax, or displacement due to the earth's annual 
 motion round the sun ; that is to say, that eleven 
 out of fourteen fast-moving stars are within a 
 measurable distance of the earth, and therefore 
 near us when compared with the great majority 
 of the stars which are not within measurable 
 distance, or, at least, are beyond our present 
 methods of measurements. 
 
 In the case of small groups of stars, we may 
 assume that the real motions of the individual 
 stars take place indifferently in all directions, and 
 that consequently, taking an average of all the 
 motions of the stars composing the group, the 
 effects due to the real motions will destroy each 
 other, and there will remain, as the most reliable 
 criterion, the effect due to the sun's motion in 
 space. If, however, we compare the proper 
 motions of groups situated in different parts of 
 the sky, there is a consideration which, to a great 
 extent, vitiates this conclusion ; for, near the 
 point of the heavens towards which the sun and 
 earth are moving, known as the apex of the solar 
 way, and probably situated somewhere near the 
 bright star Vega (as indicated by recent re- 
 searches); and near the point from which the 
 sun is moving known as the ant-apex, about 
 20 south of Sirius there will be no apparent 
 
CONSTRUCTION OF VISIBLE UNIVERSE 169 
 
 displacement due to the solar motion in space, 
 as this motion takes place in the line of sight 
 with reference to these points of the sky. The 
 observed proper motions at these points will, 
 therefore, be solely due to the real motion of the 
 stars in those regions. In other parts of the 
 heavens, however, the total proper motion will 
 be a combination of the apparent and real motions 
 of the stars, and for stars in different parts of the 
 heavens it will not follow that stars having equal 
 proper motions are necessarily at the same dis- 
 tance from the earth. To make this point clearer, 
 let us assume that there are two stars at abso- 
 lutely the same distance from our eye, one situated 
 at or near the solar apex, and the other at a point 
 90 from the apex, and let us suppose that both 
 are moving through space with exactly the same 
 velocity, and in the same direction say at right 
 angles to the direction of the solar motion. Then, 
 in the case of the star near the "apex," the 
 observed " proper motion " will be solely due 
 to the star's real motion, and in the star 90 
 distant from the apex the "proper motion" will 
 be solely due to the solar motion. Now, unless 
 the stellar motion and the solar motion happen to 
 be equal, the observed " proper motions " will not 
 be equal, although both stars are at the same 
 distance from the earth, and are moving with the 
 same velocity. If both stars are really at rest, the 
 star at the "apex" will have no proper motion, 
 
170 STUDIES IN ASTRONOMY 
 
 while the star 90 distant will have an apparent 
 motion due to the sun's motion. To overcome 
 this source of error in estimating the distance 
 of a star from its proper motion, Professor 
 Kapteyn made use of another measure which 
 is independent of the solar motion. This is the 
 component of the proper motion measured at 
 right angles to a great circle of a sphere passing 
 through a star and the solar apex. The amount 
 of motion in this direction will evidently not be 
 affected by the sun's motion ; and from a discussion 
 of the stars contained in the " Draper Catalogue of 
 Stellar Spectra," which were observed by Bradley 
 (and of which the proper motions are now known 
 with accuracy), Professor Kapteyn finds that this 
 motion is "nearly inversely proportional to the 
 distance ; " that is, the greater the motion the 
 less the distance of the stars, and the smaller the 
 motion the greater the distance. Excluding stars 
 with proper motions greater than half a second of 
 arc per annum, Professor Kapteyn found that for 
 stars at various distances from the Milky Way 
 this component of the "proper motion" forms a 
 good measure of distance. 
 
 As the result of his investigations on this 
 interesting question, Professor Kapteyn arrives 
 at the following conclusions : 
 
 Neglecting stars with small or imperceptible 
 proper motions, we have a group of stars which 
 no longer show any condensation in a plane. 
 
CONSTRUCTION OF VISIBLE UNIVERSE 171 
 
 Stars with very small or no proper motions shoAv 
 a condensation towards the plane of the Milky 
 Way. This applies to stars of the 2nd, or solar, 
 type, as well as to those of the 1st, or Sirian, type 
 of spectrum, and evidently indicates that the stars 
 composing the Milky Way lie at a great distance 
 from the earth. The extreme faintness of the 
 majority of the stars composing the Galaxy seems 
 to confirm this conclusion. The condensation of 
 stars of the 1st, or Sirian, type is more marked 
 than those of the 2nd, and this agrees with the 
 fact found by Professor Pickering, that the 
 majority of the brighter stars of the Milky Way 
 show spectra of the 1st type. Judging from the 
 ease with which the fainter stars of the Galaxy 
 can be photographed, he concludes that most of 
 these fainter stars are bluish, and probably have 
 spectra of the 1st type, like Sirius and Vega, 
 which are bluish-white. From an enumeration 
 of the stars included in the " Draper Catalogue," 
 I find that 63 per cent, of the stars on the Milky 
 Way, as drawn by Heis, have spectra of the 1st 
 type. 
 
 Professor Kapteyn finds that this condensation 
 of stars with small proper motions is very percep- 
 tible, even for the stars visible to the naked eye, 
 and is as well marked in those stars which have 
 spectra of the 2nd type as for all stars of the 9th 
 magnitude; but for stars of the 1st type the 
 condensation is still more marked. He considers 
 
172 STUDIES IN ASTRONOMY 
 
 that this condensation is either partly real, or 
 that there is a real thinning out of stars near the 
 pole of the Milky Way. As I have shown else- 
 where, M. Celoria's observations with a small 
 telescope, compared with Sir William Herschel's 
 observations with a large telescope, indicate 
 clearly that there is a real thinning out of stars 
 near the poles of the Milky Way. 
 
 Professor Kapteyn concludes that the arrange- 
 ment of the stars suggested by Struve has no real 
 existence. He attributes the fallacy in Struve' s 
 hypothesis to the fact that the mean distance of 
 stars of a given magnitude in the Milky Way and 
 outside it is not the same. He finds that the 
 vicinity of the sun is almost exclusively occupied 
 by stars of the 2nd, or solar, type, a conclusion 
 which reminds us of Dr. Gould's " solar cluster." 
 He thinks that the number of Sirian-type stars 
 increases gradually with the distance, and that 
 beyond a distance corresponding to a proper 
 motion of about one-fourteenth of a second per 
 annum the Sirian stars largely predominate. 
 
 In the group of stars known as the Hyades, 
 however of which Aldebaraii is the leading 
 brilliant the components of which have a common 
 proper motion, both in amount and direction, stars 
 of the 1st and 2nd type seem to be mixed, and 
 Professor Kapteyn assumes that the two types 
 represent different phases of evolution, and that, 
 as the brightest stars of the group are chiefly of 
 
CONSTRUCTION OF VISIBLE UNIVERSE 173 
 
 the solar type, these stars must be the largest of 
 the cluster. From this fact he concludes that the 
 solar- type stars are in a less advanced stage of 
 evolution than those of the Sirian type. This 
 does not, however, agree with the generally 
 accepted view. Sir William Huggins and Pro- 
 fessor Vogel consider the Sirian stars to represent 
 an earlier stage of stellar evolution. Proctor held 
 the same opinion. In Sir Norman Lockyer's 
 hypothesis of increasing and decreasing tempe- 
 ratures in stars of various types of spectra, he 
 places the Sirian stars at the summit of the heat 
 curve, and the sun and solar stars just below them 
 on the descending branch of the curve. 1 This 
 view is in conformity with the current opinion 
 that the sun is a cooling body. In the case of the 
 Pleiades, which form a more evident cluster than 
 the Hyades, I find from the " Draper Catalogue " 
 that the great majority of the brighter stars have 
 spectra of the Sirian type. Most of these stars 
 have a very similar proper motion, both in direc- 
 tion and amount, and there can be little or no 
 doubt that they form a connected system. The 
 superior brilliancy of the stars composing the 
 Hyades would perhaps indicate that they are 
 nearer to the earth than the Pleiades group, and 
 they may possibly form members of " the solar 
 cluster." 
 
 Assuming that the distances of the stars are 
 1 " The Meteoritic Hypothesis," pp. 380, 381. 
 
174 STUDIES IN ASTRONOMY 
 
 inversely proportional to their proper motions, 
 Professor Kapteyn computes the relative volumes 
 of the spherical shells which contains the stars 
 with different proper motions (from one-tenth of a 
 second to one second of arc, and more). Comparing 
 these volumes with the corresponding number of 
 stars, we arrive at an estimate of the density of 
 star distribution at various distances. The result 
 of this calculation shows that the distribution of 
 stars of the Sirian type approaches uniformity 
 when a large number of the faint stars (the 9th 
 magnitude) are considered. With reference to 
 stars of the 2nd, or solar, type, however, the larger 
 the proper motion the greater the number of the 
 stars, or, in other words, the 2nd type, or solar 
 stars, are crowded together in the sun's vicinity. 
 Evidence in favour of this conclusion is afforded 
 by the fact that of eight stars having the largest 
 measured parallax (and whose spectrum has been 
 determined), I find that seven have spectra of the 
 solar type. The exception is Sirius, which is 
 evidently an exceptional star, with reference to 
 its brightness and comparative proximity to the 
 earth, no other star of the 1st magnitude having 
 so large a parallax. 
 
 Professor Kapteyn finds that the centre of 
 greatest condensation of the solar-type stars lies 
 near a point situated about 10 to the west of the 
 great nebula in Andromeda, and that this centre 
 nearly coincides with the point which, according 
 
CONSTRUCTION OF VISIBLE UNIVERSE 175 
 
 to Struve and Herschel, represents the apparent 
 centre of the Milky Way considered as a ring. 
 This would indicate that the sun and solar system 
 lie a little to the north of the plane of the Milky 
 Way, and towards a point situated in the northern 
 portion of the constellation of the Centaur. The 
 fact is worth noting that the nearest fixed star to 
 the earth, Alpha Centauri, lies not very far from 
 this point. Possibly there may be other stars in 
 this direction having a measurable parallax. The 
 southern portion of the heavens has not yet been 
 thoroughly explored for parallax. 
 
 For stars of equal brightness, Professor Kapteyn 
 finds that those of the Sirian type are, on an 
 average, about 2f times farther from the earth 
 than those of the solar type. As light varies 
 inversely as the square of the distance, this would 
 imply that the Sirian stars are intrinsically over 7 
 times brighter than those of the solar type. This 
 conclusion is confirmed by the great brilliancy of 
 Sirius, and other stars of the same tppe in pro- 
 portion to their mass. I have shown in the 
 chapter on the "Suns of Space" that Sirius is 
 about 31 tunes brighter than our sun would be 
 if placed at the same distance, although its mass 
 is only 2*36 times the sun's mass, as computed from 
 the orbit of its satellite. 
 
 The general conclusions to be derived from the 
 above results seem to be that the sun is a member 
 of a cluster of stars possibly distributed in the 
 
176 STUDIES IN ASTRONOMY 
 
 form of a ring, and that outside this ring, at a 
 much greater distance from us than the stars of 
 the solar cluster, lies a considerably richer ring- 
 shaped cluster, the light of which, reduced to 
 nebulosity by immensity of distance, produces the 
 Milky Way gleam of our midnight sky. 
 
XVII 
 
 The Secular Variation of Starlight 
 
 THE "secular variation" of the stars, that 
 is, the slow increase or decrease of their 
 light in the course of ages is an interesting 
 subject, and has an obvious bearing on the question 
 of stellar evolution. This secular variation must 
 not be confused with the periodic variation to 
 which a large number of stars are subject, nor 
 with the "irregular" variation to which some 
 stars are liable. Almost all the "long period" 
 and " irregular " variables have spectra of the 3rd 
 type, or " fluted spectra," and seem to belong to a 
 distinct class probably suns passing through the 
 last stages of their life-history. "Short period" 
 variables have usually spectra of the 2nd, or solar, 
 type, and the "Algol variables" which are not 
 true variables, but merely eclipse stars have 
 spectra of the 1st type. Stars affected by " secu- 
 lar variation" must be looked for among those 
 with spectra of the 1st and 2nd types. When a 
 star reaches the 3rd type it seems to become 
 either periodically or irregularly variable. Stars 
 
178 STUDIES IN ASTRONOMY 
 
 with secular variation might be expected to be 
 apparently constant in their light, the secular 
 variation being so slow that no change can be 
 detected in a few years' observation. This change 
 may, however, possibly become appreciable in the 
 course of centuries, at least in some cases. Accord- 
 ing to Lane's law, a mass of incandescent gas in 
 contracting rises in temperature, and therefore 
 probably in light, "as long as the gaseous con- 
 dition is retained," but when condensation has 
 further advanced the mass would begin to cool 
 and the light of the star would then slowly 
 diminish. 
 
 In my examination of Al-Sufi's " Description of 
 the Fixed Stars," written in the 10th century, I 
 have noticed a number of cases in which a star 
 seems to have either increased or diminished in 
 brightness. In addition to his own estimates, 
 Al-Sun gives the magnitudes as rated by Ptolemy 
 (or Hipparchus), and these are valuable, as 
 Ptolemy's magnitudes, given in all the editions of 
 the "Almagest," now extant, are quite untrust- 
 worthy. Ptolemy's magnitudes are probably 
 those of Hipparchus, and this would take us back 
 to B.C. 127, or over 2000 years ago. A period of 
 2000 years is, of course, very short in the life- 
 history of a star, but as, according to the theory 
 of stellar evolution, a star would, beginning with 
 the nebulous stage, go on for ages increasing in 
 light, then remain stationary for a long period, 
 
SECULAR VARIATION OF STARLIGHT 179 
 
 and after that slowly diminish in brightness, it 
 seems possible that some perceptible changes may 
 have taken place in certain stars since the time of 
 Hipparchus. We know that some stars have cer- 
 tainly decreased in brightness since they were 
 observed by Al-Sufi. For example, there can be 
 no doubt that the star /3 Leonis (Denebola) has 
 diminished from the 1st to the 2nd magnitude 
 since the 10th century. Al-Sufi describes it in 
 the same words that he uses with reference to 
 Regulus, namely, "the bright and great star of 
 the 1st magnitude." The same may be said of 
 Eridani, which has faded from the 1st to the 
 3rd magnitude. Some writers have suggested 
 that the star mentioned by Al-Sufi was not 
 Eridani, but a Eridani (the so-called Achernar), 
 and that Al-Sufi merely described a Eridani from 
 the descriptions of travellers! But this Al-Sufi 
 never did in the case of any star mentioned hi his 
 book. He was much too careful and conscientious 
 an observer to do anything of the sort, and he 
 expressly states in the preface to his work that 
 he has described all the stars he speaks of "as 
 seen with my own eyes." Moreover, his descrip- 
 tion of the position of the star he observed as 
 " 1st magnitude," and the neighbouring stars in 
 Eridanus, is so clear as to leave no room for doubt 
 as to the identity of the star with 6 Eridani. That 
 this is the correct interpretation of Al-Sufi's state- 
 ment was also the opinion of Ulugh Beigh, Halley, 
 
180 STUDIES IN ASTRONOMY 
 
 Baily, and Dr. Anderson. Further, Hipparchus 
 and Ptolemy state distinctly that the "Last in 
 the River " rose above their horizon at a certain 
 time of the year, and this a Eridani could not 
 possibly have done. This seems sufficient to 
 finally settle the question in favour of & Eridani, 
 which is, therefore, the real Achernar, or " Last in 
 the River" of Ptolemy." x 
 
 The following are a few of the most interesting 
 and remarkable cases of apparent increase or 
 decrease of light which I have met with in Al- 
 Sufi's work. I will first consider stars which have 
 probably decreased in brightness. 
 
 (3 Aquilse. This is the southern of the three 
 well-known stars in Aquila, y, Altair, and j3, which 
 lie nearly in a straight line, fi was rated 3rd 
 magnitude by Ptolemy, 3-4 ("small third") by 
 Al-Sufi, 4 by Argelander, Heis, and Houzeau, and 
 was measured 3*90 with the photometers at Harvard 
 and Potsdam. The spectrum is of the 2nd type 
 X (K, Pickering), which probably indicates a cooling 
 star. Ptolemy (or Hipparchus) rated /? and y as 
 both 3rd magnitude, but at present ft is one mag- 
 nitude fainter than y. The ancients called the 
 two stars al-mizdn, "the balance," probably on 
 
 1 For further particulars, see an interesting paper by Dr. 
 Anderson on " The Story of Theta Eridani," in Knowledge, July, 
 1893. I venture to suggest that 6 Eridani should be called 
 Eschatos, to show that it is identical with the Effxaros TOV nora/uov 
 of Ptolemy and Ulugh Beigh. 
 
SECULAR VARIATION OF STARLIGHT 181 
 
 account of the equality of the stars one on each 
 side of Altair. 
 
 K Librae. Rated 4th magnitude by Ptolemy and 
 Al-Sufi, 5 by Argelander, Heis, and Houzeau, and 
 measured 4'96 at Harvard. Al-Sufi made it equal 
 to y Librae (4m., as it is at present), but K is now 
 nearly a magnitude fainter than y. Al-Sufi made 
 K one magnitude brighter than 37 Librae (5, about 
 its present brightness), but they are now equal. 
 K has therefore diminished from the 4th to the 5th 
 magnitude, while 37 has probably remained con- 
 stant in light. Both Ptolemy and Al-Sufi rated K 
 two magnitudes brighter than x, but they are now 
 practically equal. The spectrum of K is of the 
 2nd type (H, Pickering), and is probably a cooling 
 star. 
 
 7 (6) Piscium. 4 Ptolemy, 4-5 Al-Sufi, 6 Arge- 
 lander, 6-5 Heis, 5 Houzeau, 5*20 Harvard. Spec- 
 trum H. Ptolemy and Al-Sufi made it equal to 
 y Piscium, but at present it is more than a 
 magnitude fainter. Argelander made it two 
 magnitudes less than y, and there can be little or 
 no doubt that it has diminished in brightness, 
 y is still about 4th magnitude, as Ptolemy rated it. 
 
 Eridani has been already referred to. Its 
 spectrum is intermediate between the 1st and 2nd 
 types (A 2 F), and is similar to that of ft Leonis. 
 
 ft Hydrae. Rated 3rd magnitude by Ptolemy 
 and Al-Sufi, 4 by Argelander and Heis, 4-5 by 
 Houzeau, 4*40 Harvard, and 4'5 at Cordoba. 
 
182 STUDIES IN ASTRONOMY 
 
 Al-Sufi made it slightly brighter than y Hydrae 
 (3-4 Al-Sufi, 3-33 Harvard), but it is now a mag- 
 nitude fainter. This seems a certain case of 
 diminution in brightness. 
 
 Piscis Australis. 4 Ptolemy, 5-6 Al-Sufi, Ar- 
 gelander, and Heis, 6-5 Behrmann, 6 Houzeau, 6*62 
 Harvard, 6'7 Cordoba. This star seems to have 
 certainly diminished. 
 
 v) Piscis Australis. 4 Ptolemy, 5 Al-Sufi, 5'6 
 Argelander, Heis, and Houzeau, 5*47 Harvard, 5*7 
 Cordoba. 
 
 X Centauri. Rated 4-3 by Ptolemy and Al-Sufi, 
 4-5 by Houzeau, 4'54 Harvard, 4*8 Cordoba, 4'75 
 Williams. Here we have a case of certain dimi- 
 nution of light. The stars v, ft, <, and x Centauri 
 lie near each other between and Centauri. Al- 
 Sufi's remarks with reference to these four stars 
 are very interesting, and his description very clear 
 and unmistakable. He says : " The 12th (v) is 
 of large 4th magnitude [3*53 Harvard], and is on 
 the left side [of the ancient figure]. The 13th and 
 14th (/A and <) are behind the 12th [that is in 
 longitude] and near it ; as to the 13th (/A), it is the 
 more southern of the two, below the 12th (v) ; it 
 is also of large 4th magnitude [3'32 Harvard], 
 Between these two stars there is a span [about 
 47']. The 14th (<) is behind the 13th (ft), and of 
 the 4th magnitude [4'05 Harvard]. Ptolemy calls 
 larger [4-3], although it is of less brightness than 
 the 12th and 13th [as it is at present]. These 
 
* t 
 
 20 
 
 V 1 
 it 
 
 V 
 
 /S 
 
 /2 
 'J 
 
 6 
 
 STABS IN CENTAUBUS (AL-SUFl). 
 
SECULAR VARIATION OF STARLIGHT 183 
 
 stars, the 12th, 13th, and 14th (v, /*, and <) are 
 close together, forming a little triangle, and are 
 all in the right side, to the south of the 7th (d, 
 between i and v). The 15th (x) follows these three 
 stars, inclining towards the north ; between it 
 and the 14th (<) there is less than a cubit [that is, 
 less than 2 20']. It is of large 4th magnitude." 
 At present x is distinctly fainter than <, although 
 rated brighter by Al-Sufi. It is also one magni- 
 tude fainter than v, which Al-Sufi rates of the 
 same magnitude (4-3). A glance at the accom- 
 panying map will show how minute and accurate 
 Al-Sufi' s description is, leaving no doubt as to the 
 identity of the stars he refers to. 1 
 
 o Persei. This is another case of apparently 
 certain diminution of light. It was rated " small 
 3rd magnitude" by Ptolemy and Al-Sufi, 4 by 
 Argelander and Heis, and was measured 3*94 at 
 Harvard, and 3*85 at Potsdam. Ptolemy and Al- 
 Sufi agree in making it equal to [2*91 Harvard], 
 which lies near it, but the Harvard measures make 
 o about one magnitude fainter than , and their 
 present great disparity in brightness is noticeable 
 at a glance. Al-Sufi' s description of the two stars 
 is clear, and leaves no room for doubt as to their 
 identity. I have plotted Ptolemy's positions of 
 these stars, and find that they agree well with 
 Al-Sufi's description. Al-Sufi says : " The 25th (o) 
 
 1 The stars shown in the map are plotted from Ptolemy's 
 positions. 
 
184 STUDIES IN ASTRONOMY 
 
 is the preceding of two stars which are in the left 
 leg, and is found in the heel [of the ancient figure 
 of Perseus]. It is of small 3rd magnitude. The 
 26th () is the following, and between them to the 
 eye there is about a cubit (2 20'). It is also of 
 small 3rd magnitude, and these two stars are the 
 nearest to the Pleiades ; there is no star between 
 them and the Pleiades." Al-Sufi's estimates of 
 star-magnitudes in this vicinity are remarkably 
 accurate. He rated Persei 4th magnitude (4'05 
 Harvard), e, 3rd magnitude (2'96 Harvard), and v^ 
 4th magnitude (3*93 Harvard), and Ptolemy agrees 
 with his estimates. We must, therefore, conclude 
 that o Persei has diminished in brightness from 
 the 3rd to the 4th magnitude since Al-Sufi's time. 
 This is one of the most remarkable cases I have 
 met with in Al-Sufi's work. The stars are close 
 together, and can be easily compared, so that no 
 mistake as to their relative brightness seems pos- 
 sible, o Persei has recently been found to be a 
 " spectroscopic binary," w~ith a period of 4*39 days. 
 Both components are bright bodies, and Vogel 
 finds a minimum mass of f of the sun's mass. 
 The orbital velocity is about 68 miles a second, 
 and the distance between the components about 
 4 millions of miles. 
 
 Let us now consider some stars which have 
 probably increased in brightness since Al-Sufi's 
 time. 
 
 o- Serpentis. This star is not mentioned by 
 
SECULAR VARIATION OF STARLIGHT 185 
 
 Ptolemy, but Al-Sufi rated it 6th magnitude. It 
 was estimated 5th magnitude by Argelander, 
 Heis, and Houzeau, and was measured 4/80 at 
 Harvard. Al-Sufi rated it two magnitudes fainter 
 than 66 Ophiuchi, but at present the two stars 
 are almost exactly equal (4'80 and 4'81 Harvard). 
 One has apparently increased and the other 
 diminished. 
 
 o> Tauri. This star, which lies a little north of 
 the Hyades, was rated 6th magnitude by Ptolemy 
 and Al-Sufi, 6-5 by Argelander and Heis, 5 by 
 Houzeau, and was measured 4*80 at Harvard. 
 Al-Sufi made it one magnitude less than p (44) 
 Tauri, but it is now considerably brighter than 
 p, and seems to have certainly increased in 
 brightness. 
 
 y Gerninorum. Rated 3rd magnitude by Ptolemy 
 and Al-Sufi, 2-3 by Argelander, Heis, and Houzeau, 
 and measured 1*93 at Harvard. Ptolemy and Al- 
 Sufi rated it equal to 8 Geniinorum, but y is now 
 1J magnitude brighter than 8. This is one of the 
 most remarkable cases I have met with in Al- 
 Sufi's work. S is now about 3J magnitude, and 
 may perhaps have faded a little ; but it seems 
 evident that y must have increased by about 
 one magnitude. Its spectrum is of the Sirian 
 type. 
 
 /8 Canis Majoris. 3m. Ptolemy and Al-Sufi, 3-2 
 Argelander and Heis, 2 Houzeau, 1*99 Harvard. 
 Al-Sufi made it equal to (3*10 Harvard), but it 
 
186 STUDIES IN ASTRONOMY 
 
 is now a magnitude brighter. The spectrum is of 
 the " Orion type " (B 1 A, Pickering). 
 
 109 Herculis. This star was not mentioned by 
 Ptolemy, but Al-Sufi rated it a large 6th mag- 
 nitude (6-5). It was estimated 4 by Argelander 
 and Heis, 4-5 by Houzeau, and was measured 3'92 
 at Harvard. 
 
 ft Eridani. Rated 4th magnitude by Ptolemy 
 and Al-Sufi, 3 by Argelander and Heis, 3-4 Hou- 
 zeau, 2'8 at Cordoba, and 2*92 at Harvard. Al-Sufi 
 rated it equal to X Eridani (4*34 Harvard), but it 
 is now about 1| magnitude brighter than X, and 
 it seems to have certainly increased in brightness 
 since Al-Sufi's time. 
 
 To the above we may, perhaps, add 17 Tauri 
 (Alcyone in the Pleiades), which is apparently not 
 mentioned by Ptolemy or Al-Sufi, and has pro- 
 bably increased considerably in brightness since 
 ancient times. Spectrum, B 5 A, Pickering. 
 
XVIII 
 
 The Herschels and the Nebulae 
 
 A LARGE number of those interesting and 
 mysterious objects, the nebulae, were dis- 
 covered by the illustrious astronomer, Sir 
 William Herschel, and his famous son, Sir John 
 Herschel, and some account of their labours in this 
 branch of astronomy may prove of interest to the 
 general reader. 
 
 In the year 1783 Sir William Herschel began a 
 series of observations or " sweeps " of the heavens, 
 as he termed them with a view to gain some 
 knowledge respecting what he called " The Interior 
 Construction of the Universe." In the course of 
 these " sweeps " he discovered a considerable 
 number of new nebulae and clusters of stars not 
 noticed by previous observers. The instrument 
 used in this research was a Newtonian reflector of 
 18*7 inches aperture and 20 feet focal length, the 
 power used in " sweeping " being 157 diameters, 
 and the field of view 15' 4", or about half the 
 apparent diameter of the moon. Herschel's first 
 
188 STUDIES IN ASTRONOMY 
 
 catalogue of " One Thousand New Nebulae and 
 Clusters of Stars " appeared in the Philosophical 
 Transactions of the Royal Society for the year 
 1786. In this catalogue he gives the approximate 
 position of each nebula and cluster, with an 
 abridged description of its general appearance as 
 seen in his telescope, this description being dictated 
 to and written down by an assistant while the great 
 astronomer had the object actually before his eye. 
 
 The catalogue is divided into eight classes. The 
 first class includes " bright nebulae ; " the second 
 class, " faint nebulae ; " the third class, " very faint 
 nebulae ; " the fourth class, " planetary nebulae ; " 
 the fifth class, " very large nebulae ; " the sixth 
 class, "very compressed and rich clusters of 
 stars ; " the seventh class, " pretty much com- 
 pressed clusters of large or small stars ; " and the 
 eighth class, " coarsely scattered clusters of stars." 
 In the Philosophical Transactions for 1789 he 
 gives a second catalogue of 1000 new nebulae and 
 clusters which form a continuation of the first 
 catalogue. These two catalogues include 215 
 objects of the first class, 768 of the second, 747 of 
 the third, 58 of the fourth, 44 of the fifth, 35 of 
 the sixth, 55 of the seventh, and 78 of the eighth. 
 An example or two taken from each class and 
 compared with modern observations of these 
 objects may prove of interest to the reader. 
 
 No. 6 of Class I. lies following the star 64 
 (Flamsteed) Virginis, and is thus described by Sir 
 
THE HERSCHELS AND THE NEBULA 189 
 
 William Herschel : " Very bright, pretty large, 
 gradually much brighter in the middle." 
 
 No. 47 of Class I. follows 1 Aquilse, and Herschel 
 says it is " bright, very large, of an irregular figure, 
 easily resolvable, stars visible." Webb says, 
 " Beautiful resolvable nebula." 
 
 No. 162 of Class II. lies a little preceding the star 
 34 Virginis, and is described by Herschel as " not 
 very faint, pretty large, irregularly round, a little 
 brighter towards following side." 
 
 Nos. 129 and 130 of Class III. lie north following 
 the star a- Bootis, and are described as " two, about 
 6 minutes distant, both extremely faint, very small, 
 round, verified with 240." 
 
 No. 26 of Class IV., or " planetary nebulse," lies 
 about 4 following the star y Eridani, and is 
 described by Herschel as " very bright, perfectly 
 round, or very little elliptical, planetary but ill- 
 defined disc./ Second observation, resolvable on 
 the borders, and is probably a very compressed 
 cluster of stars at an immense distance." Admiral 
 Smyth describes it in 1837 as " a splendid though 
 not very conspicuous object, of a greyish white 
 colour; it is somewhat like a large star out of 
 focus, with a planetary aspect." Lassell described 
 this curious nebula as " the most interesting and 
 extraordinary object of the kind " he had ever 
 seen, d' Arrest, like Herschel, found the edges 
 resolvable, and Huggins finds that the spectrum is 
 not gaseous. 
 
190 STUDIES IN ASTRONOMY 
 
 No. 43 of Class V. is described by Herschel as 
 " very brilliant, 15' long, running into very faint 
 nebulosity extending a great way." / Smyth says, 
 " A large white nebula ... a noble-sized oval . . . 
 with a brightish nucleus in its southern portion ; 
 the lateral edges are better denned than the ends." 
 Webb says, " Large, oval, bright, best denned at 
 edges, nucleus south, like Andromeda nebula on a 
 small scale. Spectrum continuous." It lies a little 
 south of the star 3 Canum Venaticorum. 
 
 Herschel describes his No. 10 of Class VI., which 
 lies a little north preceding a Scorpii (Antares), as 
 " a very compressed and considerably large cluster 
 of the smallest stars imaginable, all of a dusky 
 _red ; the next step to an easily resolvable nebula." 
 That all the stars should be of a red colour is 
 remarkable. Sir John Herschel says, with refer- 
 ence to this cluster, " Pretty large, oval, gradually 
 brighter in middle, resolvable." l 
 
 No. 30 of Class VI. was discovered by Miss 
 Caroline Herschel, Sir William Herschel' s famous 
 sister, and is described as " a beautiful cluster of 
 very compressed small stars, very rich." It lies a 
 little south of p Cassiopeise, and between that star 
 and cr Cassiopeise. Admiral Smyth describes it 
 (1835) as "a very glorious assemblage, both in 
 extent and richness, having spangly rays of stars 
 which give it a remote resemblance to a crab, the 
 claws reaching the confines of the space in view, 
 1 " Cape Observations," p. 111. 
 
THE HERSCHELS AND THE NEBULA 191 
 
 under an eyepiece magnifying 185 times . . . The 
 crab itself is but a mere condensed patch in a vast 
 region of inexpressible splendour, spreading over 
 many fields." Photographs taken by Dr. Roberts 
 in 1892 and 1898 show " lines, wreaths, and curves 
 of stars," which he thinks " give evidence of their 
 formation, or arrangement, by vortical movements 
 similar in character to those shown in the spiral 
 nebulae." The individual stars are distinctly seen 
 on the negative, and could be easily counted. 
 
 No. 2 of Class VII. lies north following the star 
 8 Monocerotis, and is described by Herschel as " a 
 beautiful cluster of scattered stars, the first large, 
 the second arranged in winding lines. Contains 
 the 12th Monoceros." / Webb says, " Beautiful, 
 visible to the naked eye ; including 12, 6th magni- 
 tude, yellow ; and many 7 and 8 magnitude stars. 
 The smallest, 14th magnitude, run in rays. Small 
 pair near centre." Near this cluster Professor 
 Lewis Swift sees " a wonderful nebulous ring." 
 
 No. 16 of Class VIII. follows < Cygni, and is 
 described by Herschel as " a cluster of not very 
 compressed stars, closest in the middle. It may be 
 called (if the expression be allowed) a forming 
 cluster, or one that seems to be gathering." 
 
 A few of Herschel' s nebulae may have been 
 telescopic comets. With reference to No. 7 of 
 Class I., which lay a little following the star 49 
 Leonis, and which is described in the catalogue as 
 " very bright, large, and round," Herschel says, in 
 
192 STUDIES IN ASTRONOMY 
 
 the notes to his first catalogue, " This remarkable 
 appearance being 110 longer in the place it has been 
 observed, we must look upon it as a very consider- 
 able telescopic comet. It was visible in the finder, 
 and resembled one of the bright nebulae of the 
 Connoissance des Temps so much that I took it for 
 one of them till I came to settle its place ; but this 
 not being done till a month or two after the obser- 
 vation, the opportunity of pursuing and investi- 
 gating its track was lost ; " and he says that No. 6 
 of Class II. was also probably " a telescopic comet, 
 as I have not been able to find it again, notwith- 
 standing the assistance of a drawing which repre- 
 sents the telescopic stars in its neighbourhood." 
 
 In the preface to his second catalogue, Herschel 
 refers to the nebulae " as being no less than whole 
 sidereal systems." But this conclusion must now 
 be modified to a considerable extent, as spectro- 
 scopic observations show that many of them are 
 notliing but masses of glowing gas, and may lie 
 well within the limits of our own sidereal system. 
 With reference to the " planetary nebulae," Her- 
 schel says they " may be looked upon as very 
 aged, and drawing on towards a period of age or 
 dissolution ; " but as several of the " new " or 
 " temporary stars " discovered in recent years 
 have apparently changed into *' planetary nebulas," 
 it would seem probable that these curious objects 
 represent rather an early stage, and not a late one, 
 of nebular formation. Herschel seems to have 
 

 THE HERSCHELS AND THE NEBULAE 193 
 
 had an idea that there was some relation between 
 planetary nebulae and the phenomenon of a new 
 star, for in a previous paper l he says, " If it were 
 not, perhaps, too hazardous to pursue a former 
 surmise of a renewal in what I frequently called 
 the laboratories of the universe, the stars forming 
 these extraordinary nebulae, by some decay or 
 waste of nature being no longer fit for their 
 former purposes, and having their projectile 
 forces, if any such they had, retarded in each 
 other's atmosphere, may rush at last together, 
 and, either in succession or by one general tre- 
 mendous shock, unite into a new body. Perhaps 
 the extraordinary and sudden blaze of a new star 
 in Cassiopeia's Chair in 1572 might possibly be of 
 such a nature." But the reverse of this seems 
 now more probable, a planetary nebula being 
 formed from a temporary star, not a temporary 
 star from a planetary nebula. 
 
 In the Philosophical Transactions for 1791 there 
 is a paper by Sir William Herschel on " Nebulous 
 Stars," in which he expresses his opinion that the 
 nebulosity surrounding these curious objects is 
 not composed of stars. He says, "View, for 
 instance, the 19th cluster of my 6th class, and 
 afterwards cast your eye on this cloudy star. . . . 
 Our judgment, I may venture to say, will be that 
 the nebulosity about this star is not of a starry 
 nature" 2 Among the nebulous stars described 
 1 Phil Trans., 1785, pp. 265, 266. 2 The italics are Herschel's. 
 
STUDIES IN ASTRONOMY 
 
 by Herschel in this paper, the following may be 
 mentioned : 
 
 " January 17, 1787. A star with a pretty strong 
 milky nebulosity, equally dispersed all round; 
 the star is about the 9th magnitude. A memo- 
 randum to the observation says that, having just 
 begun, I suspected the glass to be covered with 
 damp, or the eye out of order ; but yet a star of 
 the 10th or llth magnitude, just north of it, was 
 free from the same appearance. A second obser- 
 vation calls it one of the most remarkable phse- 
 nomena I have ever seen, and like my northern 
 planetary nebula in its growing state. The con- 
 nection between the star and the milky nebulosity 
 is without all doubt." Sir John Herschel describes 
 it as an 8th magnitude star, " exactly in the centre 
 of an exactly round, bright atmosphere 25" in 
 diameter." Webb found a "bluish nebulosity, 
 quite like a telescopic comet," and says, " the Earl 
 of Rosse saw a marvellous object a star sur- 
 rounded by a small circular nebula, in which, 
 close to the star, is a little black spot. This 
 nebula is encompassed, first by a dark then by a 
 luminous ring, very bright, and always flickering ; 
 perhaps a spiral. ... A mass of luminous gas." 
 This wonderful object lies south following the 
 star 63 Geminorum. 
 
 " March 5, 1790. A pretty considerable star of 
 the 9th and 10th magnitude, visibly affected with 
 a very faint nebulosity of little extent all around. 
 
THE HERSCHELS AND THE NEBULAE 195 
 
 A power of 300 showed the nebulosity of greater 
 extent. The connection is not to be doubted." 
 This object lies a little south preceding the star 
 22 Monocerotis. 
 
 "November 13, 1790. A most singular phse- 
 iiomenon! A star of about the 8th magnitude, 
 with a faint luminous atmosphere, of a circular 
 form, and about 3' in diameter. The star is 
 perfectly in the centre, and the atmosphere is so 
 diluted, faint, and equal throughout, that there 
 can be no surmise of its consisting of stars ; nor 
 can there be doubt of the evident connection 
 between the atmosphere and the star. Another 
 star, not much less in brightness, and in the same 
 field with the above, was perfectly free from any 
 such appearance." This object will be found about 
 2 north of the star \f/ Tauri. 
 
 With reference to the constitution of these 
 curious objects, Herschel rejects the idea that the 
 luminous atmosphere is composed of small stars, 
 and says, "We therefore either have a central 
 body which is not a star, or have a star which is 
 involved in a shining fluid, of a nature totally 
 unknown to us," and he adds, " But what a field 
 of novelty is here opened to our conceptions ! A 
 shining fluid, of a brightness sufficient to reach us 
 from the remote regions of the 8th, 9th, 10th, llth, 
 or 12th magnitude, and of an extent so considerable 
 as to take up 3, 4, 5, or 6 minutes in diameter! 
 Can we compare it to the coruscations of the 
 
196 STUDIES IN ASTRONOMY 
 
 electric fluid in the aurora borealis? or to the 
 more significant cone of the zodiacal light as we 
 see it in spring or autumn? The latter, notwith- 
 standing I have observed it to reach at least 90 
 from the sun, is yet of so little extent and bright- 
 ness as possibly not to be perceived even by the 
 inhabitants of Saturn or the Georgian planet, and 
 must be utterly invisible at the remoteness of the 
 nearest fixed star." 
 
 Herschel suggests that large nebulae, like that 
 in Orion, may possibly be composed of this 
 luminous matter an hypothesis which accounts, 
 he says, " much better for it than clustering stars 
 at a distance." This was a happy foresight of 
 Herschel's into the existence of gaseous nebulae, 
 the truth of which has been fully proved in recent 
 years by means of the spectroscope. The great 
 nebula in Orion is now known to consist of 
 glowing gas, as Herschel surmised. He further 
 expresses his opinion that "planetary nebulae" 
 may also be composed of luminous gaseous matter 
 and the spectroscope has confirmed the truth of 
 this hypothesis also. 
 
 In the Philosophical Transactions for 1802, 
 Herschel gives a further catalogue of 500 " new 
 nebulae, nebulous stars, planetary nebulae, and 
 clusters of stars." The numbers in each class are 
 continued from the former catalogues, and bring 
 the totals up to : First class, 228 ; second class, 
 907 ; third class, 978 ; fourth class, 78 ; fifth class, 
 
THE HERSCHELS AND THE NEBULA 197 
 
 52 ; sixth class, 42 ; seventh class, 67 ; and eighth, 
 88. In the preface to this catalogue he gives some 
 interesting remarks on the various kinds of 
 nebulae. With reference to the globular clusters 
 of stars he says there must clearly be a centre of 
 attraction, either empty or occupied by a massive 
 body, round which all the stars resolve. With 
 reference to the nebulae, properly so called, he con- 
 siders that some, at least, may be clusters of stars 
 rendered nebulous in appearance from the effects 
 of immense distance, and that in some cases 
 their light may possibly take nearly two millions 
 of years to reach us ! But this conclusion 
 seems now improbable. Take a globular cluster 
 like w Centauri, and suppose that it contains 
 10,000 stars, each of the same size and intrinsic 
 brightness of our sun, and that it is placed at 
 such a distance that its light would take even 
 one million of years to reach us. Its parallax 
 would then be about 307^00 ^ a second of arc, 
 which would imply a distance of about 63,000 
 million times the sun's distance from the earth! 
 Placed at this enormous distance the sun would, 
 I find, be reduced to a star of the 27th magnitude ! 
 and a cluster of 10,000 suns would shine as a star 
 of the 17th magnitude, or a faint point of light 
 barely visible in the great Lick telescope! But 
 many of these globular clusters are comparatively 
 bright objects, even in telescopes of moderate 
 power, and a few, like w Centauri, are easily visible 
 
198 STUDIES IN ASTRONOMY 
 
 to the naked eye. Herschel admits, however, that 
 "milky nebulosity," such as that in the great 
 nebula in Orion, is probably not due to clusters of 
 stars. The spectroscope has now proved this 
 conclusion to be correct. 
 
 In a paper in the Philosophical Transactions 
 for 1811, relating to the " Construction of the 
 Heavens," Herschel further considers the nebulae. 
 He says, " An equal scattering of the stars may be 
 admitted in certain calculations; but when we 
 examine the Milky Way, or the closely compressed 
 clusters of stars, of which my catalogues have 
 recorded so many instances, the supposed equality 
 of scattering must be given up. We may also 
 have surmised nebulae to be no other than clusters 
 of stars disguised by their very great distance, 
 but a longer experience and better acquaintance 
 with the nature of nebulae will not allow a 
 general admission of such a principle, although 
 undoubtedly a cluster of stars may assume a 
 nebulous appearance when it is too remote for us 
 to discern the stars of which it is composed." * 
 He gives in a table a list of 52 spots in the heavens, 
 in which there is " diffused milky nebulosity " 
 over a considerable area. These regions should be 
 examined and photographed with our large 
 modern telescopes. 2 
 
 1 Phil. Trans., 1811, p. 270. 
 
 2 This has recently been done by Dr. Isaac Roberts, and in most 
 of the cases he finds no nebulosity. 
 
THE HERSCHELS AND THE NEBUL.E 199 
 
 Herschel says, with reference to the great 
 nebulse in Orion, that " we can hardly have a doubt 
 of it being the nearest of all the nebulse in the 
 heavens." And of "double nebulse," or nebulsB 
 having two centres of condensation, he suggests 
 that in the course of ages they may divide and 
 form two separate and distinct nebulse close 
 together. Dr. See has recently suggested that 
 this may have been the origin of binary or 
 revolving double stars. 
 
 With reference to what Herschel calls " cometic 
 nebulse," which show " a gradual and strong 
 increase of brightness towards the centre of a 
 nebulous object of a round figure," he says. Their 
 great resemblance to telescopic comets, however, 
 is very apt to suggest the idea that possibly 
 such small comets as often visit our neighbour- 
 hood may be composed of nebulous matter, or 
 may, in fact, be such highly condensed nebulse." 
 
 In the Philosophical Transactions for 1814, 
 Herschel continues his observations "relating to 
 the sidereal part of the heavens and its connec- 
 tion with the nebulous part." He considers the 
 apparent connection in many cases between stars 
 and nebulse, and shows that this connection is 
 probably real and not merely apparent. From 
 this he concludes that the stars were originally 
 formed by the condensation of nebulous matter. 
 This, of course, agrees with the nebular theory of 
 Laplace. 
 
200 STUDIES IN ASTRONOMY 
 
 In the Philosophical Transactions for 1818 his 
 last paper on the subject Herschel considers the 
 probable distance of clusters of stars. Taking the 
 power of his 20-feet Newtonian telescope to pene- 
 trate into space as 61*18 times that of the naked eye, 
 which he assumes can see stars of the 12th order 
 that is 12 times the distance of stars like Capella 
 and Vega he concludes that several of the com- 
 pressed clusters of stars lie at a distance of 734 
 times the distance of Capella or Vega. Some 
 clusters he finds to be placed at the " 900th order 
 of distance," while in one case he concludes that 
 the cluster lies at 950 times the distance of Capella 
 or Vega. I find that if Capella or Vega were 
 placed at 734 times their present distance what- 
 ever that may be they would be reduced in 
 brightness to stars of about 14 magnitude, and 
 this would be about the faintest star visible in 
 Herschel' s telescope. Herschel' s conclusions, of 
 course, depend on the assumption that the stars 
 of the clusters referred to are of the same size 
 as the brighter stars, and that their faintiiess is 
 due merely to their great distance from the earth. 
 This assumption, however, cannot be considered 
 as certainly true, for their faintiiess may possibly 
 be due to small size as well as to great dis- 
 tance. Probably both causes combine to make 
 them faint. 
 
 In the year 1825 Sir John Herschel, the famous 
 son of Sir William Herschel, commenced a series 
 
THE HERSCHELS AND THE NEBULAE 201 
 
 of observations on the nebulae in the northern 
 hemisphere with a 20-feet reflector. The result 
 of these observations are given in a catalogue 
 published in the Philosophical Transactions for 
 1833. This catalogue contains 2306 nebulae and 
 clusters, of which 1781 were observed by Sir 
 William Herschel and others, the remaining 525 
 being new. Among the latter, Sir John Herschel 
 says there is "only one very conspicuous and 
 large nebula, and only a very few entitled to 
 rank in his (father's) first class, or among the 
 * bright nebulae.' By far the greater proportion 
 of them are objects of the last degree of faintness, 
 only to be seen with much attention and in a 
 good state of the atmosphere and instrument." 
 He gives a series of plates with some beautiful 
 drawings of the various kinds of nebulae. Some 
 of the forms depicted are very curious. One of 
 them is spindle-shaped with a vacuity in the 
 middle. It looks like a circular ring seen very 
 obliquely, and reminds one of Saturn's ring, if we 
 imagine the planet removed. This remarkable 
 object is No. 19 of Sir William Herschel's Fifth 
 Class, and is described by Sir John Herschel as 
 " an extraordinary ray 3' or 4' long ; 40" broad ; 
 very large ; very much elongated ; pretty faint ; 
 has a chink or dark division in the middle and 
 two stars. Position with meridian 21'2. A 
 wonderful object." In the drawing the stars 
 mentioned are placed at each end of the central 
 
202 STUDIES IN ASTRONOMY 
 
 opening, and certainly look as if they were con- 
 nected with the nebula. A photograph, taken 
 by Dr. Roberts in December, 1891, agrees well 
 with the above description. He says, "The de- 
 scriptions given by the observers just cited will 
 also apply to the photograph, and the suggestion 
 by Sir John Herschel that the nebula is a thin, 
 flat ring of enormous dimensions, seen very 
 obliquely, receives strong confirmation." 
 
 With reference to the well-known "dumb-bell 
 nebula," of which he gives a drawing, Sir John 
 Herschel says, " The outline is filled up elliptically 
 with a faint nebulosity, as in figure, which, I 
 think, leaves ansse, as if inclined to form a ring." 
 This view of its construction is shown to be 
 correct by a photograph taken by Dr. Roberts in 
 the year 1888, which shows that the nebula is 
 really a globe surrounded by a darker ring. Dr. 
 Roberts says, " The nebula is probably a globular 
 mass of nebulous matter which is undergoing the 
 process of condensation into stars, and the faint 
 protrusions of nebulosity on the south folloiving 
 and north preceding ends are the projections of 
 a broad ring of nebulosity which surrounds the 
 globular mass. This ring, not being sufficiently 
 dense to obscure the light of the central region 
 of the globular mass is dense enough to obscure 
 those parts of it that are hidden by the increased 
 thickness of the nebulosity, thus producing the 
 ' dumb-bell ' appearance." 
 
THE HERSCHELS AND THE NEBULAE 203 
 
 Another curious object is a small round nebula 
 surrounding a triple star, the stars forming an 
 equilateral triangle, of which the sides are about 
 4" in length, and the stars of the llth, 12th, and 
 14th magnitude. " The nebula surrounds the star 
 like an atmosphere." This is No. 261 of Sir 
 William Herschel's First Class, and lies a little 
 south following the star < Aurigse. Webb saw 
 four stars with a 91-inch speculum in 1873 and 
 1876; D' Arrest five stars; Lord Rosse saw six 
 stars, and found the nebula oval, with branches. 
 
 Another interesting object is one discovered by 
 Sir John Herschel, of which he gives a drawing, 
 and describes it as "a most remarkable pheno- 
 menon. A very large space, 20 or 30 minutes 
 broad in polar distance, and 1 minute or 2 minutes 
 in right ascension full of nebula and stars mixed. 
 The nebula is decidedly attached to the stars, and 
 as decidedly not stellar. It forms irregular lace- 
 work marked out by stars, but some parts are 
 decidedly nebulous, wherein no stars can be seen. 
 A figure represents general character, but not 
 the minute details of this object, which would 
 be extremely difficult to give with any degree 
 of fidelity." It lies about 3| preceding the star 
 Cygni. 
 
 In the year 1834 Sir John Herschel went to 
 the Cape of Good Hope to observe the southern 
 heavens, and in an interesting and classical work, 
 known as the "Cape Observations," he gives a 
 
204 STUDIES IN ASTRON 7 OMY 
 
 catalogue of 1707 nebulae, most of which are south 
 of the equator. Of these, 89 are identical with 
 objects in his first catalogue, mentioned above, 
 and 135 are included in Sir William Herschel's 
 catalogues. The instrument used by Sir John 
 Herschel in these observations was a reflector 
 of the same size as that used by Sir William 
 Herschel. 1 From this southern catalogue of Sir 
 John Herschel's I select the following interesting 
 objects, which, so far as I know, have not been 
 described in popular books 011 astronomy. 
 
 h. 2345. Nebula about 7 south of the star 
 (3 Ceti. It is described by Sir John Herschel as 
 " very, very bright ; exceedingly large ; very 
 much elongated ; 30 minutes long, 3 or 4 minutes 
 broad; has several stars in it; gradually much 
 brighter in the middle to a centre elongated like 
 the nebula itself. / The nebula is somewhat streaky 
 and knotty in its constitution, and may perhaps be 
 resolvable ; " and in a second observation he says, 
 " A superb object. The light is somewhat streaky, 
 but I see no stars in it, but four large and one 
 very small one, and these seem not to belong to 
 it, there being many near." This nebula is No. 1 
 of Sir William Herschel's Fifth Class, and was 
 discovered by Miss Caroline Herschel in 1783. 
 
 h. 2878. A nebula situated near the northern 
 edge of the Nubecula Major, or " greater Magel- 
 lanic Cloud." Sir John Herschel describes it as 
 1 See next chapter. 
 
THE HERSCHELS AND THE NEBULA 205 
 
 "very bright; very large; oval; very gradually 
 pretty much brighter in the middle ; a beautiful 
 nebula ; it has very much resemblance to the 
 Ntibecula Major itself as seen with the naked eye, 
 but is far brighter and more impressive in its 
 general aspect, as if the nubecula were at least 
 doubled in intensity. . . . And who can say 
 whether in this object, magnified and analyzed 
 by telescopes infinitely superior to what we now 
 possess, there may not exist all the complexity 
 of detail that the nubecula itself presents to our 
 examination .' ' 
 
 h. 3315. A little north preceding the great 
 nebula in Argo (which surrounds the variable 
 star rj Argus). Sir John Herschel describes it as 
 " a glorious cluster of immense magnitude, being 
 at least two fields in extent every way. The 
 stars are 8, 9, 10, and 11 magnitude, but chiefly 
 10 magnitude, of which there must be at least 
 200. ' It is the most brilliant object of the kind 
 I have ever seen." In another observation he 
 says, "A very large round, loosely scattered 
 cluster of stars 8 ... 12 m. stars, which fills two 
 or three fields. A fine bright object." And in 
 a third observation he says, " A superb cluster, 
 which has several elegant double stars, and many 
 orange-coloured ones." 
 
 As in his first catalogue, he gives beautifully 
 executed drawings of some of the nebulae he 
 observed. Some of the forms shown are very 
 
206 STUDIES IN ASTRONOMY 
 
 curious and interesting, and several are very 
 similar in shape to objects observed in the northern 
 hemisphere. He also gives elaborate drawings 
 of the great nebula in Orion, the nebula round 
 i) Argus, and the various objects contained in the 
 Nubecula Major. His detailed descriptions of 
 these wonderful objects are very valuable for 
 comparison with photographs which have been 
 taken or will be taken in the future. He also 
 gives a catalogue of the objects visible in the 
 Nubecula Major and Nubecula Minor, so that any 
 future change in the brightness or position of any 
 of these objects can be easily detected. 
 
 Many new nebulae have of course been dis- 
 covered since the days of the Herschels, but these 
 are, for the most part, exceedingly faint objects, 
 and we may say that the heavens were thoroughly 
 explored by the Herschels so far as the power 
 of their telescopes would permit. They were 
 excellent observers, and no greater astronomers 
 ever lived. 
 

 XIX 
 
 A Chapter in the History of Astronomy 
 
 IN the year 1825 the famous astronomer Sir 
 John Herschel commenced a re-examination 
 of the nebulae and clusters of stars discovered 
 by his illustrious father, Sir William Herschel. 
 This work was carried on for about eight years, 
 and the results were presented to the Royal A 
 Society, and were published in their Philo- 
 sophical Transactions in the form of a catalogue. 
 This work contained observations of 2306 nebulae 
 and clusters of stars, of which 525 were new. In 
 addition to the nebulae and clusters, many double 
 stars were also observed, and the observations 
 of these were published in the Transactions of 
 the Royal Astronomical Society. All these ob- 
 servations were made with the aid of a reflect- 
 ing telescope of 18^ inches clear aperture and 
 20 feet focal length, and the practice thus acquired, 
 combined with the interest of the subject, induced 
 Sir John Herschel " to attempt the completion of 
 a survey of the whole surface of the heavens, and 
 
208 STUDIES IN ASTRONOMY 
 
 for this purpose to transport into the other hemi- 
 sphere the same instrument which had been 
 employed in this, so as to give a unity to the 
 results of both portions of the survey, and to 
 render them comparable with each other." 
 
 In pursuance of this scheme, the indefatigable 
 astronomer packed up his large reflector and also 
 an equatorically mounted achromatic telescope of 
 5 inches aperture and 7 feet focal length, made 
 by Tully, with other apparatus, and sailed with 
 his family from Portsmouth on board the East 
 India Company's ship Mount Steivart Elphinstone, 
 on November 13, 1833. After a pleasant voyage 
 he landed safely with his instruments at Table 
 Bay, Cape of Good Hope, on January 16, 1834. 
 He then looked out for a residence in a suitable 
 locality, and soon found one at a place called 
 Feldhuysen, or Feldhausen, about 6 miles from 
 Cape Town, near the base of Table Mountain. 
 In this favourable position, sheltered on one side 
 by Table Mountain, and on the other by oak and 
 fir trees, a building was erected for the equatorial 
 instrument, and on May 2, 1834, "a series of 
 micrometrical measures of southern double stars 
 was commenced by the measurement of a Centauri, 
 the chief among them." At a short distance from 
 this building the large reflector was erected in 
 the open air. The exact position of this astro- 
 nomical station was in south latitude 33 58' 56"'55, 
 longitude 22 h 46 m 9 s - 11 from Greenwich. Its height 
 
A CHAPTER IN ASTRONOMY 209 
 
 above the mean sea-level of Table Bay was about 
 142 feet. / 
 
 The reflector was provided with three mirrors, 
 one made by Sir William Herschel, and used by 
 him in his 20-f eet " sweeps " of the northern 
 heavens, and the other two constructed by Sir 
 John Herschel himself. With this instrument 
 observations were made "in search of new 
 objects" in "sweeps" of three degrees in breadth 
 in polar distance, on clear moonless nights. The 
 months from May to October, the winter of the 
 southern hemisphere, and especially June and 
 July, proved most suitable for observation, and 
 nights after heavy rain were found to be the best 
 for the purpose. 
 
 At this favourably situated station the dis- 
 tinguished astronomer carried on his observations 
 during the years 1834 to 1838, and a short account 
 of the results he obtained may prove of interest 
 to the reader. 
 
 The first portion of these results is contained 
 in a splendid work which was published in 1847, 
 at the expense of the Duke of Northumberland, 
 and consists of a catalogue of nebulae and clusters 
 of stars observed in " sweeps " with the 20-inch 
 reflector. The positions of these interesting 
 objects are t?arefully noted, and a short descrip- 
 tion of each is given. In addition to the cata- 
 logue, which contains 1707 objects, separate 
 drawings were made of the most remarkable and 
 
 p 
 
210 STUDIES IN ASTRONOMY 
 
 interesting nebulae and clusters. These include 
 drawings of the great nebula in Orion ; the 
 " trifid " nebula ; the looped nebula, 30 Doradus ; 
 the nebula surrounding the variable star 77 Argus ; 
 the clusters w Centaur i and 47 Toncani ; that 
 surrounding K Crucis, and other remarkable and 
 interesting objects. These drawings are beauti- 
 fully executed in black on a white ground, and 
 exhibit some of the most striking and extra- 
 ordinary forms visible in the southern heavens. 
 An elaborate drawing is also given of the larger 
 " Magellanic cloud," showing the brighter clusters 
 and nebulae, and the stars down to the 10th 
 magnitude included in this wonderful object. 
 This drawing will be of great use for comparison 
 with future photographs of this marvellous cluster, 
 which contains all forms of sidereal objects from 
 stars to irresolvable nebulae. In addition to the 
 drawing, a catalogue is given of the objects in 
 the Nubecula Major and Minor, as the "Magel- 
 lanic clouds" are termed by astronomers. The 
 beautiful drawing of the great nebula in Orion, 
 given by Sir John Herschel, agrees fairly well in 
 its principal details with modern photographs; 
 but owing to the long exposure required to bring 
 out the fainter portions of the nebula, the brighter 
 portions are, in the photographs, always over- 
 exposed, and render a comparison with the draw- 
 ing a matter of some difficulty. 
 
 Following the catalogue of nebulae is a catalogue 
 
A CHAPTER IN ASTRONOMY 211 
 
 of double stars observed with the 20-feet re- 
 flector. This list includes 2102 objects, and is 
 followed by a list of inicrornetrical measures of 
 417 doubles made with the 7-feet equatorial. To 
 these measures are added a series of notes de- 
 scribing the appearance and character of the 
 various objects measured. Some of these measures 
 have been found very useful in calculating the 
 orbits of some of the southern binary or revolving 
 double stars, the angular motion of some of these 
 interesting objects having been considerable since 
 the date of Sir John Herschel's observations. 
 These measures of double stars are followed by 
 notes on the most remarkable of these objects. 
 They include very interesting observations of the 
 famous binary star y Virginis, and an investiga- 
 tion of the orbit of this remarkable stellar system. 
 The period of revolution found by Sir John 
 Herschel (182 years) agrees well with the best 
 recent determinations. 
 
 Another work undertaken by Sir John Herschel 
 was the determination of the relative brilliancy 
 of the brighter stars in the southern hemisphere. 
 These determinations were chiefly made by obser- 
 vation with the naked eye, without the aid of 
 any instrument. A form of photometer was tried, 
 but the results obtained with it did not prove 
 very satisfactory. The naked eye observations 
 were made by the method of sequences a method 
 which consists in arranging the stars in lists in 
 
STUDIES IN ASTRONOMY 
 
 the order of brightness, combining these sequences, 
 and then reducing the observations to a uniform 
 scale. Sir John Herschel says, " I am disposed 
 to rely mainly for the formation of a real scale 
 of magnitudes on comparisons made by the 
 unassisted judgment of the naked eye," and 
 although photometers have been in recent years 
 most successfully used for this purpose, still, for 
 small differences of brightness between neigh- 
 bouring stars, the eye alone could, with experi- 
 enced observers, probably hold its own against 
 any photometer. The work of Sir John Herschel, 
 like the whole of his work at the Cape, was 
 carried out in continuation of the work done by 
 his illustrious father, Sir William Herschel, in 
 the northern hemisphere. The elder Herschel' s 
 results will be found in the Philosophical Trans- 
 actions of the Royal Society for the years 1796, 
 1797, and 1799, and form a valuable record in 
 connection with suggested variability in any of 
 the brighter stars. Sir John Herschel gives his 
 sequences in detail, and the reduced magnitudes 
 of the stars observed. In his reduced list he 
 gives the following as the twelve brightest stars 
 in order of magnitude : Sirius, Canopus, a Ceii- 
 tauri, Arcturus, Capella, a Lyrse (Vega), Rigel 
 Procyon, a Eridani, a Orionis, Aldebaraii, and 
 (3 Centauri. 
 
 While at the Cape, Sir John Herschel made a 
 careful examination of the " general appearance 
 
A CHAPTER IN ASTRONOMY 213 
 
 and telescopic constitution of the Milky Way in 
 the southern hemisphere," and his resiilts, which 
 are very interesting, form a valuable contribution 
 to our knowledge of this wonderful zone. From 
 the telescopic aspect of the Galaxy in this region, 
 he concludes that " it consists of portions differing 
 exceedingly in distance, but brought by the effect 
 of projection into the same, or nearly the same, 
 visual line; in particular, that at the anterior 
 edge of what we have called the main stream, 
 we see foreshortened a vast and illimitable area 
 scattered over with discontinuous masses, and 
 aggregates of stars in the manner of the cumuli 
 of a mackerel sky, rather than a stratum of 
 regular thickness and homogenous formation ; 
 and that in the enclosed spaces insulated from 
 the rest of the heavens by the preceding and 
 following streams, and the ' bridges ' above 
 spoken of as connecting them (as, for instance, 
 in that which includes \ Scorpii), we are, in fact, 
 looking out into space through vast chimney- 
 form or tubular vacancies, whose terminations 
 are rendered nebulous by the effect of their ex- 
 ceeding distance, and, at the same tune, are 
 brought by that of perspection to constitute the 
 interior borders of apparent vacuities." Recent 
 observations and photographs, however, now 
 render this conclusion more than doubtful, and 
 the weight of evidence seems in favour of the 
 hypothesis, that the Milky Way is in reality what 
 
STUDIES IN ASTRONOMY 
 
 it seems to be, namely, a system of a roughly 
 circular section, the most distant parts being, 
 comparatively, not much farther from us than 
 the nearest, and that the differences in luminosity 
 are due rather to differences in aggregation, and 
 in the absolute sizes of the component stars, than 
 to difference of distance. 
 
 While Sir John Herschel was at the Cape the 
 famous comet of Halley returned to the sun's 
 vicinity. It was carefully observed by the great 
 astronomer, and he gives full details of his obser- 
 vations in the work above referred to, and some 
 beautiful drawings of the appearance presented 
 by the nucleus or head. He also made observa- 
 tions of the satellites of Saturn, and the solar 
 spots which are interesting for comparison with 
 modern observations. 
 
 After Sir John Herschel's departure from the 
 Cape an obelisk was erected by some friends to 
 mark the site occupied by the large reflector. 
 This column bears the following inscription : 
 "Here stood, from 1834 to 1838, the reflecting 
 telescope of Sir John Herschel, Baronet; who, 
 during a residence of four years in this colony, 
 contributed as largely, by his benevolent exer- 
 tions, to the cause of education and humanity 
 as by his eminent talents to the discovery of 
 scientific truth." 
 
XX 
 
 Messier's Nebulae 
 
 IN the year 1771, Messier, the famous discoverer 
 of comets, published a catalogue of 68 
 nebulae in the "Memoirs of the French 
 Academy of Science." In the years 1781, 1782 
 this list was republished in the Connaissance des 
 Temps, and 35 other nebulae were added, thus 
 bringing the total up to 103 nebula3. Most of 
 these nebulae have been carefully re-observed 
 since Messier's time, and some of them have 
 proved to be most wonderful and interesting 
 objects. As these observations and descriptions 
 are scattered through various publications, some 
 of them almost inaccessible to the general reader, 
 I have compiled the following list, with short 
 descriptions of the appearance and character of 
 each nebula. The positions are given for 1900-0. 
 There are a few of Messier's nebulae, of which I 
 have been unable to find any account. These 
 are Nos. 40, 45, 48, and 102. 
 
 Messier 1 : R.A. 5 h 28 m '5, N. 21 57'. This is the 
 famous "Crab nebula" in Taurus. It lies about 
 
216 STUDIES IN ASTRONOMY 
 
 1 north preceding the star Tauri. It was first 
 seen by Bevis in 1731. It was again seen by 
 Messier in 1758, while observing the comet of that 
 year, and its re-discovery induced him to form 
 his catalogue of nebulae, to help observers in dis- 
 tinguishing these objects from comets. Sir John 
 Herschel thought it was a cluster of stars at a 
 distance " of about the 980th order," that is 980 
 times the distance of Capella or Vega. Lord 
 Rosse's great telescope was supposed to have 
 resolved it into stars, but photographs, taken by 
 Dr. Isaac Roberts in 1892 and 1896, show a very 
 nebulous appearance, and but little of the " crab- 
 like " form depicted in Lord Rosse's drawing. 
 
 M. 2 : 21 h 28 m '3, S. 1 15'. In Aquarius, about 
 5 north of the star ft Aquarii. A globular cluster 
 of 5' or 6' in diameter. Sir John Herschel com- 
 pared it to a heap of white sand ; and Admiral 
 Smyth says, " This magnificent ball of stars 
 condenses to the centre, and presents so fine a 
 spherical form that imagination cannot but 
 picture the inconceivable brilliance of their visible 
 heavens to its animated myriads." But that each 
 of these points of light should have planets 
 revolving round it seems very doubtful. Sir 
 William Herschel, with his 40-feet telescope, could 
 see the individual stars even in the centre of the 
 cluster. A photograph by Dr. Roberts, taken in 
 1891, shows the centre of the cluster involved 
 in dense nebulosity, and he thinks that it was 
 
MESSIER'S NEBULAE 217 
 
 probably evolved from a spiral nebula. The stars 
 composing the cluster are very faint, probably 
 not brighter than the 15th magnitude. Seen as 
 
 
 
 a star, it was measured 7*69 magnitude at the 
 Harvard Observatory, U.S.A. Assuming these 
 magnitudes as correct, I find that the cluster 
 contains about 800 stars. 
 
 M. 3: 13 h 37 m -6, N. 28 53'. A fine globular 
 cluster in the constellation Canes Venatici, about 
 12 north preceding Arcturus./ Messier, who dis- 
 covered it in 1764, described it as "a nebula 
 without a star, brilliant and round." Sir William 
 Herschel, with his 20-feet reflector, found it "a 
 beautiful cluster of stars 5' or 6" in diameter." 
 Sir John Herschel describes it as very large and 
 bright, with stars about llth to 15th magnitude. 
 Smyth called it " a noble object," and thought it 
 contained " not less than 1000 small stars." Lord 
 Rosse saw it as a cluster blazing in the centre 
 with rays of stars running out from it on all 
 sides. A photograph by Dr. Roberts, in May, 
 1891, "confirms the general descriptions" of 
 previous observers. No less than 132 variable 
 stars have been detected among the outliers of 
 this cluster ! 
 
 M. 4 : 16 h 17 m -5, S. 26 17'. A large but rather 
 faint object, about 1J preceding Antares. Dis- 
 covered by Messier in 1763. In 1783 Sir William 
 Herschel resolved it into stars. Smyth describes 
 it as " a compressed mass of small stars . . . with 
 
218 STUDIES IN ASTRONOMY 
 
 outliers, and a few small stellar companions in 
 the field." 
 
 M. 5 : 15 h 13 m -5, N. 2 27'. A globular cluster of 
 stars of lltli to 15th magnitudes, lying closely 
 north preceding the 5th magnitude star 5 Ser- 
 pentis. Messier was " certain that it contained 
 no star ! " Sir William Herschel resolved it, and 
 estimated the number of stars at 200, but it pro- 
 bably contains many more. Smyth describes it 
 as " a noble mass, refreshing to the senses after 
 sweeping for faint objects." A photograph by 
 Dr, Roberts shows stars to 15th magnitude " with 
 dense nebulosity about the centre." No less than 
 85 variable stars have been detected among the 
 outliers of this cluster. 
 
 M. 6 : 17 h 33 m , S. 32 10'. In Scorpio. Sir John 
 Herschel calls it " a fine large discrete cluster of 
 stars, 10, 11 ; one star is 7 m ; one is7'8. Fills field." 
 From the position given by Sir John Herschel for 
 the nebula Dunlop, 612, it should be in the same 
 low-power field with M. 6 ; but observing in the 
 Punjab I could only see one cluster near the place, 
 with stars in "zigzag lines," which agree with 
 Herschel's description of Dunlop's nebula. 
 
 M. 7: 17 h 44 m , S. 30 39'. Sir John Herschel 
 describes it as " A highly condensed nebulous mass, 
 3' diameter, on an irregular round nebula ; pretty 
 much brighter in the middle ; resolvable." 
 
 M. 8 : 17 h 57 m> 6, S. 24 22'. A very fine object in 
 the Milky Way in Sagittarius. Visible to the 
 
MESSIER'S NEBULAE 219 
 
 naked eye. A beautiful drawing of it, showing 
 nebulous streaks and loops, is given by Sir John 
 Herschel in his Cape Observations, and he calls it 
 " a superb nebula." I found it plainly visible to 
 the naked eye in the Punjab, and a glorious object 
 even with a 3-inch refractor. It has been photo- 
 graphed by Dr. Roberts (Knowledge, June, 1900). 
 Secchi found the spectrum that of a gaseous 
 nebula. 
 
 M. 9: 17 b 13 m '3, S. 18 25'. Between rj and 58 
 Ophiuchi ; nearer to 77. Discovered in 1764 by 
 Messier, who described it as a nebula " unaccom- 
 panied by any star ; " but it was resolved into 
 stars by Sir William Herschel in 1784 with his 
 20-f eet reflector. Sir John Herschel, at the Cape, 
 found it about 4' in diameter, and " resolved into 
 stars of the 14th magnitude." Smyth says, " This 
 fine object is composed of a myriad of minute 
 stars, clustering into a blaze in the centre, and 
 wonderfully aggregated, with numerous outliers 
 seen by glimpses." 
 
 M. 10 : 16 h 51 m '9, S. 3 57'. About 10 east of the 
 stars 8 and e Ophiuchi. Sir John Herschel describes 
 it as "a fine large cluster . . . diameter 5', with 
 stragglers, several of which are of larger size to 
 about 12' ; all resolved into stars 11-15 mag- 
 nitude ; very compressed." A photograph by Dr. 
 Roberts in June, 1891, shows the cluster " nearly 
 free from nebulosity." 
 
 M. 11 : 18 h 45 m *7, S. 6 23'. In the so-called " Shield 
 
220 STUDIES IN ASTRONOMY 
 
 of Sobieski," which forms the southern portion 
 of Aquila. Discovered by Kirch in 1681. Sir 
 William Herschel saw stars of about llth mag- 
 nitude divided into 5 or 6 groups, and he found 
 it just visible to the naked eye. Sir John Herschel 
 called it round and rich, with stars 9-11 mag- 
 nitude. Smyth compared it to "a flight of wild 
 ducks," and says it " is a gathering of minute stars 
 with a prominent 8th magnitude star in the 
 middle and two following." A photograph by Dr. 
 Roberts agrees well with the descriptions, and 
 shows that the cluster is free from nebulosity. 
 
 M. 12 :, 16 h 42 m '0, S. 1 48'. Between 8 and 41 Op- 
 hiuchi. "it was discovered by Messier in 1764, and 
 described by him as " a round nebula, unaccompanied 
 by any star." Sir John Herschel found it resol- 
 vable into stars of 10th magnitude. Smyth called 
 it " a fine rich globular cluster, with a cortege of 
 bright stars and many minute straggling outliers." 
 A photograph taken by Dr. Roberts in June, 1892, 
 shows the stars visible to the centre with " a trace 
 of nebulosity." It should be stated, however, that 
 Professor Barnard finds that there is no trace of 
 nebulosity in any of the great globular, clusters, 
 as seen in the great Yerkes telescope. 
 
 M. 13 : 16 h 38 m 'l, N. 36 37'. This is the famous 
 globular cluster in Hercules, between rj and , and 
 considered to be one of the finest of its class. / It 
 was discovered by Halley in 1714. Messier, who 
 observed it with a 4-feet Newtonian reflector and 
 
THE GREAT GLOBULAR CLUSTER IN HERCULES (I. 13). 
 From a Photograph by W. E. Wilson, F.R.S. 
 
MESSIER'S NEBULA 
 
 power of 60, was " assured that it contained no 
 star ! " Sir William Herschel described it as " a 
 most beautiful cluster of stars, exceedingly com- ' 
 pressed in the middle and very rich." He esti- 
 mated the number of stars at 14,000. Sir John 
 Herschel saw thousands of stars in it. Smyth 
 calls it " a superb object," and Dr. Nichol remarked 
 that " Perhaps no one ever saw it for the first 
 time through a telescope without uttering a shout 
 of wonder." Lord Rosse found 3 dark streaks, or 
 lanes, which were also seen by Buffham. Photo- 
 graphs taken by Dr. Roberts in 1887 and 1895, 
 show the dark lanes seen by Lord Rosse, and he 
 thinks it was probably evolved from a spiral 
 nebula. From a photograph taken in America by 
 Mr. H. K. Palmer, with an exposure of two hours, he 
 finds the number of stars to be 5482, of which 1016 
 are " bright," and 4466 " faint." The dark lanes 
 are also clearly visible in Palmer's photograph. 
 
 M. 14: 17 h 32 m '3, S. 3 11'. Between r and 41 
 Ophiuchi. / Sir John Herschel found it " a most 
 beautiful and delicate globular cluster ; not very 
 bright, but of the finest star-dust ; all well resolved 
 . . . excessively rich. All the stars equal 15 or 
 16 m." Lord Rosse found the stars very close and 
 very small. A photograph by Dr. Roberts in 
 August, 1897, shows "curves and lines of stars 
 radiating in all directions outwards from the 
 dense cluster ... as well as vacancies, within the 
 nebulous centre." 
 
222 STUDIES IN ASTRONOMY 
 
 M. 15 : 21 h 25 m 'l, N. 11 43'. A globular cluster 
 in Pegasus in a comparatively vacant space about 
 4 north following 8 Equulei. It was discovered 
 in 1745 by Miraldi, who thought it contained 
 " many stars." Sir John Herschel estimated the 
 component stars about 15th magnitude. Lord 
 Rosse described it as a globular cluster with 
 bright and faint stars. A photograph by Dr. 
 Roberts " confirms the general descriptions," and 
 shows the stars arranged in " curves, lines, and 
 patterns," and the centre nebulous. 
 
 M. 16 : 18 h 13 m '2, S. 13 49'. South of " Sobieski's 
 Shield." Sir John Herschel described it as a 
 cluster containing at least 100 stars, and Smyth 
 called it " A scattered and fine large cluster." But 
 a photograph by Dr. Roberts in August, 1897, 
 shows that it is really " a large bright nebula, with 
 a cluster apparently involved in it." 
 
 M. 17: 18 h 15 m '0, S. 16 14'. This is the so-called 
 " horse-shoe," or " Omega " nebula. It lies about 
 2| south of Messier 16. It was described by Sir 
 John Herschel as a magnificent object, bright, and 
 very large. It was also described and drawn by 
 Lord Rosse, Lassell, Holden, and Trouvelot. 
 Smyth gives a drawing of it which very much 
 resembles a horse-shoe, but this form is nearly 
 lost in a photograph by Dr. Roberts in August, 
 1893. Sir William Huggins finds the spectrum 
 gaseous. 
 
 M. 18 : 18 h 14 m 'l, S. 17 10'. About one degree 
 
MESSiEITS NEBULA 
 
 south of M. 17. Webb says, "Glorious field in 
 a very rich vicinity; south lies a region of sur- 
 passing splendour." 
 
 M. 19 : 16 h 56 m '4, S. 26 7'. Between 39 Ophiuchi 
 and Antares, nearer the former. Sir William 
 Herschel resolved it into stars. Sir John Herschel 
 describes it as " superb. A globular cluster ; very 
 bright ; round ; diameter 10' ; resolved into stars 
 14th, 15th, 16th magnitude." Smyth called it " A 
 fine insulated globular cluster of small and very 
 compressed stars," and he says it " is near a large 
 opening, or hole, about 4 broad, in the Scorpion's 
 body, which Sir William Herschel found almost 
 destitute of stars." 
 
 M. 20 : 17 h 56 m '3, S. 23 2'. This is the well-known 
 " Trifid nebula," which lies closely north of the 
 star, 4 Sagittarii. A very curious object, with three 
 dark lanes radiating from the centre. It has been 
 well drawn by Sir John Herschel and Trouvelot, 
 and it was photographed at the Lick Observatory 
 with the Crossley reflector. 1 Although it has a 
 very nebulous appearance, the spectrum seems to 
 be not gaseous. 
 
 M. 21 : 17 h 58 m '7, S. 22 30'. Between /* and 4 
 Sagittarii and a little north following the " Trifid 
 nebula" (M. 20). Smyth describes it as "A coarse 
 cluster of telescopic stars in a rich gathering 
 galaxy region " 
 
 M. 22: 18 h 30 n "3, S. 23 59'. About midway 
 1 Astrophyaical Journal, May, 1900. 
 
STUDIES IN ASTRONOMY 
 
 between p and <r Sagittarii. It seems to have 
 been seen by Hevelius before 1665, and it was 
 drawn by Le Gentil in 1747. Messier, who 
 observed it in 1764, thought that it contained 
 no stars ! Sir John Herschel describes it as "a 
 globular cluster, very bright ; very large ; very 
 much compressed, 7' diameter. The stars are of 
 two magnitudes, 15-16 magnitude and 12 mag- 
 nitude ; and what is very remarkable, the largest 
 of these latter are visibly reddish." And in an- 
 other observation he says, "Consists of stars of 
 two sizes, 11 and 15, with none intermediate, 
 as if it consisted of 2 layers, or one shell over 
 another. A noble object." Webb found it " very 
 interesting from visibility of components 10 and 
 11 magnitude, 1 which makes it a valuable object 
 for common telescopes, and a clue to the struc- 
 ture of many more distant or difficult nebulse." 
 Observing with 3-inch refractor in the Punjab, 
 the present writer found the larger stars well 
 seen; but the greater portion of the cluster is 
 nebulous with a telescope of this size. 
 
 M. 23 : I7 h 51 m -0, S. 19 0'. 'Smyth describes it 
 as " A loose cluster ... an elegant sprinkling 
 of telescopic stars over the whole field, under 
 a moderate magnifying power." Webb says, 
 " Grand low-power field." 
 
 M. 24 : 18 h 12 m -6, S. 18 27'. About 3 north of 
 /A Sagittarii. Sir John Herschel describes it as 
 1 Probably Sir John Herscliel's brighter components. 
 
THE DUMB-BELL NEBULA. 
 
 From a Photograph ~by W. E. Wilson, F.R.S. 
 
MESSIER^ NEBULAE 225 
 
 * 
 pretty large and very rich, with stars of 11-20 
 
 magnitude. Lord Rosse saw some unresolved 
 nebulous light in it, but a photograph by Dr. 
 Roberts, taken in February, 1894, shows the stars 
 free from nebulosity. On this photograph the 
 streams of stars surrounding the centre seem to 
 be arranged in spirals, and strongly suggest that 
 the cluster has been evolved from a spiral nebula. 
 
 M. 25 : 18 h 24 m -l, S. 19 2'. X Smyth describes it 
 as "a loose cluster of large and small stars in the 
 Galaxy." Closely south following this nebula is 
 the short-period variable star V Sagittarii, which 
 varies from 7*0 to 8'3 magnitude with a period of 
 about 6f days. It was discovered by Schmidt in 
 1866. 
 
 M. 26 : 18 h 39""7, S. 9 29'. A little south, follow- 
 ing the star 2 Aquilae in " Sobieski's Shield." ^Sir 
 John Herschel describes it as large and pretty 
 rich ; stars 12th to 15th magnitude. Smyth calls 
 it " a small and coarse, but bright cluster of 
 stars," and this is confirmed by a photograph 
 taken by Dr. Roberts in August, 1892. 
 
 M. 27 : 19 h 55 m -3, N. 22 27'. The well-known 
 " Dumb-bell " nebula. It lies a little south of the 
 star 14 Vulpeculse. / It has been drawn by Sir 
 John Herschel, d' Arrest, Lord Rosse, and Lassell. 
 At one time Lord Rosse thought that it might 
 be resolvable into stars, but Huggins found a 
 gaseous spectrum. Photographs by Dr. Roberts 
 and Dr. ^Wilson show it as a globular mass 
 
 Q 
 
226 STUDIES IN ASTRONOMY 
 
 surrounded by a broad and darker ring which 
 gives it the dumb-bell appearance. From recent 
 photographs taken by Schaeberle he finds that 
 " this object is a great counter clock- wire spiral, 
 at least half a degree in diameter, the well-known 
 nebula occupying the central area." 1 
 
 M. 28 : I8 h 18-M, S. 24 55'. A globular cluster. 
 It lies about 1 north preceding the star \ Sagit- 
 tarii. Messier described it as a nebula without a 
 star, but Sir William Herschel resolved it, and 
 Sir John Herschel describes it as " very bright ; 
 round ; very much compressed . . . resolved into 
 stars 14 ... 16 m. ; a fine object." This nebula 
 with Messier's numbers 8, 16, 17, 18, 20, 21, 22, 23, 
 24, and 25, all lie in a comparatively small area 
 of the sky surrounding the 4th magnitude star 
 fjL Sagittarii. 
 
 M. 29: 20 h 20 m '5, N. 38 11'. About 2 south of 
 y Cygni. Smyth describes it as " a neat but small 
 cluster of stars." 
 
 M. 30 : 21 h 34 m -7, S. 23 38'. Closely north pre- 
 ceding the 5th magnitude star 41 Capricorni/ 
 There is an 8th magnitude star close to it. It 
 was resolved into stars by Sir William Herschel 
 in 1783, and Sir John Herschel describes it as " A 
 globular cluster ; bright, 4' long by 3' broad ; all 
 resolved into stars 16 m., besides a few 12 m." 
 Lord Rosse found a spiral arrangement of the 
 stars. 
 
 1 Astronomical Journal, No. 547, September 30, 1903. 
 
MESSIER'S NEBULAE 
 
 M. 31 : O h 37 ra> 3, N. 40 43'. " The great nebula 
 in Andromeda." ' Plainly visible to the naked 
 eye, a little west of the star v Andromedse, and ' 
 quite a conspicuous object even in a binocular 
 field-glass. Al-Sufi refers to it as a familiar 
 object in his time (10th century). This magni- 
 ficent nebula has been frequently drawn, and has 
 been so often described in astronomical books 
 that a detailed description is unnecessary here. 
 Photographs by Dr. Roberts and others show its 
 real character. At first it was supposed to be 
 annular, but Dr. Roberts says, " That the nebula 
 is a left-hand spiral, and not annular as I at first 
 supposed, cannot now be questioned ; for the 
 convolutions can be traced up to the nucleus, 
 which resembles a small bright star at the centre 
 of the dense surrounding nebulosity; but, not- 
 withstanding its density, the divisions between 
 the convolutions are plainly visible 011 negatives 
 which have had a proper degree of exposure ; " 
 and he thinks that " these photographs throw a 
 strong light on the probable truth of the Nebular 
 Hypothesis, for they show what appears to be 
 the progressive evolution of a gigantic stellar 
 system." He finds its apparent diameter about 
 2|. It seems to be not gaseous, as Huggins 
 finds a continuous spectrum. A small " new," 
 or " temporary star" suddenly appeared near the 
 nucleus in August, 1885. It had faded to the 
 16th magnitude in February, 1890. 
 
228 STUDIES IN ASTRONOMY 
 
 M. 32 : O h 37 m , N. 40 18'. A small bright round 
 nebula a little south of the nucleus of the great 
 nebula in Andromedse (M. 31).' It was discovered 
 by Le Gentil in 1749. It is said to have been 
 resolved into stars by Lord Rosse's 3-f eet telescope. 
 Its spectrum is similar to that of the great nebula. 
 
 M. 33 : l h 28 m -2, N. 30 9'. Between ft Andro- 
 medse and a Arietis, nearer to ft Andromedse. x It 
 was described by Sir John Herschel as a remark- 
 able object, extremely large, round and very rich, 
 and resolvable into stars ; but a photograph by 
 Dr. Roberts, in November, 1895, shows it to be 
 really a spiral nebula. There is a nucleus of 
 " dense nebulosity," with about 20 stars involved, 
 and the other parts of the nebula contain hundreds 
 of faint nebulous stars of about 16th or 17th 
 magnitude. It is of considerable apparent size, 
 measuring about 1 long by in width. 
 
 M. 34 : 2 h 35 m -6, N. 42 21'. About 5 North pre- 
 ceding Algol (ft Persei). Just visible to the naked 
 eye in a clear sky. Messier called it "a mass of 
 small stars." Sir John Herschel described it as 
 bright, very large, and but little compressed, with 
 scattered stars of the 9th magnitude. Smyth 
 says " it is a scattered but elegant group of stars 
 from the 18th to the 13th degree of brightness on 
 a dark ground, and several of them form close 
 pairs." A photograph taken by Dr. Roberts in 
 December, 1892, shows a loose cluster of stars 
 down to the 15th magnitude ; but it is not very 
 
THE SPIRAL NEBULA; 33 MESSIER TRIANGULI. 
 
 From a Photograph by W. E. Wilson, F.R.S. 
 
MESSIER^ NEBULA 229 
 
 rich, and the stars can be easily counted. On the 
 print the densest part of the cluster does not 
 apparently contain more than 100 stars. But 
 there may be more on the original negative. 
 
 M. 35 : 6 h 2 m '7, N. 24 21'. A little north pre- 
 ceding the star 17 Geminorum. Just visible to the 
 naked eye./ Sir John Herschel described it as a 
 very large, rich cluster, with stars 9th to 16th 
 magnitude. Lord Rosse called it "magnificent," 
 and he counted 300 stars in a field of 26 minutes 
 of arc, or less than the moon's apparent diameter. 
 Lassell described it as a marvellously "striking 
 object." On a photograph taken in February, 1893, 
 by Dr. Roberts, he counted 620 stars in a field of 
 26 minutes, or more than double the number seen 
 by Lord Rosse. But a glance at the photographs 
 shows that it is not nearly so rich in stars as the 
 globular clusters. 
 
 M. 36: 5 h 29 m *7, N. 34 4'. About 2 following 
 the star < Aurigae. - x Sir John Herschel describes it 
 as bright, very large, and very rich, with stars of 
 the 9th and llth magnitudes. Lord Rosse called 
 it a coarse cluster. Smyth says "a splendid 
 cluster ... a rich though open splash of stars 
 from the 8th to the 14th magnitude, with numerous 
 outliers." But these descriptions are somewhat 
 misleading, as in a photograph taken by Dr. 
 Roberts in February, 1893, the stars seem com- 
 paratively few in number, and do not much exceed 
 100, at least on the print. Compared with the 
 
230 STUDIES IN ASTRONOMY 
 
 following cluster M. 37 in the same constellation 
 M. 36 is comparatively poor. 
 
 M. 37: 5 h 45 m -7, N. 32 31'. About midway 
 between < and K Aurigse. Sir John Herschel 
 describes it as a rich cluster with large and small 
 stars. Smyth calls it "a magnificent object, the 
 whole field being strewed as it were with spark- 
 ling gold-dust; and the group is resolvable into 
 about 500 stars from the 10th to the 14th magni- 
 tude, besides the outliers." This description is 
 confirmed by a photograph taken by Dr. Roberts 
 in February, 1893, in which the stars are shown 
 down to about the 16th magnitude. The sur- 
 rounding region is pretty rich in stars. 
 
 M. 38 : 5 h 22 m -0, N. 35 45'. About 1^ north of 
 <j> Aurigae. 'Smyth describes it as " A rich cluster 
 of minute stars. ... It is an oblique cross with a 
 pair of large stars in each arm and a conspicuous 
 single one in the centre." Webb says, " Glorious 
 neighbourhood." 
 
 M. 39 : 21 h 29 m , N. 47 59'. About 3 south pre- 
 ceding IT Cygni. Smyth describes it as " A loose 
 cluster, or rather splashy galaxy field of stars, in 
 a very rich vicinity." 
 
 M. 41 : 6 h 42 m *7, S. 20 38'. About 4 south of 
 Sirius. Just visible to the naked eye. It is 
 referred to by Aristotle in his " Meteorologies " as 
 a star " with a tail." Messier described it as a 
 mass of small stars. Smyth calls it " a scattered 
 cluster . . . divided into five groups ; " and Webb 
 
a > 
 
 r - 
 
 s. -" 
 
MESSIER'S NEBULAE 231 
 
 says, " Superb group. . . . Larger stars in curves 
 with ruddy star near centre," which Espin suspects 
 to be variable in light. 
 
 M. 42: 5 h 30 m -4, S. 5 27'. This is "the great 
 nebula in Orion," which has been so fully described, 
 drawn, and photographed that a detailed account 
 is unnecessary here. 
 
 M. 43. This is the small nebula closely north of 
 the great nebula in Orion, M. 42. / 
 
 M. 44 : 8 h 34 m '3, N. 20 20'. This is the Prsesepe 
 of the old astronomers in the constellation Cancer. 
 A scattered cluster to the naked eye. ' 36 stars 
 were counted in it by Galileo, but of course it 
 contains many more. It has been photographed 
 by Dr. Roberts, who thinks that the brighter 
 stars are " nebulous." 
 
 M. 46 : 7 h 37 m '2, S. 14 35'. A little preceding 
 the star 2 Puppis (Argo). Sir John Herschel called 
 it "a superb cluster of stars, 12 ... 16m." It 
 includes a planetary nebula (Herschel), which 
 Lassell, Lord Rosse, and Dr. Roberts found to be 
 annular. Smyth describes it as " A noble though 
 rather loose assemblage of stars from the 8th to 
 the 13th magnitude." 
 
 M. 47: 7 h 50 m -2, S. 15 9. This lies about 3 
 following the preceding, M. 46. Sir John Herschel 
 says, " Place from Wollaston's Cat." 
 
 M. 49 : 12 h 24 m -7, N. 8 33'. It lies to the west of 
 8 and e Virginis, and nearly forms an equilateral 
 triangle with those stars. / Smyth calls it "A 
 
STUDIES IN ASTRONOMY 
 
 bright, round, and well-defined nebula." Webb 
 says, "Faint haze in beautiful position between 
 two 6th magnitude stars." 
 
 M. 50: 6 h 58 m 'l, S. 8 12'. Between Sirius and 
 Procyon, and about 4 south of the star 19 Mono- 
 cerotis. / Sir John Herschel described it as a 
 remarkable cluster, very large and rich, with stars 
 12th to 16th magnitude. Smyth says it "is an 
 irregularly round and very rich mass," and Webb 
 calls it a "brilliant cluster." A photograph by 
 Dr. Roberts in March, 1893, shows that it is not 
 very rich. On his print the stars may be easily 
 counted, and probably do not exceed 200 in the 
 main body of the cluster. 
 
 M. 51 : 13 h 25 m '7, N. 47 43'. This is the wonderful 
 spiral nebula in Canes Venatici. It lies about 3 
 south preceding 17 Ursae Majoris. / Sir John Her- 
 schel described it as a double nebula, the larger 
 with a nucleus and ring round it. Its spiral 
 character was discovered by Lord Rosse, and his 
 drawing agrees well in general outlines with 
 photographs taken in April, 1889, and May, 1896, 
 by Dr. Roberts, who finds "both nuclei of the 
 nebula to be stellar, surrounded by dense nebu- 
 losity, and the convolutions of the spiral in this 
 as in other spiral nebulae are broken up into star- 
 like condensations with nebulosity around them." 
 The so-called second "nucleus" seems to be a 
 portion which is being detached from the parent 
 mass, probably by the "centrifugal force" of 
 
MESSIEITS NEBULAE 233 
 
 rotation. The nebula has also been well photo- 
 graphed by Dr. W. E. Wilson. 
 
 M. 52 : 23 h 19 m '8, N. 61 3'. About 1 south of 
 the star 4 Cassiopeise. It is described by Sir John 
 Herschel as large, rich, round, and much com- 
 pressed, with stars 9th to 13th magnitude. 
 Admiral Smyth saw it of a triangular form and 
 "resembling a bird with outspread wings," and 
 adds that " the field is one of singular beauty 
 under moderate magnification." Lord Rosse 
 thought it contained about 200 stars. A photo- 
 graph by Dr. Roberts confirms the descriptions 
 given by previous observers. 
 
 M. 53 : 13 h 8 m '0, N. 18 42'. A little north pre- 
 ceding the star a Comae Berenices. / It was de- 
 scribed by Sir John Herschel as a bright globular 
 cluster, very compressed in the centre, and stars 
 of the 12th magnitude. Smyth calls it " a 
 brilliant mass of minute stars from the llth to 
 the 15th magnitude, and from thence to gleams 
 of star-dust with stragglers." 
 
 M. 54 : 18 h 48 ra '4, S. 30 39'. About 2 preceding 
 2 Sagittarii. Sir John Herschel described it as 
 " a globular cluster ; bright ; round ; gradually 
 brighter in the middle ; 2-|' diameter ; resolved 
 into stars 15m., with a few outliers 14m." 
 
 M. 55: 19 h 32 m , S. 31 13'. Sir John Herschel 
 describes it as "globular; a fine large, round 
 cluster; 6' diameter; all clearly resolved into 
 stars, 11, 12, 13m." Observing it with 3-iiich 
 
234 STUDIES IN ASTRONOMY 
 
 refractor in the Punjab, the present writer saw 
 glimpses of stars in it with power 40 ; it will not 
 bear higher powers with this aperture. 
 
 M. 56: 19 h 12 m '7, N. 30 0'. Between ft Cygiii 
 and y Lyrse, nearer the former. 'A. globular 
 cluster in a rich region. Sir John Herschel 
 says resolved into stars of 11-14 magnitude. A 
 photograph by Dr. Roberts shows it to be a 
 globular cluster with rays of stars projecting 
 from it. 
 
 M. 57 : 18 h 49 m '9, N. 32 54'. This is the well- 
 known "annular nebula" between ft and y Lyrae. 
 Drawings by Sir John Herschel and Lord Rosse 
 agree well with photographs by Dr. Roberts ; but 
 it is more elliptical in shape than the drawings 
 show. The central opening is not quite dark, but 
 is filled in with faint nebulous light. A faint 
 star in the centre is suspected of variable light. 
 Lord Rosse, Secchi, and Chacornac thought this 
 nebula might be resolvable into stars, but Huggins 
 finds a gaseous spectrum. Recent photographs 
 by Schaeberle show that it is really a spiral 
 nebula. 1 (See next Chapter.) 
 
 M. 58: 12 h 32 m -6, N. 12 22'. In the nebulous 
 region in Virgo/ Sir John Herschel says, " Bright ; 
 large ; irregularly round ; very much brighter in 
 middle ; resolvable." 
 
 M. 59: 12 h 37 m , N. 12 13'. A little north of 
 p Virginis. / Smyth calls it "a bright little 
 1 Astronomical Journal, 539, 547. 
 
MESSIER^ NEBULAE 235 
 
 nebula." It is in the same low-power field with 
 M. 60. 
 
 M. 60 : 12 h 38 m '6, N. 12 6'. A little north of , 
 p Virginis, and in a low-power field with M. 59. 
 Smyth describes it as " A double nebula . . . about 
 2' or 3' from centre to centre, the preceding one 
 being extremely faint." There is another small 
 nebula near. 
 
 M. 61 : 12 h 16 m '8, N. 5 2'. A little north of the 
 star 16 Virginis. / Smyth describes it as "A large 
 pale-white nebula, but so feeble as to excite sur- 
 prise that Messier detected it with his 3|-foot 
 telescope in 1779." Webb says, "Faint; bright 
 centre." Lord Rosse found it spiral. A photo- 
 graph by Dr. Roberts in May, 1899, shows it to 
 be " a right-hand spiral." l 
 
 M. 62 : 16 h 54" U 8, S. 29 56'. About 6 following 
 and a little south of the star r Scorpii. Messier 
 described it as " resembling a little comet." Sir 
 William Herschel resolved it into stars, and de- 
 scribed it as a miniature of M. 3. Sir John 
 Herschel says, " Globular cluster, bright ; large ; 
 round; superb; about 7' diameter; all resolved 
 into stars 15 mag., very equal. The most con- 
 densed part is a perfect blaze, but not quite in 
 the centre." 
 
 M. 63 : 13 h ll m -3, N. 42 34'. A little north of the ^ 
 star 20 Canum Venaticorum. '' Sir John Herschel 
 describes it as very bright and large, with a 
 1 Knowledge, August, 1901. 
 
236 STUDIES IN ASTRONOMY 
 
 bright nucleus. A photograph by Dr. Roberts in 
 May, 1896, with an exposure of 2 hours and 25 
 minutes, shows it to be a spiral nebula " with a 
 bright stellar nucleus in the centre of dense nebu- 
 losity." Huggins finds a continuous spectrum. 
 
 M. 64 : 12 h 51 m -8, N. 22 13'. In Coma Berenices, 
 about 1 north following the star Flamsteed 35. 
 Sir John Herschel described it as very bright and 
 large, and thought it resolvable. Lord Rosse found 
 a dark spot on one side. Smyth says, "It is 
 magnificent in size and brightness." Webb calls 
 it a " Magnificent large bright nebula, blazing to 
 a nucleus." A photograph by Dr. Roberts shows 
 it to be a spiral nebula, ''with a large bright 
 stellar nucleus; one of the convolutions is very 
 bright, with a dark space between it and the 
 nucleus, which is free from nebulosity, thus pro- 
 ducing the effect of contrast between dark and 
 light spaces." 
 
 M. 65: ll h 13 m '6, N. 13 38'. A little south pre- 
 ceding the star 73 Leonis (south of 6 Leonis). A 
 photograph by Dr. Roberts shows it " to be a left- 
 hand spiral, with the external outline so regularly 
 formed that it resembles an annular nebula with 
 rings encircling it; but the spiral form must be 
 the true interpretation, and the rings of nebu- 
 losity, with the dark spaces between them, and 
 the nebulous star-like condensations, together form 
 parts of the convolutions ; the dark spaces being 
 the intervals between them." 
 
MESSIER'S NEBULAE 237 
 
 M. 66. ll h 15 m , N. 13 32'. In the same low- 
 power field with M. 65 (above). - Lord Rosse found 
 it to be spiral, and this is confirmed by a photo- 
 graph taken in February, 1894, by Dr. Roberts, 
 who says, " The photograph shows the nebula to 
 be a spiral, with a well-defined stellar nucleus, 
 which forms the pole of the convolutions in which 
 I have counted fourteen nebulous star-like con- 
 densations." Smyth says that at a short distance 
 following M. 65 and M. 66 there is another elliptical- 
 shaped nebula of even larger apparent dimensions. 
 
 M. 67: 8 h 45 m -8, N. 12 ll f . About 2 preceding 
 a Cancri. /Sir John Herschel described it as a 
 remarkable object, very bright, large, and rich, 
 with stars 10th to 15th magnitude. Smyth calls 
 it a rich but loose cluster. A photograph by 
 Dr. Roberts shows that the cluster is not very 
 rich. It seems to contain about 200 stars, as 
 estimated by Sir William Herschel. 
 
 M. 68 : 12 h 34 m -2, S. 26 12'. In Hydra, about 3} 
 south of /? Corvi, and near a 5| magnitude star. 
 Sir John Herschel describes it as "A globular 
 cluster, irregularly round; gradually brighter in 
 the middle. . . . All clearly resolved into stars 
 12 m. ; very loose and ragged at the borders." 
 
 M. 69: 18 h 24 m '5, S. 32 25'. About 2f north 
 following e Sagittarii. Sir John Herschel described 
 it as " A globular cluster ; bright ; round ; 3' 
 diameter; all clearly resolved into stars, 14-15 
 mag. A blaze of stars." 
 
238 STUDIES IN ASTRONOMY 
 
 M. 70 : 18 h 36 m -3, S. 32 25'. About 2J following 
 M. 69 (above). / Sir John Herschel describes it as 
 " A globular cluster ; bright ; round ; resolved into 
 stars 15 mag." 
 
 M. 71 : 19 h 49 m -3, N. 18 31'. Between y and 3 
 Sagittse. / Resolved into stars by Sir William 
 Herschel. Sir John Herschel describes it as a very 
 large and very rich cluster, with stars 11 to 16 
 magnitude. A photograph by Dr. Roberts shows 
 a cluster in which the " curves and arrangements 
 of stars resemble those of a spiral nebula." The 
 surrounding region is "densely crowded with 
 stars down to about 17th magnitude, arranged in 
 remarkable curves and lines, which are very 
 suggestive of having been produced by the effects 
 of spiral movements." 
 
 M. 72 : 20 h 48 m , S. 12 54'. In Capricornus. A 
 globular cluster discovered by Messier in 1780 as 
 a nebula. ' Sir William Herschel resolved it into 
 stars with his 20-feet reflector. He called it " a 
 very bright object," and estimated its diameter 
 at 1' 53"'6. 
 
 M. 73: 20 h 53 m '5, S. 13 1'. In Capricornus, a 
 little following M. 72. Sir John Herschel says, 
 " Cluster ? ? ; extremely poor ; very little com- 
 pressed ; no nebula." 
 
 M. 74: l h 31 m -3, N. 15 6'. A little following 
 Y] Piscium. Sir John Herschel thought this to be 
 a globular cluster, very large and round, brighter 
 in the middle, and partially resolved into stars ; 
 
MESSIER'S NEBULAE 239 
 
 but Lord Rosse found it to be a spiral nebula, and 
 a photograph by Dr. Roberts, taken in December, 
 1893, confirms this, and shows it to be a perfect 
 and beautiful spiral, " with a central stellar 
 nucleus," and numerous "star-like condensations" 
 in the convolutions of the spiral. 
 
 M. 75 : 20 h O m '2, S. 22 12'. Messier thought it 
 to be a mass of very small stars. Sir William 
 Herschel resolved it, and Smyth calls it a globular 
 cluster. Webb says, "Bright nucleus, with low 
 power." 
 
 M. 76 : l h 36 m -0, N. 51 4'. A little north of the 
 star 54 Anclrornedae. A double nebula, the com- 
 panion being No. 193 of Herschel' s 1st Class. 
 Messier thought it was a cluster, but Sir William 
 Herschel considered it irresolvable. Lord Rosse 
 found it spiral, with two centres. Webb says, 
 " Curious miniature of M. 27 [the dumb-bell], and 
 like it, gaseous." A photograph by Dr. Roberts 
 " shows the two nebulae to be one only. . . . The 
 figure of the nebula suggests that it is a broad 
 ring seen edgewise." 
 
 M. 77 : 2 h 37 m -6, S. 26'. About 1 south follow- 
 ing 8 Ceti. Sir John Herschel describes it as 
 pretty large and irregularly round, brighter in 
 the middle, with a nucleus, and partly resolved. 
 Lord Rosse and Lassell thought it spiral. A 
 photograph by Dr. Roberts shows " a stellar 
 nucleus, with projecting ansce of dense nebulosity 
 . . . and surrounding the ansce is a zone of faint 
 
240 STUDIES IN ASTRONOMY 
 
 nebulosity surrounded by a broad nebulous ring, 
 which is studded with strong condensations re- 
 sembling stars with irregular margins." The 
 surrounding region is very devoid of stars. 
 
 M. 78 : 5 h 41 m -6, N. I 1 . About 2f north fol- 
 lowing Orionis.' Webb says, "Singular ' wispy' 
 nebula." Lord Rosse thought it possibly a spiral. 
 
 M. 79 : 5 h 20 m 'l, S. 24 37'. About 3J south of 
 fi Leporis. ' Seen by Sir William Herschel as a 
 cluster about 3' in diameter. Webb says, " Toler- 
 ably bright with my 64, blazing in centre ; higher 
 powers showed it mottled." 
 
 M. 80: 16 h ll ra -l, S. 22 44'. About midway be- 
 tween a and /? Scorpii, and a little north of 19 
 Scorpii. Sir William Herschel thought it the richest 
 and most condensed mass of stars in the heavens. 
 Sir John Herschel, in his " Cape Observations " 
 (p. Ill), describes it as " a globular cluster ; round ; 
 suddenly very much brighter in the middle to a 
 blaze . . . stars =14 m. ; all resolved. Fine 
 object." In May, 1860, a temporary star of about 
 the 7th magnitude blazed out in the centre of this 
 nebula, and by its light completely obscured the 
 cluster. On June 16 of the same year it had com- 
 pletely disappeared, and has not been seen since. 
 A little north, following the nebula, are two 
 known variable stars, R and S Scorpii. 
 
 M. 81 : 9 h 47 m -3, N. 69 32'. In Ursa Major, about 
 10 north-west of a. Described by Sir John Her- 
 schel as a remarkable, extremely bright nebula 
 
MESSIEITS NEBULAE 241 
 
 with a nucleus. Lord Rosse thought it very like 
 the great nebula in Andromeda, and this is con- 
 firmed by a photograph taken by Dr. Roberts in 
 March, 1889, with an exposure of 3J hours ; but it 
 is very much smaller in apparent size. This 
 photograph shows the nebula " to be a spiral, with 
 a nucleus which is not well defined at its boundary, 
 and is surrounded by rings of nebulous matter." 
 Huggins finds a continuous spectrum, like that of 
 the great nebula in Andromeda. 
 
 M. 82 : 9 h 47 m '6, N. 70 10'. About half a degree 
 north of M. 81 (above). Sir John Herschel de- 
 scribed it as a very bright ray, and very large. 
 Lord Rosse called it a most extraordinary object. 
 A photograph by Dr. Roberts shows that it " is 
 probably seen edgewise with several nuclei of 
 nebulous character involved." Huggins find the 
 spectrum to be continuous. This is a character- 
 istic of the spiral nebula. 
 
 M. 83 : 13 h 31 m '4, S. 29 22'. Sir John Herschel 
 says, " Very bright ; very large ; suddenly much 
 brighter in middle to a nucleus ; large 3-branched 
 spiral." 
 
 M. 84: 12 h 20 m '0, N. 13 26'. Closely preceding 
 M. 86. /Sir John Herschel says, "Very bright; 
 pretty large; round; pretty suddenly much 
 brighter in middle ; resolvable." 
 
 M. 85: 12 h 20 m '4, N. 18 45'. A little east of 
 11 Comse Berenices. / 
 
 M. 86: 12 h 21 m , N. 13 30'. In "the nebulous 
 
 R 
 
242 STUDIES IN ASTRONOMY 
 
 region" in Virgo. Sir John Herschel thought it 
 probably resolvable. 
 
 M. 87 : 12 h 25 m '8, N. 12 56'. In the nebulous 
 region in Virgo. A little preceding M. 89. Sir 
 John Herschel says, " Very bright ; very large ; 
 round ; much brighter in the middle." 
 
 M. 88 : 12 h 26 m '9, N. 14 58'. A long and faint 
 nebula in the nebulous region in Virgo. Smyth 
 called it " A long, elliptical nebula." Lord Rosse 
 thought it possibly spiral. 
 
 M. 89: 12 h 30 m '6, N. 13 6'. In the nebulous 
 region in Virgo. A little following M. 87 (above). 
 Sir John Herschel says, " Pretty bright ; round ; 
 gradually much brighter in the middle." 
 
 M. 90 : 12 h 31 m '9, N. 13 42'. In the nebulous 
 region in Virgo/ A little north of M. 89 (above). 
 Sir John Herschel says, " Pretty large ; nucleus." 
 
 M. 91. In the nebulous region in Virgo. Sir 
 John Herschel doubted the existence of this 
 nebula. 
 
 M. 92 : 17 h 14 m ;l, N. 43 15'. In a rather blank 
 space in Hercules, about 6 north of ir. Messier 
 found it easily visible with a telescope of only one 
 foot in length. Sir William Herschel found it a 
 brilliant cluster of 7' or 8' in diameter. Sir John 
 Herschel described it as " a globular cluster, very 
 bright and large, and well resolved into small 
 stars." Webb says, " A very fine cluster, though 
 not equal to M. 13 ; less resolvable ; intensely 
 bright in centre." A photograph by Dr. Roberts 
 
MESSIER^ NEBULAE 243 
 
 in May, 1891, shows the cluster " involved in dense 
 nebulosity." But Professor Barnard finds that 
 there is no trace of any real nebulosity in any of 
 the great globular clusters as seen with the great 
 Yerkes telescope. The nebulosity visible on the 
 photographs is therefore probably due to a photo- 
 graphic effect. 
 
 M. 93: 7 h 40 m '2, S. 23 28'. A little following 
 | Puppis (Argo). / Sir William Herschel resolved it 
 into stars 8 to 13 magnitude. Smyth calls it " A 
 small galaxy cluster," and says, " This neat group 
 is of a star-fish shape, the south preceding portion 
 being the brightest. Webb says, " Bright cluster 
 in a rich neighbourhood." 
 
 M. 94 : I2 h 46 m -2, N. 41 40'. A little to the north 
 of a line joining a and fi Canum Venaticorum. / 
 Sir John Herschel describes it as large, very 
 bright, and irregularly round, with bright nucleus 
 and barely resolvable. Lord Rosse saw a dark 
 and bright ring round the nucleus, and thought it 
 probably spiral. A photograph by Dr. Roberts 
 in May, 1892, shows the stellar nucleus and rings, 
 but, he says, "I am unable to trace any spiral 
 structure on the photograph." 
 
 M. 95 : 10 h 38 m '7, N. 12 13'. About 4 north fol- 
 lowing p Leonis. Discovered by Mechain in 1781. 
 Sir John Herschel says, " Bright ; large ; round ; 
 pretty gradually much brighter in middle to a 
 nucleus." Lord Rosse saw two ellipses, and the 
 centre perhaps resolvable. 
 
244 STUDIES IN ASTRONOMY 
 
 M. 96 : 10 h 4175, N. 12 21'. About 40' east of 
 M. 95 (above). Discovered by Mechain in 1781. 
 Sir John Herschel says, " Very bright ; very large ; 
 a little elongated ; very suddenly very much 
 brighter in the middle, barely resolvable." Smyth 
 says, speaking of this and M. 95, " Another round 
 but not equally defined nebula, large and of a 
 pale white colour." A little north of M. 96 are 
 two faint nebulae, Nos. 17 and 18 of Sir William 
 Herschel's Class I. 
 
 M. 97: ll h 9 m -0, N, 55 34'. About 2 south fol- 
 lowing (3 Ursae Majoris. / Described by Sir John 
 Herschel as a remarkable planetary nebula, very 
 bright, very large, and about 160 seconds of arc in 
 diameter. Lord Rosse resembled it to the face of 
 an owl, and it has hence been known as " the owl 
 nebula." A photograph by Dr. Roberts in April, 
 1895, shows it of an elliptical shape, about 208 
 seconds in diameter, with a 15th magnitude star 
 in the centre, but no other stars in the nebula. 
 He says, " The star seen by both Lord Rosse and 
 Dr. Robinson has disappeared." 
 
 M. 98: 12 h 8 m -7, N. 15 27'. Closely preceding 
 the star 6 Comas Berenices. Smyth says, " A fine 
 and large but rather pale nebula . . . elongated 
 in the direction of two stars." 
 
 M. 99 : 12^ 13 m -7, N. 14 58'. A little south fol- 
 lowing the star 6 Comae Berenices. Sir John Her- 
 schel describes it as a very remarkable object, 
 bright, large, and round. Lord Rosse found it to 
 
MESSIER^ NEBULJS, 245 
 
 be a wonderful spiral. A photograph by Dr. 
 Roberts confirms the spiral character, and shows 
 "many star-like condensations in the convolutions." 
 M. 100: 12 h 17 m -9, N. 16 23'. About 2 north 
 following 6 Comae Berenices. Discovered by 
 Mechain in 1781. Smyth thought it " globular," 
 but Lord Rosse found it to be a spiral, with the 
 centre a planetary nebula. A photograph by Dr. 
 Roberts in May, 1896, shows it to be " a strikingly 
 perfect" spiral, with a sharply stellar nucleus 
 " in the midst of faint nebulosity." 
 
 M. 101 : 13 h 59 m '6, N. 54 30'. About 6 east of 
 Ursae Majoris. Sir John Herschel describes it as 
 pretty bright, very large, and irregularly round. 
 Lord Rosse found it to be a large spiral nebula, 
 14' in diameter; and his drawing agrees fairly 
 well with a photograph taken by Dr. Roberts in 
 May, 1892, which shows "a well-defined stellar 
 nucleus," with 'the usual convolutions and "star- 
 like condensations." 
 
 M. 103 : l h 26 m -6, N. 60 11'. About 1 north-east 
 of S Cassiopeiae. Sir John Herschel describes it as 
 a bright cluster, pretty large, round and rich, 
 with stars 10th and llth magnitudes. A photo- 
 graph by Dr. Roberts shows stars down to about 
 the 15th magnitude. It contains bright and faint 
 stars, but the cluster does not seem to be a very 
 rich one. Smyth called it a " fan-shaped group," 
 and the photograph confirms this description. 
 
XXI 
 
 The Ring Nebula in Lyra 
 
 THIS annular nebula lies between the stars 
 ft and y Lyras, and about 6| south and a 
 little to the east of Vega. It is a favourite 
 object with amateur telescopists, being probably 
 the only object of its class within the range of 
 moderate-sized instruments. It was discovered 
 by Darquier in 1779, and it is No. 57 in Messier's 
 Catalogue of Nebulae; but neither of these ob- 
 servers seems to have noticed its real form. To 
 Sir William Herschel it appeared as a luminous 
 ring of nebulous light with a dark centre. Schroter, 
 in 1797, and Sir John Herschel, in the nineteenth 
 century, saw a fainter light filling up the interior 
 of the ring ; and photographs with long exposures 
 show so much nebulous light within the ring as 
 almost completely to obliterate its annular aspect. 
 Secchi and Chacornac thought it resolvable into 
 small stars, but Sir William Hug-gins found a 
 gaseous spectrum. 
 
 / The ring is not perfectly circular, but oval in 
 shape. According to measures made by Professor 
 
THE RING NEBULA IN LYRA 247 
 
 Barnard in 1 893-94 l its apparent dimensions are 
 as follows : 
 
 Outer Major diameter 80"-89 
 
 Inner 36"*52 
 
 Outer Minor diameter 58"'81 
 
 Inner 29"'36 
 
 In apparent size, therefore, it is considerably 
 larger than Saturns' rings. 
 
 From the above measures it appears that the 
 width of the ring varies from about 14" to 22". 
 The ring is not uniform in brightness, but some- 
 what fainter at the ends of the longer axis, and a 
 little brighter at the ends of the shorter axis. 
 This feature, which was noticed by Sir William 
 Herschel in 1785, is, however, only noticeable on 
 photographs taken with short exposures. Barnard 
 finds it a " beautiful object " with the great Yerkes 
 telescopes of 40 inches aperture, and discovered 
 another nebula of a spiral form distant about 4' 
 from the centre of the ring. With reference to 
 the larger nebula he says, " Under the best con- 
 ditions the interior of the ring has appeared of 
 unequal brightness. The light of the ring itself, 
 however, blinds one's eye to the details on the 
 interior, so that it is not possible to speak with 
 certainty of the form of the details." 
 
 The real shape of this wonderful object is pro- 
 bably that of an ellipsoid of revolution. This 
 ellipsoid being hollow, or partly so, and nearly 
 1 Monthly Notices, R.A.S., January, 1900. 
 
248 STUDIES IN ASTRONOMY 
 
 transparent, gives it the appearance of a ring. 
 Professor Schaeberle has recently found indications 
 on photographs that the nebula forms the centre 
 of a larger spiral structure. 1 The spectrum of the 
 nebula shows a number of bright lines. These 
 indicate the presence of hydrogen and another 
 substance, but helium seems to be absent. 
 
 There are many faint stars in the same field 
 with the nebula, and one very faint one in the 
 centre, which has been suspected of variable light. 
 It seems to have been first seen by Von Hahm in the 
 eighteenth century, for he speaks of its disappear- 
 ance in 1800. It was apparently not seen by Sir 
 William Herschel, nor by Sir John Herschel, but 
 it was observed by Lord Rosse in 1848, and by 
 Secchi in 1855. It was, however, missed by Pro- 
 fessor Asaph Hall in 1877 with the 26-inch re- 
 fractor of the Washington Observatory, and by 
 Vogel with 27 inches in 1883. But it is visible 011 
 all photographs, which shows that its light, like 
 that of the nebula, is strongly actinic. An ex- 
 posure of a few minutes' duration is sufficient to 
 show it. In 1899, MM. Bourget, Montaiigerand, 
 and Bailland published observations tending to 
 show that this small star had increased in bright- 
 ness in recent years ; but Barnard, from over 10 
 years' observations with the Lick and Yerkes tele- 
 scopes, thinks that there has been 110 change in its 
 light. In July, 1899, he estimated it as 15* or 16th 
 1 Astronomical Journal, No. 539. 
 
THE RING NEBULA IN LYRA 249 
 
 magnitude, 1 and Buruham, in 1891, rated it 15*4 m. 
 The photographic and spectroscopic evidence shows 
 that this small star is most probably connected 
 with the nebula, and Barnard, Keeler, and Scheiner 
 agree that it forms the nucleus of the nebula. 
 Keeler says, " It is as clearly defined as are other 
 stars outside the nebula;" but according to Dr. 
 Roberts, other nebulae have a stellar nucleus. 
 There is another extremely faint star just inside 
 the ring. This small star is distinctly visible on a 
 photograph taken by Keeler with an exposure of 
 10 minutes. He says, " It is at the very limit of 
 vision with the 36-inch, and must, like the central 
 star, possess unusual photographic energy." 2 
 
 Recent measures made by Dr. Burt L. Newkirk 
 to determine the parallax of the central star gave 
 a result of about ^ of a second of arc. He used 
 sixteen comparison stars on different sides of the 
 nebula, and he thinks this parallax is worthy of 
 confidence, and that we are justified in regarding 
 the nebula " as one of our nearest celestial neigh- 
 bours." 3 Assuming this parallax, we have, from 
 Barnard's measures, the outer length of the ring 
 about 809 times the sun's distance from the earth, 
 and its outer width 588 times the same distance. 
 The inner length and width would be about 365 and 
 294 times the sun's distance, and as Dr. Newkirk 
 
 1 Monthly Notices, E.A.S., January, 1900, p. 253. 
 
 2 Astrophysical Journal, June-December, 1899, p. 195. 
 
 3 Publications of the Astronomical Society of the Pacific, Feb- 
 ruary 10, 1904. 
 
250 STUDIES IN ASTRONOMY 
 
 says, " The whole solar system could be put into 
 the ring with plenty of room to spare." He 
 might have said several solar systems, as the dia- 
 meter of Neptune's orbit is only a little over 60 
 times the sun's distance from the earth. The 
 parallax found by Newkirk indicates a "light 
 journey" of about 33 years, and it also follows 
 that light would take about 4 days 16 hours 
 to pass from one extremity of the ring to the 
 opposite end. Placed at the distance of the ne- 
 bula, our sun would shine as a star of about the 
 5th magnitude, or over 15,000 times brighter than 
 the central star. This small star must therefore 
 be a comparatively faint object. Seen from the 
 extremity of the ring, it would, I find, shine as a 
 star of about 3 magnitude, or between Venus 
 and Jupiter in brightness. 
 
 The ring nebula lies about 6J from the bright 
 star Vega, for which Dr. Elkin finds a parallax of 
 0"'082, or only a little less than that found by 
 Dr. Newkirk for the nebula. From the relative 
 distances indicated by these parallaxes, I find that 
 the distance of the nebula from Vega is about one- 
 fourth of its distance from the earth. Hence, it 
 would follow that the ring nebula, as seen from 
 Vega, or rather from a planet revolving round 
 Vega, if such there be, would shine 16 times 
 brighter than we see it. This represents 3 stellar 
 magnitudes, so that it might perhaps be visible to 
 the naked eye, or at least with a very small telescope. 
 
XXII 
 
 A Great Belgian Astronomer 
 
 ALTHOUGH perhaps unknown to the general 
 public, one of the greatest Belgian astrono- 
 mers of the nineteenth century was Jean 
 Charles Houzeau. He accomplished much excellent 
 work in astronomy, which has rendered his name 
 for ever immortal in the history of the " sublime 
 science." Houzeau was born in the suburb of 
 Havre near Mons, on October 7, 1820, on a small 
 property owned by his parents. He was the elder 
 of two sons, and his brother, Auguste Houzeau, 
 became professor at the School of Mines at Mons, 
 and a member of the Chamber of Representatives. 
 Like many other great men, Houzeau was a 
 precocious child. Before he could read or write 
 he showed an evident taste for astronomy, and 
 it is said of him that with the sweetmeats given 
 him by his parents and friends he used to make 
 figures representing the constellations on a table ! 
 In his studies at the college of Mons he achieved 
 a brilliant success, and was awarded a silver medal 
 for his zeal and hard work. He entered the 
 
252 STUDIES IN ASTRONOMY 
 
 University of Brussels in 1837, but here he was not 
 so successful, either from indifference to honours 
 and distinctions or on account of his taste for 
 original investigation. Disappointed by his want 
 of success at the University, his parents brought 
 him home to Mons, and here he was free to follow 
 his astronomical studies. He built with his own 
 hands a small observatory on a hill called Panisel, 
 situated near his father's residence. It merely 
 consisted of a wooden hut, and contained a mural 
 circle, a transit instrument, and a telescope mounted 
 equatorially. The tubes of these instruments 
 were of zinc, and the lenses, which were purchased 
 in Paris, were not even achromatic ! This equip- 
 ment, although a very imperfect one for the study 
 of astronomy, shows the taste and aptitude of the 
 young astronomer, who was then only eighteen. 
 Shortlyafter this,iii the years 1838 to 1841,Houzeau 
 became a journalist, and wrote a considerable 
 number of articles in a Brussels paper called 
 Emancipation. The papers were on various 
 subjects, such as the conservation of forests, the 
 use of air as a motor, the application of geology 
 to agriculture, civil architecture, steam-engines, 
 navigable canals, the improvement of railways, 
 artesian wells, etc., a remarkable series of articles 
 for so young a man. He also wrote a small work 
 on turbines, their construction and application to 
 industrial purposes, but this work is now un- 
 fortunately lost. 
 
A GREAT BELGIAN ASTRONOMER 253 
 
 During the years 1840 and 1841, young Houzeau 
 studied a course of science at Paris. He returned 
 to Belgium in 1842 and resumed his astronomical 
 studies, in which he was encouraged by Quetelet, 
 then Director of the Brussels Observatory, who 
 permitted him to act as a voluntary assistant in 
 certain observatory work. 
 
 About this time Houzeau sent a paper to the 
 Astronomische Nachrichten on the Zodiacal Light, 
 and this note was referred to by Humboldt in his 
 famous " Cosmos." Up to this Houzeau was quite 
 unknown in the astronomical world. During the 
 years 1843 and 1844, he paid many visits to Paris, 
 and studied astronomical works in the National 
 Library. In 1844 he published an important paper 
 in the Astronomische Nachrichten on the binary 
 stars, 61 Cygni, and 70 Ophiuchi. With reference 
 to the latter star he showed that there was an 
 irregularity in its apparent motion which seemed 
 to indicate that either the component stars did 
 not follow Newton's laws of gravitation, or else 
 that the centre of motion is not the centre of 
 gravity of the masses. This irregularity in the 
 motion of 70 Ophiuchi is now well known, but 
 has not yet been very satisfactorily explained. 
 Houzeau ascribed it to an effect caused by the 
 aberration of light, but this view was contested 
 by Sir John Herschel in the Astronomische Na 
 chrichten (No. 520). 
 
 From the year 1844, Houzeau made meteoro- 
 
254 STUDIES IN ASTRONOMY 
 
 logical as well as astronomical observations at the 
 Brussels" Observatory. On August 3 of that year 
 he presented to the Belgian Academy of Sciences 
 an important paper on the August meteors, and 
 showed how to determine the "radiant" point. 
 In the year 1845 he sent some papers on comets 
 to the Belgian Academy, but for some reason 
 these were not published. About this period 
 he seems to have first thought of compiling 
 an astronomical bibliography. His idea was to 
 continue the work of Lalande, which stopped in 
 1802. This great work, on which he spent an 
 enormous amount of labour, was completed and 
 published in Brussels in 1887, the year before his 
 death. 
 
 On September 30, 1846, Houzeau was appointed 
 assistant astronomer at the Brussels Observatory, 
 on the small salary of 1400 francs, or about 56 a 
 year. He occupied this position for about three 
 years, and during that period he communicated 
 no papers to the Academy, his whole time being 
 devoted to the duties of his office. His labours 
 and zeal were much thought of by Quetelet, who 
 speaks highly of Houzeau in the Annals de 
 V Observatoire de Bruxelles (1851). During his 
 stay at the Observatory, Houzeau observed the 
 transits of Mercury across the sun's disc, which 
 took place on May 8, 1845, and November 8, 1848. 
 He also observed a comet, discovered by Colla 
 at Parma, and computed its orbit. In October 
 
A GREAT BELGIAN ASTRONOMER 255 
 
 and November, 1846, he undertook a series of 
 observations of the planet Neptune which had 
 then been recently discovered. In 1848 and 1849 
 he published a number of articles of a democratic 
 and republican character, and on account of his 
 political views he was dismissed from his post on 
 April 6, 1849, notwithstanding Quetelet's efforts 
 on his behalf. 
 
 In September, 1849, Houzeau left Belgium, 
 accompanied by two friends, for an excursion in 
 Germany, Switzerland, and France. They travelled 
 partly on foot and partly by railroad and diligence, 
 and visited several places of interest. In May, 
 1850, he went to Paris and remained there till 
 1855. During this period he had no regular occu- 
 pation, but studied in the National Library and 
 accumulated an enormous number of notes on all 
 sorts of subjects. He then went to England, and, 
 assisted by his brother, made some experiments 
 on the possibility of optical telegraphy by means 
 of lights, but his labours ended in no practical 
 result. 
 
 In the years 1851 to 1854, Houzeau wrote several 
 papers on physical geography and geodesy. In 
 November, 1854, he was appointed astronomer to 
 the Belgian War Department to assist in the 
 topographical survey of the country. This work 
 was carried on in summer in the field, and Houzeau 
 passed the winter in Paris, reducing his observa- 
 tions, and, in his leisure hours, studying in the 
 
256 STUDIES IN ASTRONOMY 
 
 School of Mines. His work on the survey of 
 Belgium was continued until May, 1857, when, 
 from want of funds, the work was stopped. 
 Notwithstanding his arduous work in these years 
 he still found time to write newspaper articles on 
 various subjects, some astronomical. In 1857 
 he published an important work on physical 
 geography, entitled " Histoire du Sol de 1'Europe." 
 In maps illustrating this volume he shows the 
 varying heights of the ground by " contour lines," 
 or lines of equal height and by tints, and this 
 method of map-making, now frequently employed, 
 seems to have been invented by Houzeau. 
 
 After losing his post in the War Department, 
 Houzeau returned to Mons, and prepared for a 
 visit to America, a journey which he had long 
 contemplated. On June 21, 1857, he started for 
 Brussels, and 011 July 1 he proceeded to London, 
 and resided there for two months in order to im- 
 prove his knowledge of English. On September 10 
 he sailed from Liverpool on board the sailing-ship 
 Metropolis for New Orleans. The voyage lasted 
 seven weeks, and after some rough weather, and 
 rather scanty fare, the ship arrived at New 
 Orleans on October 28. Although his intention 
 was, on leaving England, to spend only a few 
 months in the United States, his visit to America 
 extended over a period of nearly twenty years ! 
 During this period of his life a large portion of 
 his astronomical work was done. He also wrote a 
 
A GREAT BELGIAN ASTRONOMER 257 
 
 number of papers on the manners, customs, and 
 institutions of the United States. These accounts 
 were communicated to a periodical called Revue 
 Trimestrielle, during the years 1858 to 1868, and 
 included a discussion on the abolition of slavery, 
 a subject in which he always took the deepest 
 interest. 
 
 After spending a short time in New Orleans, he 
 proceeded, in company with a caravan of farmers, 
 to Texas, a country almost absolutely unknown 
 at that time. After numerous adventures on the 
 prairies, Houzeau arrived on May 21, 1858, at the 
 small town of San Antonio. While here, he was 
 employed by a company to do some survey work 
 for irrigation purposes. He then joined another 
 caravan, and proceeded on an excursion to the Rio 
 Grande, the large river which forms the boundary 
 between Texas and Mexico. The journey there 
 and back lasted from September 1 to October 15, 
 1858, and during that tune he made some interest- 
 ing meteorological observations. It was then that 
 the famous comet of Donati shone so brilliantly in 
 the evening sky. It was first seen by Houzeau on 
 September 19, and he remarked that it passed over 
 the stars <r and p Bootis without obscuring their 
 light. Soon after his return to San Antonio, 
 Houzeau was again employed by the company 
 mentioned above to make some explorations to 
 the west of Texas. This work occupied him for 
 four years, during which he passed a wandering 
 
258 STUDIES IN ASTRONOMY 
 
 life, living chiefly in the open air, investigating 
 the climate and the mineral and other resources of 
 a country as large as France. 
 
 At the beginning of the year 1861, the war 
 between the Northern and Southern States broke 
 out. At this time Houzeau was about to under- 
 take a geological excursion to the most distant 
 part of the prairie, and on the completion of this 
 work he returned to San Antonio. After a short 
 rest he proceeded on a second geological expedition 
 to the Rio Pecos, but owing to the political state 
 of the country he was obliged to return. After a 
 little, Houzeau left San Antonio and went to 
 a town called Austin. Here some of his friends 
 tried to induce him to join the staff of the Con- 
 federate Army in the capacity of an engineer, but 
 he firmly refused to have anything to do with 
 upholding the cause of slavery. After a short 
 residence at Austin, he returned to San Antonio, 
 and occupied his time with his intellectual labours. 
 But owing to his sympathy with the negroes, he 
 was not long permitted to remain in peace ; and 
 as the authorities tried to compel him to join the 
 militia, in spite of his protestations made through 
 the Belgian Consul at New Orleans, he determined 
 to leave San Antonio and proceed to Mexico. 
 After several adventures 011 the journey, he 
 arrived safely at Matamoros, near the mouth of 
 the Rio Grande, on March 20, 1862. Here he 
 remained for some months, and supported himself 
 
A GREAT BELGIAN ASTRONOMER 259 
 
 by gardening and architectural work on buildings 
 in the town. Wishing to visit the United States, 
 he succeeded, after some difficulty, in obtaining a 
 free passage on an American warship bound for 
 New Orleans, and arrived in that city on January 
 31, 1863. Here he lived for five years with the 
 exception of a visit of four months to the city of 
 Philadelphia. During his sojourn in New Orleans 
 he wrote many articles, under the name of Dalloz, 
 for a journal called the Union, published in the 
 interests of the negroes, for whom Houzeau always 
 had the greatest sympathy. He soon became 
 editor of this paper; and during his absence in 
 Philadelphia its name was changed to the Tribune. 
 On his return to New Orleans he was appointed 
 director of the journal, and for over three years 
 he continued to champion with energy the cause 
 of the negroes. The number of articles he wrote 
 for the Tribune was prodigious and sufficient to 
 fill several large volumes. His labours in the 
 negro cause aroused the animosity of the planters, 
 and he experienced much persecution on account 
 of his views. Some disagreement between Houzeau 
 and the administrators of the Tribune led to his 
 resignation, which was accepted on January 18, 
 1868, and on April 25 of the same year the journal 
 ceased to exist. 
 
 During his residence in America, Houzeau wrote 
 several astronomical papers for European journals. 
 Among these was one "on the determination of 
 
260 STUDIES IN ASTRONOMY 
 
 the radius vector of a new planet," in which he 
 showed a new method for finding the distance of 
 a planet from the sun, and calculating the elements 
 of its orbit. He also wrote papers on the parallax 
 of the planets, and on the proper motions of the 
 stars. 
 
 His connection with journalism in New Orleans 
 having come to an end, Houzeau resolved to pro- 
 ceed to Jamaica, an idea which he had long enter- 
 tained. On May 17, 1868, he left New Orleans, 
 and on June 5 arrived at Kingston, one of the 
 principal ports of the island. Houzeau lived in 
 Jamaica for about six years, and would probably 
 have remained there for the rest of his life had not 
 the death of Quetelet, in 1874, recalled him to 
 Brussels to undertake at the urgent request of 
 his friends the Directorship of the Brussels 
 Observatory. Some of the most important work 
 of Houzeau' s active life was accomplished during 
 his residence in Jamaica. Soon after his arrival 
 at Kingston, he rented a farm a few miles from 
 that town. Here he remained only a year, and in 
 1869|he removed to a place a few miles farther 
 away, called Rose View, at the foot of the Blue 
 Mountains. His new residence was a small house, 
 to which was attached a garden of about 2J acres, 
 containing cocoanut palms, mango trees, guavas, 
 pine-apples, etc. Here, again, Houzeau found him- 
 self in the midst of a negro community, who at 
 first showed symptoms of hostility, but finding 
 
A GREAT BELGIAN ASTRONOMER 261 
 
 that Houzeau was in sympathy with them, they 
 soon became his friends. In this beautiful climate 
 his life seems to have been a happy one, free from 
 the cares and excitement of more civilized regions. 
 For servants he had a young mulatto, named 
 William Lang, who came with him from New 
 Orleans, and a young negro, named Georges Hall, 
 and both seem to have been devoted to his 
 service. 
 
 While in Jamaica, Houzeau made several excur- 
 sions into the interior of the island, and one 
 expedition undertaken in 1873 to the summit 
 of the Blue Mountains seems to have been almost 
 a "voyage of discovery," as in those days these 
 mountains were comparatively unknown to 
 travellers. 
 
 By the aid of a small printing press, and with 
 the help of his two attendants, Houzeau printed 
 several small works during his stay in Jamaica. 
 These were chiefly on mathematical subjects ; and 
 as only a few copies were printed they are now 
 extremely rare. He also wrote many other papers 
 on subjects connected with astronomy and natural 
 history. For the study of the latter subject he 
 
 seems to have always had a great aptitude ; and, 
 
 / 
 
 indeed, his work, " Etudes sur les f acultes inentales 
 des animaux comparees a celles de rhommes," 
 would alone have been sufficient to establish his 
 fame as a great philosopher and naturalist. Some 
 have even placed his writings on this subject 
 
262 STUDIES IN ASTRONOMY 
 
 in the same rank as those of the illustrious 
 Darwin. 
 
 Among his astronomical labours at Jamaica 
 may be mentioned his observations on the Zodiacal 
 Light, and his " Atlas of stars visible to the naked 
 eye." The latter work is one of considerable 
 importance, executed as it was in a beautiful 
 climate like that of Jamaica, and at a station 
 situated not far from the Equator, a position 
 which enabled this eminent observer to see nearly 
 all the stars in both hemispheres. About thirty 
 years before Houzeau commenced his survey of 
 the heavens, the famous Argelander had published 
 maps of the northern hemisphere and a portion 
 of the southern. This work was afterwards 
 revised by Jleis ; and Behrmann had published 
 a similar work for the southern hemisphere. 
 Houzeau' s work has, however, the advantage of 
 having been accomplished by one observer for 
 both hemispheres. This work was commenced on 
 February 25, 1875. At first Houzeau feared that 
 it would be an undertaking of great magnitude 
 and labour, but after a few days' experience he 
 came to the conclusion that it would be a com- 
 paratively easy task. Before three months had 
 elapsed he found that one-third of the work was 
 done. It has been estimated from Argelander' s 
 observations in the northern hemisphere that the 
 total number of stars visible to the naked eye 
 in both hemispheres would be about 4200. Hou- 
 
A GREAT BELGIAN ASTRONOMER 263 
 
 zeau's maps shows nearly 6000, an increase partly 
 explained by the clearer skies of Jamaica, and 
 partly, Houzeau thought, by the difficulty of seeing 
 southern stars near the horizon of Argelander's 
 station. To enable him to see the stars further 
 south, Houzeau went to Panama on October 16, 
 1875, and having there completed his maps, he 
 returned to Jamaica on December 16 of the same 
 year. Here he found a telegram awaiting him, 
 announcing his appointment as Director of the 
 Brussels Observatory. 
 
 Before relating Houzeau' s subsequent career, 
 let us further consider his star atlas. In addition 
 to all the stars visible to the naked eye, he added 
 a drawing of the Milky Way, shown blue on a 
 white ground. His drawing is somewhat diagram- 
 matic and deficient in detail. The method of 
 delineation adopted by Houzeau was to trace the 
 lines of equal brightness (or "isophotes," as he 
 termed them) of the various portions of the Milky 
 Way. These somewhat resemble, he says, the 
 " contour lines " on terrestrial maps, and are filled 
 hi with a blue tint, the washes of colour being 
 placed one over the other, so that " plus il y a de 
 courbes, plus 1'espace renferme dans la derniere 
 est brillant." As in Heis' drawing of the Milky 
 Way, Houzeau shows five gradations of bright- 
 ness, and these he determined by comparing the 
 brilliancy of different portions with neighbouring 
 stars of magnitudes, 6-7, 6, 5-6, 5 and 4-5. In 
 
264 STUDIES IN ASTRONOMY 
 
 making this comparison he was guided by the 
 appearance or disappearance of the luminous 
 patches of Milky Way light in the twilight 
 or moonlight simultaneously with the stars of 
 comparison. It seems doubtful, however, whether 
 this method is susceptible of any great accuracy, 
 the comparison of a bright point like a star 
 with a nebulosity extending over a considerable 
 area being evidently a matter of much difficulty 
 and considerable uncertainty. The visibility of 
 the star and the adjoining nebulosity might not, 
 in all cases, be equally affected by varying 
 atmospheric conditions, and the gradations of 
 light in the different portions of the galaxy 
 are so gradual, numerous and complicated, that 
 many of the smaller details would unavoidably 
 be lost. Houzeau seems to have been con- 
 scious of the uncertainty of his method, for 
 he says : " Cependant il ne serait pas exact 
 d'en conclure que ces plaques brillants doiinent 
 autant de lumiere qu'uiie nappe continue d'etoiles 
 du 5m ordre, il est incontestable que leur etendue 
 aide a les apercevoir, et que leur visibilete ne 
 repose pas uniquement sur leur eclat specifique." 
 The drawing being, however, the work of a single 
 observer, and so accomplished an astronomer as 
 Houzeau, and moreover executed from observa- 
 tions made in a favourably situated station, like 
 Jamaica, possesses a value to which it might not 
 otherwise be entitled. 
 
A GREAT BELGIAN ASTRONOMER 265 
 
 As has been said, Houzeau was, in December, 
 1875, offered the appointment of Director of the 
 Brussels Observatory. But some of the Belgian 
 Ministers had opposed his nomination, owing to 
 his well-known republican opinions. They even 
 induced the king to cancel his nomination. How- 
 ever, these difficulties were surmounted by his 
 friends, and in the beginning of 1876 his appoint- 
 ment to the Observatory was definitely decided. 
 Houzeau left Jamaica on March 25, 1876, and on 
 June 17 of the same year he took over charge of the 
 Observatory. He at once commenced a thorough 
 reorganization of the establishment, which had 
 for some years become much out of date both as 
 to its instruments and its management. During 
 the six years he remained in charge of the 
 Observatory he made many changes. On his 
 arrival there were only four assistants, but when 
 he retired in 1883 the number was sixteen. In 
 the way of instruments he added equatorial tele- 
 scopes of 6 and 15 inches aperture, constructed by 
 Cooke of York, a meridian circle of 6 inches, the 
 work of Repsold, and other instruments. During 
 his superintendence of the Observatory he laboured 
 as usual with great zeal, and the amount of work 
 accomplished was very considerable. Many works 
 were published during this period, including the 
 star atlas already referred to. 
 
 In 1882, Houzeau, accompanied by two assistants, 
 went to his former place of residence, San Antonio 
 
266 STUDIES IN ASTRONOMY 
 
 in Texas, to observe the transit of Venus, which 
 took place in December of that year. His observa- 
 tions were only partially successful, owing to the 
 presence of clouds during the early phases of 
 the phenomenon. On his return to Europe he 
 remained for some time at Orthez, near Pau, and 
 afterwards at Blois. In November, 1883, he 
 resigned the Directorship of the Brussels Observa- 
 tory, and in 1886 he returned to Brussels and 
 resided there till his death. 
 
 After his retirement from the Observatory, his 
 time was chiefly devoted to the completion of his 
 "Bibliographic Astronomique," a work already 
 referred to in the beginning of this sketch. His 
 health, never very robust, became much impaired, 
 and after considerable suffering, he expired on 
 July 12, 1888. His remains were conveyed to his 
 native place, Mons, and there interred on July 15. 
 He was twice married, but left no children. 
 
 Houzeau possessed many noble traits of 
 character. He was charitable, honourable, just, 
 modest, and frank. Outwardly he was somewhat 
 reserved in manner, but he had a warm heart 
 and was a good and constant friend. He was 
 held in high esteem by the members of the 
 Observatory. His object in life seems to have 
 been to help in the cause of humanity and science. 
 His studies included almost all branches of human 
 knowledge. He was a veritable encyclopaedia. 
 During his active life he gave his attention to 
 
A GREAT BELGIAN ASTRONOMER 267 
 
 astronomy, meteorology, geography, geodesy, 
 philosophy, literature, political economy, etc. 
 Although he made 110 great discovery in 
 astronomy, his published works show great know- 
 ledge and judgment, and an original treatment 
 of his subject, which renders them very interest- 
 ing and instructive, not only to scientific 
 students, but also to the general reader. The 
 famous French astronomer, Flammarion said of 
 him, " Houzeau was a laborious student, an in- 
 dependent man, a noble heart, and a grand 
 character. He always placed the love of science 
 and truth above personal interest, and the vain 
 ambitions to which many students sacrifice their 
 lives. His name will remain nobly associated 
 with the history of contemporary astronomy, of 
 which he was one of the most genuine repre- 
 sentatives. His beautiful career, alas ! too short, 
 was wholly devoted to the cause of Progress." 
 
XXIII 
 
 Some Recent Advances in Stellar Astronomy 
 
 A1TRONOMICAL discoveries are now being 
 rapidly made. This is partly due to the 
 large telescopes which have been recently 
 constructed, and partly to the increased interest 
 now taken by amateurs and the general reader in 
 "the sublime science." Almost daily we hear of 
 something new, and books on astronomy become 
 rapidly out of date. In the following pages I 
 propose to consider the most important advances 
 which have been made in the department of 
 stellar astronomy during the seven years 1894 to 
 1900, 1 the closing years of the nineteenth century. 
 Let us first consider the results of investiga- 
 tions made on the distance of the stars from the 
 earth. For the five stars in the Plough, ft, y, S, e, 
 and , which are known to have a common proper 
 motion that is, that they are apparently travel- 
 ling through space in the same direction and at 
 nearly the same rate Dr. Hbffler finds a parallax 
 
 1 For an account of advances prior to 1894, see my work " The 
 World of Space." 
 
ADVANCES IN STELLAR ASTRONOMY 
 
 of 0"-0165. This makes their distance from the 
 earth about 200 years' journey for light. On Mr. 
 Monck's scale, of which the unit is the distance 
 of a star with a parallax of one second of arc, their 
 distance would be represented by 60*6. Placed 
 at this vast distance, the sun would, I find, be re- 
 duced in brightness to a star of about 8J magni- 
 tude, and would therefore be quite invisible to 
 the naked eye! Prom this it will be seen that 
 they must be very large and brilliant suns. If 
 the observed parallax is correct, the actual dis- 
 tance from |8 to would be at least four million 
 times the sun's distance from the earth. Such is 
 the scale on which the heavens are constructed. 
 The spectra of all five stars are of the first or 
 Sirian type, a fact which probably indicates an in- 
 trinsically brighter body than our sun. Dr. Hbffler 
 thinks that c is 40 times brighter than Sirius. 
 
 From a series of measures made in different 
 years, Sir David Gill finds that the parallax of 
 Sirius is 0"'370, and he thinks that the parallax 
 of this brilliant star has now been satisfactorily 
 determined. He finds that the parallax of a Cen- 
 tauri the nearest of all the stars to the earth 
 certainly lies between 0"*74 and 0"*75. This im- 
 plies a distance of about 275,000 tunes the sun's 
 mean distance from the earth, or about 25 billions 
 of miles. Dr. Gill thinks that the parallax of the 
 bright star Rigel is not more than 0"*01, which 
 implies that the star's distance is certainly greater 
 
270 STUDIES IN ASTRONOMY 
 
 than 20 million times the sun's distance, and a 
 light journey of 325 years. And yet it is one of 
 the brightest stars in the sky about seventh on 
 the list. For Canopus, Gill finds no measurable 
 parallax, a result which is very remarkable, as, 
 next to Sirius, it is the brightest star in the 
 heavens. 
 
 Parallaxes have been found at the Yale Obser- 
 vatory (U.S.A.) for the ten brightest stars in the 
 northern hemisphere, viz. Arcturus, Capella, Vega, 
 Procyon, Betelgeuse, Altair, Aldebaran, Pollux, 
 Regulus, and a Cygni, with the result that Pro- 
 cyon is the nearest and a Cygni the furthest from 
 the earth. Arcturus, the brightest of the ten, has 
 a very small parallax, and must therefore be a 
 sun of enormous size. 
 
 With reference to stellar motions, it had been 
 for many years considered that the star Groom- 
 bridge 1830 the so-called "runaway star" 
 had the largest proper motion about 7 seconds 
 of arc per annum ; but now Mr. R. T. A. Innes 
 and Professor Kapteyn have discovered that a 
 star of the 8th magnitude, in the southern constel- 
 lation Pictor, has a proper motion of 8*7 seconds 
 per annum. The faintness of this new " runaway " 
 is remarkable. Sir David Gill finds a parallax of 
 0"'312. At the distance indicated by this compara- 
 tively large parallax, the sun would shine as a 
 star of about 2' 6 magnitude. This makes the 
 sun about 144 times brighter than the star. 
 
ADVANCES IN STELLAR ASTRONOMY 271 
 
 As is now well known, the actual velocity of a 
 star in the line of sight can be measured with the 
 spectroscope. Some large velocities have recently 
 been found in this way by Professor Campbell, 
 now director of the Lick Observatory. For 
 /x Cassiopeiae he finds 60 miles a second ; for 
 e Andromedae, 52 ; for /* Sagittarii, 47 ; rj Cephei, 
 46 ; Herculis, 33| ; and for the planetary nebula 
 G.C. 4373, 31^ miles all approaching the earth. 
 For those receding, 6 Canis Majoris and 8 Leporis 
 show a .velocity of 59 miles a second. That a 
 gaseous mass like a planetary nebula should be 
 rushing through space with a velocity of over 31 
 miles a second, seems very extraordinary. 
 
 A variable velocity in the line of sight has been 
 observed in a number of stars, which suggests 
 that they are binary stars, with the components 
 so close together that no telescope could divide 
 them. Among these may be mentioned Capella, 
 Castor, UrsaB Majoris, ft Aurigae, ft Herculis, 
 -f] Pegasi, o Leonis, x Draconis, Geminorum, t and 
 K Pegasi, Draconis, \ Andromedae, Ursae Majoris, 
 o> Draconis, ft Capricorni, Centauri, ft Scorpii, 
 TT Cephei, and the Pole Star. Of these Capella is a 
 most interesting object. According to the spectro- 
 scopic observations, the relative velocity of the 
 components is about 37 miles a second, and the 
 period of revolution about 104 days. Attempts to 
 see the star visually double, made with the great 
 telescope of the Lick Observatory, have failed; 
 
272 STUDIES IN ASTRONOMY 
 
 but several of the Greenwich observers have seen 
 the star "elongated" with the 28-inch refractor. 
 The observed changes in the relative positions of 
 the components agree well with the period of 104 
 days found with the spectroscope, and from the 
 measures made it has been computed that the 
 combined mass of the components is about 18 
 times the mass of our sun. The measures also 
 show that the parallax found by Dr. Elkin (0"'081) 
 is about correct. Professor Campbell finds that 
 one component of Capella has a spectrum of the 
 solar type, while the other is of the type of 
 Procyon. The components do not differ much 
 in brightness. (See chapter on Spectroscopic 
 Binaries.) 
 
 The Pole Star has also been found to be a binary 
 star, with a period of about 4 days. 1 The orbit is 
 nearly circular, and in dimensions about the same 
 as that of the moon round the earth. The presence 
 of a third body is suspected. 
 
 The brighter component of the well-known 
 double star Castor was found by Dr. Belopolsky 
 to be a close spectroscopic binary. The period is 
 about 3 days, and the relative orbital velocity 
 about 20*7 English miles a second. 
 
 From spectroscopic measures of motion in the 
 line of sight of the famous binary star y Virginis 
 Belopolsky finds a parallax of 0"'051, and a com- 
 bined mass equal to 15 times the mass of the sun. 
 1 More exactly, 3 days, 23 hours, 14'3 minutes. 
 
ADVANCES IN STELLAR ASTRONOMY 273 
 
 The system is receding from the earth at the rate 
 of nearly 13 miles a second. He makes a similar 
 calculation with reference to the binary star 
 y Leonis, finding a parallax of 0"'0197, and a mass 
 of 6^ times the sun's mass ; but he seems to be 
 unaware of the fact that the orbit of y Leonis is 
 very uncertain. 
 
 Belopolsky finds that the velocity of 61 Cygni 
 (the nearest star to the earth hi the northet^n 
 hemisphere), as derived from spectrum photo- 
 graphs, is about 26*8 miles a second towards the 
 earth. Assuming a parallax of 0"'5, and a proper 
 motion of 5'2 seconds, the velocity across the line 
 of sight would be about 22*6 miles a second. 
 Combining these velocities, he finds an actual 
 velocity through space of 35 miles a second. 
 
 With reference to double and binary stars, 
 some interesting results have been found. Pro- 
 fessor Barnard, observing with the great 40-inch 
 telescope of the Yerkes Observatory (U.S.A), in 
 1897, found a faint star near Vega, which was not 
 seen with the Lick telescope. In 1864, Winnecke 
 found a small star at the same distance (55 
 seconds) from Vega, and not far from it; but 
 Barnard's new companion is much fainter than 
 Winnecke's, which is rated 14^ magnitude. 
 Curious to say, Struve's well-known companion 
 (10th magnitude) is also at the same distance from 
 Vega, but in a different quadrant. A faint and 
 close companion to the bright star Procyon has 
 
274 STUDIES IN ASTRONOMY 
 
 been discovered by Schaeberle. It is evidently 
 revolving round the bright star, and Dr. See finds 
 a period of 40 years. He finds the masses of the 
 two stars in the ratio of one to five. With Elkin's 
 parallax of 0"*266, the semi-axis major of the 
 orbit is 21*2 times the earth's distance from the 
 sun, or a little larger than the orbit of Uranus. 
 The combined mass of the two stars is about 
 6 times the sun's mass, and hence, as in the case 
 of Sirius, the faint companion has about the same 
 mass as the sun. 
 
 Numerous and interesting additions have been 
 made to the list of variable stars. A very interest- 
 ing variable of the type of Algol "the slowly 
 winking star " was discovered at Potsdam by 
 Messrs. Miiller and Kempf . It varies from about 
 6*9 to 8'0 magnitude, and it has a secondary mini- 
 mum of 7*35. These magnitudes give the relative 
 brightness at the maximum and minima in the 
 ratio of three, two, and one ; and if the eclipses 
 are central, it is easy to show that the phenomena 
 may be satisfactorily explained by supposing two 
 components of equal size, of which one is twice as 
 bright as the other. It has been computed that 
 the two stars revolve in their orbit in a period of 
 3 days, 23 hours, 49 minutes, and 32'7 seconds. 
 The "Algol variable," W Delphini, has the 
 greatest variation of this class known, namely 
 2'71 magnitudes that is, the light of the star at 
 maximum is about 12 times the light at minimum. 
 
ADVANCES IN STELLAR ASTRONOMY 275 
 
 Next coines U Cephei, which varies 2' 44 magni- 
 tudes. The variation of Algol itself is only 1*2 
 magnitude, and U Ophiuchi only varies 0*66 mag- 
 nitude. Several other variables of the Algol type 
 have recently been detected. 
 
 A variable, remarkable for its large variation 
 and comparatively short period, was discovered in 
 1896 by Miss Louisa Wells near Schmidt's Nova 
 Cygni. It varies from 7*2 to 11*2 magnitude, with 
 a period of about 40 days. Here the light at 
 maximum is 40 times the light at minimum. It 
 lies about half a degree north folloiving the star 
 75 Cygni. 
 
 A number of variable stars have been discovered 
 by Mrs. Fleming at the Harvard Observatory from 
 an examination of photographs of stellar spectra. 
 A number of these interesting objects have also 
 been found by Dr. Anderson, the discoverer of 
 Nova Aurigce and Nova Persei. 
 
 Dr. W. J. S. Lockyer has undertaken a discussion 
 of the variations of the well-known variable star 
 f] Aquitae. He found about 12,000 observations 
 available for the purpose. Of these, over 7000 
 were made by the late Dr. Julius Schmidt, of the 
 Athens Observatory, discoverer of the Nova Cygni 
 of 1876. Dr. Lockyer finds that Argelander's mean 
 value for the period cannot be much improved 
 upon at present. He finds, however, that there 
 are oscillations of a few hours in the times of 
 maxima and minima. These cause a variation in 
 
276 STUDIES IN ASTRONOMY 
 
 the period between 7 days, 4 hours, 14 minutes, 
 40 seconds, and 7 days, 4 hours, 13 minutes, 28 
 seconds. Dr. Lockyer finds that one secondary 
 maximum (among others) occurs 15 hours after 
 the principal minimum. From spectroscopic ob- 
 servations of the star for motion in the line of 
 sight, Belopolsky finds evidence of orbital motion 
 in a period of 7 days, 4 hours, but he thinks that 
 the variation of light cannot be caused by an 
 eclipse (as in the case of Algol), as the time of 
 observed minima does not coincide with the time 
 of an eclipse in the computed orbit. He finds a 
 somewhat similar result in the case of the variable 
 star 8 Cephei. A small variable star with a re- 
 markably short period known as U Pegasi was 
 discovered by Dr. Chandler in 1894. According to 
 Chandler, the period is about 4J hours, but from 
 photometric measures made by Wendell, Professor 
 Pickering makes it about 9 hours, or double the 
 period found by Chandler. Another variable star 
 of very short period was found by Professor Bailey 
 in the globular cluster o> Centauri. The period is 
 about 7 hours, 11 minutes, so that this curious star 
 goes through all its light changes three times in 
 24 hours ! 
 
 A large number of variable stars have been 
 found in globular clusters. Professor Bailey 
 has found at least 87 in the cluster Messier 3 in 
 Canes Venatici. In some cases the variation of 
 light is two magnitudes or more, and some have 
 
ADVANCES IN STELLAR ASTRONOMY 277 
 
 very short periods, only a few hours. In the 
 cluster No. 5272 of the New General Catalogue, 
 Bailey found 113 variable stars. In Messier 5, 85 
 have been found out of 750 stars, and in o> Centauri 
 122! Variables have also been found in other 
 clusters, but in the well-known cluster in Hercules, 
 Messier 13, there are very few, if any. 
 
 With reference to the probable temperature of 
 stars of the "Orion type," it has been found by 
 Kayser and Runge that in the spectrum of mag- 
 nesium the triplet of lines known as 6 cannot exist 
 at a very high temperature, and as they are absent 
 in the spectra of Rigel and other stars of the 
 Orion type, it has been inferred that the tempera- 
 ture of these stars must be higher than that of 
 the electric spark. This agrees well with the 
 great brilliancy of Rigel, notwithstanding its 
 great distance. 
 
 The presence of oxygen has been determined in 
 the spectra of ft Crucis and /? and e Canis Majoris, 
 also hydrogen, helium, and probably carbon and 
 magnesium. Sir William Huggins finds that in 
 stars whose spectra show strong lines of helium, 
 such as Bellatrix and Rigel, there are dark lines 
 which probably coincide with the lines of nitrogen. 
 
 As is well known, the nature of the substance 
 giving the three well-known lines in the spectrum 
 of the gaseous nebulae has not yet been determined. 
 For this unknown substance the name " nebulium " 
 has been suggested by Sir William Huggins, and 
 
278 STUDIES IN ASTRONOMY 
 
 the term has been adopted by Sir William 
 Crookes. 
 
 Professor Barnard finds that there is no trace 
 of any nebulosity in any of the great globular 
 star clusters, as seen in the great Yerkes telescope. 
 
 The 3-feet reflector, presented by Mr. Crossley 
 of Halifax to the Lick Observatory, has been used 
 for photographing stars and nebulae, and with 
 considerable success. The photographs show stars 
 and nebulae " far beyond the range of any visual 
 telescope." Keeler thought that the total number 
 of nebulae which would be shown in the whole sky 
 would much exceed 120,000 ! And it is remarkable 
 that most of these nebulae seem to be spiral. 
 
 It has been found by Schaeberle that photo- 
 graphs taken by Dr. Max Wolf with a Voightlaiider 
 lens of 6 inches aperture, show as many stars as 
 the 36-inch telescope of the Lick Observatory ! 
 This is remarkable, as Max Wolf's station is near 
 the level of the sea, whereas the Lick telescope is 
 placed at a height of about 4000 feet above sea- 
 level. This shows the power of photography to 
 reveal faint stars. 
 
 Several of those interesting and mysterious 
 objects, known as "new" or "temporary stars," 
 have been discovered during the last few years by 
 Mrs. Fleming from an examination of photographs 
 of stellar spectra taken at the Harvard Observatory 
 (U.S.A.). One of about the 9th magnitude seems 
 to have appeared in the constellation Perseus in 
 
ADVANCES IN STELLAR ASTRONOMY 279 
 
 1887, but it could not be found with the telescope 
 in after years. One in the southern constellation 
 Norma appears to have reached the 7th magnitude. 
 Its spectrum was similar to that of Anderson's 
 new star in Auriga (1892), and, like that star, it 
 seems to have faded into a planetary nebula. 
 Another, of about the 8th magnitude, was found 
 by Mrs. Fleming in the southern constellation 
 Argo. It seems to have appeared between 
 March 5 and April 8, 1895. The spectrum was 
 apparently the same as that of the new stars in 
 Auriga and Norma. Another star of the same 
 kind was also found by Mrs. Fleming on photo- 
 graphs of the constellation Centaurus. It was 
 about the 7th magnitude, and blazed out some 
 time between June 14 and July 8, 1895. It was 
 observed visually on December 16, 1895, by Pro- 
 fessor O. C. Wendell, with a 15-inch telescope, and 
 had then faded to the llth magnitude. It was 
 situated in the outskirts of a small nebula (N. G. 
 Cat., No. 5253). The spectrum was not similar to 
 those of the temporary stars hi Auriga, Norma, 
 and Argo; but, like those stars, "it appears to 
 have changed into a gaseous nebula." Early in 
 the year 1898, or possibly towards the end of 1897, 
 a new star appeared in the constellation Sagit- 
 tarius. It was detected by Mrs. Fleming on 
 photographic plates taken in March and April, 
 1898. These photographs show that on March 8 
 it was about 4*7 magnitude, and on March 13, 
 
280 STUDIES IN ASTRONOMY 
 
 5'0 magnitude, so that on those dates it must 
 have been easily visible to the naked eye. On 
 April 3 it had faded to 8'2 magnitude, and, with 
 some slight fluctuations of light, remained about 
 this brightness during the month of April. It was 
 observed visually on March 13, 1899, by Wendell, 
 and he estimated it 11*37 magnitude 011 the photo- 
 metric scale. A photograph of the spectrum, 
 taken on April 19, 1898, shows the hydrogen lines 
 bright, and some other narrow bright lines, which 
 appear to be identical with lines in the spectrum 
 of Anderson's new star in Auriga. When observed 
 by Wendell on March 13, 1899, its light was found 
 to be nearly monochromatic (that is, of nearly one 
 colour), showing "the chief nebular line," and a 
 faint continuous spectrum. It would seem, there- 
 fore, that this star like other " new " stars has 
 " changed into a gaseous nebula." 
 
 Another small " Nova," also discovered by Mrs. 
 Fleming, appeared in April, 1899, in the constella- 
 tion Aquila. It was of the 8th magnitude in 
 April, 1899, and in July, 1900, it was found to be 
 " a nebula of the 12th magnitude." 
 
 The great new star of 1901 will be described in 
 the next chapter. The discovery of so many of 
 these temporary stars in the last few years 
 suggests the idea that the phenomena may not 
 be so rare as is generally supposed. But unless a 
 new star becomes clearly visible to the naked eye 
 it might very easily escape detection. It is an 
 
ADVANCES IN STELLAR ASTRONOMY 281 
 
 interesting fact that most of these temporary 
 stars have blazed out in or near the Milky Way. 
 The principal exceptions to this rule are : the star 
 of 76 B.C. in the Plough, the star recorded by 
 Hepidanus in A.D. 1012, and the " Blaze Star " in 
 Corona Borealis in 1866. , 
 
 To explain the phenomenon of temporary stars 
 several hypothesis have been advanced. Tycho 
 Brahe thought they might be formed from the 
 cosmical vapour of which the Milky Way was 
 composed, an hypothesis which was supported by 
 Kepler. Sir Isaac Newton seems to have thought 
 that they were in some way related to comets. 
 In 1865, Zollner advanced the hypothesis that the 
 phenomenon of a new star might be due to the 
 sudden rupture of a crust beginning to form on 
 the surface of a cooling-down star. This hypo- 
 thesis was supported by Vogel in 1877. Huggins 
 and Miller suggested that the outburst of light in 
 the "Blaze Star" in Corona Borealis may have 
 been due to a convulsion taking place in the body 
 of the star, causing the evolution and combustion 
 of hydrogen and other gases. Lohse, in 1887, 
 suggested chemical combinations of gases cooling 
 down as a probable cause. In the same year 
 Lockyer advanced the theory of a collision between 
 two meteoric swarms. In 1885 Mr. Monck 
 suggested with reference to the new star in 
 the Andromeda nebula that "as shooting stars 
 are know to be dark bodies rendered luminous 
 
282 STUDIES IN ASTRONOMY 
 
 for a short time by rushing through an atmo- 
 sphere, new stars are dark (or faintly luminous) 
 bodies which acquire a short-lived brilliancy by 
 rushing through some of the gaseous nebulse 
 which exist in space." A direct or " grazing " 
 collision between two dark bodies has also been 
 suggested as a possible explanation, the arrested 
 motion being converted into heat. A direct 
 collision between two large bodies moving in 
 opposite directions seems very improbable, as the 
 result would be the formation of an enormous 
 nebulous mass which would not cool down for 
 probably millions of years. The rapid fading 
 away of the light of temporary stars seems 
 directly opposed to such an hypothesis. A 
 "grazing" collision is also open to a similar 
 objection. A near approach of two dark bodies 
 might, however, produce tides in their liquid 
 interior which would probably cause an explosion 
 in one or both bodies. M. Flammarion, the 
 famous French astronomer, has suggested that 
 a body surrounded by a hydrogen atmosphere, a 
 comet for example, grazing a dark body enveloped 
 in an atmosphere of oxygen would be sufficient to 
 produce a tremendous explosion. Mr. Monck's 
 hypothesis of a dark body rushing through a 
 nebula, is perhaps as probable as any other, and it 
 seems strengthened by the discovery of the nebula 
 surrounding the new star in Perseus, an account of 
 which will be found in the following chapter. 
 
ADVANCES IN STELLAR ASTRONOMY 283 
 
 Should our sun in its journey through space 
 pass through a mass of nebulous matter, its heat 
 and light would be vastly increased by the friction 
 produced; and "the heavens being on fire" the 
 earth would be "burned up," and St. Peter's 
 prediction of a general conflagration would at 
 once be fulfilled. 
 
 Numerous other discoveries have been made in 
 the years 1901 to 1903, and the following are some 
 of the most interesting results which have been 
 obtained. 
 
 Professor Campbell (Director of the Lick 
 Observatory) finds with the spectrograph that 
 the so-called "runaway star," Groombridge 1830, 
 is approaching the earth with a velocity of 59 
 miles a second. The motion at right angles to 
 the line of light shown by its large "proper 
 motion" is considerably greater, probably 140 
 miles a second or more. 1 
 
 Prom the radial velocity (or velocity in the line 
 of light) of 280 stars measured with the spectro- 
 graph, Professor Campbell finds that the rate of 
 the sun's motion is about 11/8 miles a second. 1 
 Professor Kapteyn deduces a velocity of 11-44 
 miles a second, so it would seem that this motion 
 has now been satisfactorily determined. A curious 
 result of Professor Campbell's inquiry is that 
 "the fainter stars seem to be moving, on an 
 average, more rapidly than the brighter." 1 This 
 1 Astronomical Society of the Pacific, April, 1901. 
 
284 STUDIES IN ASTRONOMY 
 
 seems corroborated by the fact that of the 13 
 stars which have the largest proper motions 
 (above 3" per annum), 9 are fainter than the 5th 
 magnitude. 
 
 Professor Max Wolf of Heidelbergh finds "a 
 nest or cluster of nebulae" in Coma Berenices, 
 nearly due west of the star ft, and preceding it 
 about 13 minutes of time. No fewer than 108 
 nebulae seem to be gathered together within an 
 area not exceeding in extent that of the full 
 moon. They are mostly small and nearly circular 
 in form. 1 A cluster of nebulae is certainly an 
 unique object. 
 
 Sir David Gill finds that the spectrum of the 
 remarkable variable star rj Argus contains lines 
 which closely resemble lines in the spectrum of 
 Nova Aurigae. 2 This curious star would therefore 
 seem to be a sort of connecting-link between the 
 temporary stars and the long period variables. 
 Mr. R. T. A. Innes found its magnitude 7*68 in 
 1900 ; 7'78 in 1901 ; and 7'72 in 1902 ; so that an 
 increase of light has apparently not yet set in. 
 
 A remarkable variable of the Algol type has 
 been discovered in Sagitta by Professor Schwab. 
 It has a period of 3'38 days, and varies from 6 
 to 9th magnitude. At maximum it is conspicuous 
 in an opera glass, but at minimum it is quite 
 invisible in such an instrument. Its position for 
 
 1 Astronomische Nachrichten, 3704. 
 
 2 Monthly Notices, R.A.S., LXL, App. 3. 
 
ADVANCES IN STELLAR ASTRONOMY 285 
 
 1900 is R.A. 19 h 14 m 26 s , N. 19 25'-4. The presence 
 of a third body is suspected. A number of new 
 variable stars have been discovered in the last 
 three years, but they are mostly faint, even at 
 their maximum light. 
 
 Dr. A. W. Roberts finds for the southern variable 
 T Centauri, discovered by Colonel Markwick, a 
 mean period of 90*3 days, and a variation from 
 5*2 to 9'0 magnitude. 1 This is a very interesting 
 star, as it can be followed through most of its 
 phases with a binocular field glass. 
 
 From " a rough attempt to determine the total 
 light of the stars by direct observation," Professor 
 Simon Newcomb finds that " the total light of all 
 the stars is about equal to that of 600 stars of 
 zero magnitude [like a Centauri or Vega] with 
 a f ' probable error of one-fourth of the whole 
 amount." 2 From statistics relating to the number 
 of the stars and nebulae, the present writer com- 
 puted that the total light was equal to 589 stars 
 of zero magnitude (Knmvledge, August, 1901). His 
 article on this subject appeared several months 
 before Professor Newcomb' s paper was published. 
 
 From spectroscopic observations of nebulae in 
 the line of sight, Dr. Hartmann and Professor 
 Keeler found that the nebula No. 4373 of the 
 General Catalogue is approaching the earth with 
 a velocity of about 40 miles a second. Hartmann 
 
 1 Monthly Notices, R.A.S., November, 1901. 
 - Knowledge, March, 1902. 
 
286 STUDIES IN ASTRONOMY 
 
 found a slightly different velocity in the middle 
 and the edges of some nebulae, which indicate 
 " relative motions in the nebulae themselves." l 
 
 Professor Seeliger finds that stars of 11 or 11 J 
 magnitude are comparatively few in number 
 near the poles of the Milky Way, but are very 
 numerous in the Galactic zone. This is also true 
 for fainter stars, such as those seen by Sir William 
 Herschel in his "gauges." Easton thinks that 
 the stellar universe is of "a fairly thick lens 
 shape, filled with stars which are much more 
 densely congregated near the edges than near the 
 centre of the lens." He thinks that the southern 
 pole of the Milky Way is probably less rich in 
 stars than the northern. Seeliger finds that the 
 distance between the sun and the internal border 
 of the Milky Way is about 500 times the distance 
 of Sirius (parallax = 0"'37), and the external 
 border 1100 times the same distance; and he 
 thinks that the system probably contains from 27 
 to 41 millions of stars down to the 13th or 14th 
 magnitude. He does not admit any appreciable 
 extinction of light in our stellar system, but 
 extinction might cut off the light of external 
 galaxies. 2 This was suggested by the present 
 writer in his book, " The Visible Universe " (1893), 
 p. 322. 
 
 Experiments made at the Nice Observatory by 
 Perrotin, on the velocity of light by the toothed- 
 1 Nature, April 24, 1902. 2 Knowledge, July, 1902. 
 
ADVANCES IN STELLAR ASTRONOMY 287 
 
 wheel method under improved conditions, give 
 as the result of 1109 observations the final value 
 of 299,880 kilometres, or 186,339 miles a second, 
 with a probable error of less than 50 kilometres, 
 or 31 miles. This result agrees closely with pre- 
 vious determinations. According to Perrotin, the 
 value of the solar parallax, from observations of 
 the planet Eros at Nice, gives 8*805 0*011, and 
 from this he deduces a value of 20"*465 for the 
 constant of aberration, " thus confirming the value 
 adopted by the International Astronomical Con- 
 ference of 1896." l These results are also satis- 
 factory, for we have 8*805 x 20*465 = 180*194325, 
 and theoretically the product of the two constants 
 should be about 180. 
 
 M. Lau of Copenhagen finds variation of colour 
 in a. Ursae Majoris (the northern of the two 
 pointers in the " Plough "). According to his 
 observations, the colour varies from 2*7 to 5*3 (on 
 a scale of to 10) in a period of 50 days. The 
 star is usually yellow, but slightly reddish at 
 maximum. 2 About 25 years ago, Klein found a 
 similar variation, with a period of 30 days. 
 
 On March 16, 1903, a small new or temporary 
 star was discovered in the constellation Gemini 
 by Professor Turner, at the Oxford Observatory. 
 It was then about the 7th magnitude, and was 
 found on a photographic plate. It was of a red 
 
 1 Nature, December 11, 1902. 
 
 - Bulletin de to 8oc. Astronomtque de France, March, 1903. 
 
288 STUDIES IN ASTRONOMY 
 
 colour, and showed bright lines in its spectrum. 
 It was afterwards found on plates taken at the 
 Harvard Observatory (U.S.A.). These showed 
 that on March 1 it was not brighter than the 12th 
 magnitude, but on March 6 it was about the 5th 
 magnitude. Its rise in brilliancy was therefore 
 probably very rapid, as is usually the case with 
 these remarkable objects. On April 4, 1903, 
 Parkhurst found it about 9th magnitude. On 
 August 17, Professor Pickering found that its 
 spectrum had become that of a nebula. Its 
 position for 1900 is R.A. 6 h 37 m 49 s , N. 30 2' 38". 
 
 The binary star 8 Equulei has the very short 
 period (for a visual double star) of about 5*7 years. 
 Hussey finds, from spectroscopic observations, a 
 parallax of 0"'071, which he thinks must be very 
 near the truth. 1 The sun, placed at the distance 
 indicated by this parallax, would, I find, be 
 reduced to a star of about 5*8 magnitude ; and as 
 the star's photometric magnitude is 4/6, it follows, 
 that the star is about 3 times brighter than the 
 sun. Its spectrum is of the second type (P). The 
 star's brightness seems to agree well with its 
 parallax and spectrum. 
 
 The star < 2 Orionis (one of the stars in the 
 " head of Orion ") has been found by the spectro- 
 graph to be receding from the earth with the 
 great velocity of about 60 miles a second ! 2 
 
 1 Astronomical Society of the Pacific, April 10, 1903. 
 
 2 Nature, May 7, 1903. 
 
ADVANCES IN STELLAR ASTRONOMY 289 
 
 Professor Pickering finds that the total number 
 of stars down to 6*6 magnitude is about 10,000 ; 
 the number to 8'7 magnitude is, he thinks, about 
 100,000; to the llth magnitude, about 1,000,000; 
 and to magnitude 11'9, about 2,000,000. x 
 
 For the close pair of the triple and ternary star 
 Hydrse, Professor Aitken finds a period of 15*7 
 years for the close pair. The spectrograph also 
 shows it to be binary, and the observations seem 
 to indicate that "the visual and spectroscopic 
 binary systems are identical." 
 
 Prom photographs taken with the Crossley 
 reflector at the Lick Observatory, Professor 
 Schaeberle finds that the Ring nebula in Lyra 
 and the " Dumb-bell " nebula are both spirals. 
 
 A number of new spectroscopic binaries have 
 been found in the last few years. Among these 
 may be mentioned a Equulei, rj Orionis, v Andro- 
 medsB, o- Geminorum, Tauri, rj Virginis, Aurigae, 
 T Tauri, i^ Orionis, o Persei, 8 Aquilae, Aquilse, 
 a Draconis, a Coronse Borealis, /? Arietis, and 
 Ursse Majoris. According to Vogel, the period 
 of o Persei is about 4*39 days, with a maximum 
 velocity of about 65 miles a second. The Algol 
 variable 8 Librae has also been found to be a 
 spectroscopic binary, with a radial velocity vary- 
 ing from +23J to 76 miles a second. For 
 TJ Orionis, Adams finds a period of 7*9896 days. 
 
 1 Annals of Harvard College Observatory, vol. xlviii., No. v., 
 p. 179. 
 
 U 
 
290 STUDIES IN ASTRONOMY 
 
 The companion is relatively dark. " The range of 
 velocity is very great, amounting to over 285 
 kilometres," or 177 miles a second ! The star is 
 also a telescopic double (3J, 4J : 1"*24), but there 
 seems to be no relative motion in the visual pair. 
 
 Photographs taken with the Crossley reflector 
 at the Lick Observatory show that the spectrum 
 of Nova Cygni (1876) "has become continuous, 
 and that the spectrum of Nova Aurigae (1892) is 
 " approaching the continuous type." Mr. Palmer, 
 who took the photographs, says, " These complete 
 and astonishingly rapid changes of spectral type 
 observed in the cases of Nova Cygni and Nova 
 Aurigce, and likewise those observed in Nova 
 Normce, Nova Sagittarii, Nova Persei, etc., leave 
 little doubt that the masses of these objects is 
 small." l This seems to the present writer the 
 only conclusion admissible. 
 
 Herr Kostinsky, of the Pulkowa Observatory, 
 has found an absolute parallax for the bright star 
 ft Cassiopeise, from observations by himself (with 
 the prime vertical instrument) and by Dr. Nyren, 
 the mean result being 0"*14, with a probable error 
 of 0"*03. A parallax for this star was found by 
 the late Professor Pritchard, at Oxford, by pho- 
 tography. His result was 0"*15. Herr Kostinsky 
 " arrives at the conclusion that the absolute value 
 of the parallax of (3 Cassiopeise is, with great pro- 
 bability, very near +0"*1, and rather a little 
 1 Aetr&physical Journal, October, 1903. 
 
ADVANCES IN STELLAR ASTRONOMY 291 
 
 greater than less." 1 A parallax of 0"'l would 
 reduce the sun to a star of the 5th magnitude, and 
 as the photometric magnitude of ft Cassiopeise is 
 2*42, it must be a much larger sun than ours. Its 
 spectrum is of the second type (F 5 G Pickering). 
 
 Adams and Frost find that the star Persei is 
 apparently receding from the earth with the great 
 velocity of over 52 miles a second, and they think 
 it may possibly be a spectroscopic binary, as most 
 of the stars with the " Orion type " of spectrum 
 have a low radial velocity. 2 
 
 From spectroscopic observations of the spectro- 
 scopic binary ft Aurigse, M. Tikhoff, of the Pul- 
 kowa Observatory, has arrived at the conclusion 
 that the system consists of two pairs, " each pair 
 consisting of a star giving strong lines and another 
 giving weak lines, and each element making a 
 complete revolution about the centre of gravity 
 of its pair in 19'1 hours." The period of revolu- 
 tion of both pairs round their common centre of 
 gravity is 3 d 23 h 30 m '4. 3 This seems to be the first 
 example found of a spectroscopic quaternary 
 system. 
 
 From an elaborate investigation of the observa- 
 tions of the variable star c Aurigae, M. H. Luden- 
 dorff arrives at the conclusion that it is an Algol 
 variable, with a period of either 27*12 years or 
 
 1 Nature, November 12, 1903. 
 
 - Astrophysical Journal, December, 1903. 
 
 3 Nature, December 24, 1903. 
 
292 STUDIES IN ASTRONOMY 
 
 54J years. The middle of the last minimum was 
 1902, March 31. He finds the whole duration of 
 light change to be about 2 years, the duration of 
 minimum light 313 days, and the times occupied 
 in passing from maximum to minimum and from 
 minimum to maximum being each about 207 
 days. 1 According to Vogel, the star is a spectro- 
 scopic binary with probably a very long period." 
 But the spectroscopic measures do not seem to 
 agree with Ludendorif s conclusion. 
 
 In an interesting and suggestive paper by Pro- 
 fessor Arthur Schuster, on " The Evolution of Solar 
 Stars," in the Astrophysical Journal, April, 1903. 
 he considers that the difference between a solar 
 star and one having a spectrum like Arcturus, 
 "may not be one of age at all, but mass." "If 
 the Arcturian star is one which is bigger, it will 
 be able to absorb the hydrogen more completely, 
 and the final state of equilibrium will be such that 
 the hydrogen lines will be thinner than in the 
 Capellan or solar star." He says this theory 
 "gives an explanation of a very curious fact, 
 which I venture to think has not so far been 
 satisfactorily accounted for. In the case of double 
 stars, it is often found that the brighter one is 
 yellow, and gives a solar spectrum, while the 
 smaller one is blue, and gives a hydrogen spectrum. 
 The larger one, though it may have originally 
 
 1 Astronomiscfie Nachinchten, No. 3920. 
 
 2 Astrophysical Journal, April, 1903. 
 
ADVANCES IN STELLAR ASTRONOMY 293 
 
 attracted more hydrogen to itself, will be able to 
 absorb it more rapidly, and thus pass through the 
 stages of spectroscopic evolution more quickly." 
 This fact has always been a perplexing one in the 
 theory of stellar evolution, and Professor Schuster's 
 explanation seems a very probable one. 
 
 Mr. Joel Stebbins finds that the famous variable 
 star Mira Ceti is receding from the earth with a 
 constant velocity of about 41 miles a second, " and 
 this is held to be strong argument against the 
 theory that the light changes are due to the exist- 
 ence of a companion." Mr. Stebbins concludes, 
 from the spectroscopic observations, that "the 
 light changes are due to internal causes, which 
 produce effects that are, as yet, unfamiliar to 
 us." l 
 
 In December, 1903, Professor Schaeberle an- 
 nounced his discovery of a spiral structure in 
 the great cluster in Hercules (Messier 13). 2 " Nebu- 
 lous streams joining certain stars in curved lines 
 could be traced up to the very centre of the 
 cluster." There seems to be two spirals, one 
 " clock- wise," and the other " counter clock- wise." 
 " A similar structure on a much larger scale exists 
 in the stars and nebulosity surrounding y Cassio- 
 pei<z" and in Schaeberle's opinion, " the majority 
 of the stars both bright and faint within half a 
 degree of y Cas&iopeice, belong to a single physical 
 
 1 Nature, December 31, 1903. 
 
 The Astronomical Journal, No. 552. 
 
294 STUDIES IN ASTRONOMY 
 
 system." This tends to show that in many cases 
 the bright and faint stars in the Milky Way are 
 practically at the same distance from the earth. 
 
 Some experiments made by M. Fabry indicates 
 a value for the "sun's stellar magnitude" of 
 -26-7, or 60,000 million times the light of Vega. 
 This differs but little from the value of -26'5, 
 which I have adopted in the present volume. 
 
XXIV 
 
 The New Star in Perseus 
 
 ON the evening of Friday, February 22, 1901, 
 while returning home from the house of 
 a friend in Dublin, about ll h 40 m p.m., 
 Greenwich mean time, I happened to look towards 
 the constellation Perseus, and was astonished to 
 see a bright star of nearly the 1st magnitude 
 shining in a spot where I knew that no star visible 
 to the naked eye had previously existed. Next 
 morning I telegraphed to the Observatories at 
 Greenwich and Edinburgh, and also to Sir William 
 Huggins, the famous astronomer. In a reply from 
 Dr. Copeland, Astronomer Royal of Scotland, he 
 informed me that the new star had been discovered 
 011 the morning of February 22 at 2 h 40 m a.m., by 
 Dr. T. D. Anderson of Edinburgh, the well-known 
 discoverer of Nova Aurigae in 1892. The new star 
 was also independently discovered by Mr. Ivo 
 F. H. C. Gregg at St. Leonards on February 22, at 
 6" 40 m p.m., and by the Rev. T. E. Espin, the Rev. 
 S. J. Johnson in England, and by Dr. Bonnel in 
 Paris on the same evening. Also by Mr. E. B. 
 
296 STUDIES IN ASTRONOMY 
 
 Frost in America, and Mr. A. F. Miller at Toronto, 
 Canada. It seems to have been also independently 
 discovered by Mr. Laursen-Nordvig in Denmark, 
 and by Messrs. Kvasnikoff and Sviatsky in Russia. 
 It was seen on the following evening, February 23, 
 by Mr. H. Wake and Mr. W. B. Dodd at White- 
 haven, England, and by several observers in 
 France. Its rise in brightness must have been 
 very rapid. Mr. Dodd stated l that on the even- 
 ing of Thursday, February 21, he happened to 
 look at Perseus, and w^as sure that up to 12 p.m. 
 on that night there was no bright star visible in 
 the spot where the new star blazed out within 
 three hours afterwards. Three German astrono- 
 mers, Messrs. Grimmler, Plassmami, and Schwab, 
 also stated that they were observing the region on 
 February 21, from 7 h to 10 h 30 m p.m., and think 
 that no star brighter than the 3rd magnitude 
 could possibly have escaped their notice. Mr. 
 Espin states that he was observing that part of 
 the sky on February 20, and was sure that the 
 new star was not then visible. The region was 
 photographed on the night of February 19 at 
 Harvard College Observatory (U.S.A.), and the 
 photograph shows stars down to the llth mag- 
 nitude, but no trace of the new star is visible on 
 the plate. A photograph of the region, taken by 
 Mr. Stanley Williams on February 20, about 11 
 p.m. only 28 hours before its discovery by Dr. 
 1 English Mechanic, March 8, 1901, p. 77. 
 
THE NEW STAR IN PERSEUS 297 
 
 Anderson shows 110 trace of the Nova, although 
 it contains stars to about the 12th magnitude. 
 
 When first seen by Dr. Anderson to whom the 
 honour of its discovery is due he estimated that 
 it "somewhat surpassed the 3rd magnitude" in 
 brightness, and was about half a magnitude 
 fainter than the Pole Star, or about 2'7. He 
 stated that he was observing another part of 
 the heavens at the time, and happening accident- 
 ally to look towards the constellation Perseus, he 
 at once saw that a new star had appeared in the 
 sky. In a letter to a friend, Dr. Anderson re- 
 marked, " Oh, what an absurd sonnet is that in 
 which Keats brackets together the discovery of 
 an ocean, and the discovery of a new celestial 
 world. As if the finding of any terrestrial sheet 
 of water, however large, could be compared for a 
 moment as a source of joy with the first glimpse 
 of a new glory in the already glorious firmament." 
 
 The new star rapidly increased in brilliancy. 
 On the evening of February 22, when first seen by 
 the present writer, it was about the 1st magnitude, 
 and later on the same night it was estimated to 
 be still brighter by observers in America. On the 
 next evening, February 23, it had further increased 
 in brightness, and at the Harvard Observatory 
 it was thought to be "brighter and bluer than 
 aAurigae" (Capella). It was then a really bril- 
 liant object, and probably the brightest star in 
 the northern hemisphere! On the evening of 
 
298 STUDIES IN ASTRONOMY 
 
 February 24 I thought it fully equal to Capella ; 
 and on that day at Harvard it was seen with the 
 6-inch equatorial and its 2-inch finder in strong 
 sunlight. On February 25 it had faded to nearly 
 1st magnitude, and on February 26 I estimated it 
 as intermediate in brightness between Capella and 
 a Persei. On March 1 it was reduced to about 
 the 2nd magnitude, and on March 6 to about the 
 3rd. From that date it faded with some small 
 fluctuations until March 18, when it had de- 
 scended to about the 4th magnitude. From that 
 time a series of the most remarkable fluctu- 
 ations of light set in. On the evening of March 19 
 it had fallen a little below the 5th magnitude. On 
 March 20 it had risen to 3J magnitude, and on the 
 22nd it was again below the 5th magnitude. It 
 was again about the 4th magnitude on March 23, 
 and below the 5th magnitude 011 March 25. It 
 again rose to above the 4th magnitude on March 
 26, and these curious fluctuations continued with 
 more or less regularity, and with a longer period 
 of variation, until the third week in May, when 
 the star became so low on the northern horizon, 
 and the twilight so strong, that further observa- 
 tions became very difficult. During the month of 
 June, the observations show considerable fluctu- 
 ations of light, and to a smaller extent during 
 July also. In August the fluctuations of light 
 were small, the estimates of magnitude ranging 
 from about 5*7 to 6*5. During September and 
 
THE NEW STAR IN PERSEUS 299 
 
 October, the star's light seemed to fade slowly, 
 with no violent fluctuations, from about 6*2 
 to 6'7. At the end of the year it had fallen 
 to about the 7th magnitude. In March, 1902, it 
 had faded to the 8th magnitude ; in June, 1902, to 
 the 9th magnitude; and in November, 1902, to 
 about the 10th magnitude. In April, 1903, Pro- 
 fessor E. E. Barnard estimated it 10 J magnitude, 
 and on July 30, 1903, Professor Perrine found it 
 11*5 or 12 magnitude. 
 
 When first seen by Dr. Anderson he thought 
 its colour was bluish-white, and it remained of a 
 white or slightly yellow colour on February 23 
 and 24. It was of a pale yellow on February 25 
 and 26, and became orange at the beginning of 
 March. The colour during the remarkable oscil- 
 lations of brightness seems to have been orange at 
 maximum and red at minimum. Early in 1902. 
 Professor Barnard found it " greenish-white." 
 
 According to Professor E. C. Pickering, the 
 spectrum of the new star was 011 February 22 and 
 23 of the Orion type, " nearly continuous, with 
 narrow dark lines." On February 24 there was a 
 remarkable change, the spectrum having then 
 become like that of other new stars, that is, crossed 
 by dark and bright bands, the principal dark lines 
 being bordered by bright lines on the red side. 
 The observed displacement of the hydrogen lines, 
 from their normal position in the spectrum, seemed 
 to indicate a relative velocity of 700 to 1000 miles 
 
300 STUDIES IN ASTRONOMY 
 
 a second, thus suggesting the collision of two 
 bodies with high velocities ; but these enormous 
 velocities of colliding bodies seem contradicted by 
 the fact that measures of the dark lines of calcium 
 and sodium by Messrs. Adams, Campbell, Wright, 
 and Stebbins in America indicate a velocity in the 
 line of sight of only some 3 miles a second. The 
 observed high velocities may have been, however, 
 possibly due to an outburst of hydrogen gas from 
 the body of the star. Remarkable changes were 
 also observed in the spectrum during the sudden 
 fluctuations in the star's light which took place 
 in March, April, and May. An examination of 
 photographs of the spectrum taken at the Harvard 
 Observatory in July, 1901, showed that like other 
 new stars it was slowly changing into a gaseous 
 nebula, the " chief nebular line " being very bright. 
 The nebular spectrum became more marked in 
 August and September. 
 
 An apparent nebular aureole round the star was 
 found on photographs by Messrs. Antoniadi and 
 Flammarion in August, 1901, and this was con- 
 firmed afterwards by Max Wolf, Kostinsky, and 
 Von Gothard. This was explained as due to the 
 exceptionally strong ultra violet rays emitted by 
 the new star, rays for which the object glasses of 
 the telescopes used were not corrected. The 
 correctness of this explanation was proved by the 
 fact that photographs taken by reflecting tele- 
 scopes did not show the supposed aureole. 
 
THE NEW STAR IN PERSEUS 301 
 
 Dr. Max Wolf, while making an examination of 
 the supposed aureole, discovered a faint trace of 
 real nebula a little south of the new star. As his 
 telescope was not powerful enough to deal with 
 this faint object he suggested that it should be 
 photographed with a large reflector. This was 
 done by Mr. Ritchey at the Yerkes Observatory 
 with a 2-feet reflector on September 20, 1901, and 
 the photograph showed a mass of nebulous matter 
 of great extent and of an apparently spiral f orni 
 surrounding the Nova. This interesting discovery 
 was confirmed by Mr. Perrine at the Lick Obser- 
 vatory by photographs taken 011 November 7 and 
 8, and from a comparison of his plates with the 
 photograph taken by Ritchey, he found that some 
 of the principal condensations of the nebula were 
 apparently moving at the enormous rate of 11 
 minutes of arc per annum. Perrine's startling 
 result was confirmed by Ritchey. This unheard- 
 of motion in a sidereal object seemed to preclude 
 the idea of a real velocity of the nebulous matter, 
 and the theory was suggested by Professor Kap- 
 teyn and Dr. W. E. Wilson that the nebulous matter 
 shone merely by light reflected from the new star. 
 Assuming this to be the case, calculation showed 
 that the observed motion would be accounted for 
 by supposing that the new star had a parallax of 
 about 0"*011, on a light journey of about 296 
 years ! Perrine afterwards announced * that he 
 1 Lick Observatory Bulletin, January 14, 1902. 
 
302 STUDIES IN ASTRONOMY 
 
 had found a photograph of the new star taken on 
 March 29, 1901, on which the nebulosity was 
 very visible. This "reflection theory" has been 
 supported by Hinks and Seeliger, but other 
 astronomers do not agree with this explanation. 
 
 Attempts to measure its distance from the earth 
 have not proved very satisfactory. From measures 
 made from small stars near it, Dr. Hartwig of 
 Bamberg, and Dr. Chase of Yale College Obser- 
 vatory, found a negative parallax, which would 
 mean that the new star is further from the earth 
 than the comparison stars used in the observations. 
 Bergstrand, however, finds from photographic 
 plates an absolute parallax of 0"*033. This would 
 imply a journey for light of about 99 years, and 
 would fix the real date of the catastrophe about 
 the year 1802. 
 
 Professor W. H. Pickering thinks that " As far 
 as the observations go the collision theory has 
 been rendered untenable, and the explosion theory 
 has been corroborated." l 
 
 On one of the earlier photographs of the region 
 taken at the Harvard Observatory a very faint 
 star was found very close to the place of the Nova 
 by Father Zwack of Georgetown College Obser- 
 vatory (U.S.A.). From measurements of photo- 
 graphs taken in the years 1890 to 1900, Professor 
 Pickering finds that this small star was variable 
 to the extent of about one magnitude, and that 
 1 Astrophysical Journal, XIII., 4. 
 
THE NEW STAR IN PERSEUS 303 
 
 its position agrees closely with that of the new 
 star. The same small star was also found by Mr. 
 S. Blajko on a photograph taken by him on 
 January 30, 1899. Professor Pickering says, " We 
 may therefore conclude that a star whose light 
 varied from the 13th to the 14th magnitude was 
 visible for several years within 1 or 2 seconds of 
 arc of the Nova, the difference in position being 
 less than the errors of measurement." 1 
 
 The position of the Nova is for 1900'0 R.A. 
 3 h 24m 24Sj N> 43 o 33 , 39 ^ It lieg between the stars 
 
 K and v Persei, a little nearer to the latter star, 
 and a little to the north of the line joining these 
 two stars. 
 
 1 Harvard College Observatory Circular, No. 66, October 31, 
 
 1902. 
 
XXV 
 
 The Coming Comet 
 
 THE return of Halley's comet will take place 
 in the year 1910. This is the most re- 
 markable and interesting of all the comets 
 with known periods. Its period is about 75 years, 
 and its returns have been traced back to B.C. 11. 
 Other returns were recorded in the years A.D. 6(5, 
 141, 989, 1066, 1145, 1223, 1301, 1378, 1456, 1531, 
 1607, 1682, 1759, and 1835. It is the comet depicted 
 in the Bayeux tapestry as having appeared at the 
 time of the Norman conquest of England in 1066. 
 At this return it seems to have been of great 
 brilliancy, as its head is described as being equal 
 to the full moon in size ! with a tail of about 
 60 in length. It was also very bright in the 
 years 1145, 1223, and 1301. It was observed in 
 1531 by Pierre Apian at Ingolstadt, and is said 
 to have been then of a " bright gold colour." In 
 1607 it was observed by Kepler and Longomoii- 
 tanus, and on this occasion its colour is described 
 as " dark and livid," although in brightness it is 
 said to have exceeded all the brightest stars, and 
 
THE COMING COMET 305 
 
 even Jupiter, with a long and thick tail. At its 
 return in 1682 it was well observed by Flamsteed, 
 Halley, Hevelius, La Hire, and Picard. Halley 
 having computed its orbit found that the comets 
 of 1531, 1607, and 1682 were identical, and pre- 
 dicted its return in 1758-59. Lalande, assisted by 
 Madame Lepaute, also computed the orbit, and 
 predicted a return in the spring of 1759. Halley 
 did not live to see his prediction fulfilled, but 
 the comet duly returned, and was first seen 
 by Palitzch, an amateur astronomer living near 
 Dresden, on Christmas Day, 1758. Its appearance 
 in 1759 was described by Dr. Nicholas Munckley 
 as " large, but very ill defined," and " very evident 
 to the naked eye," even in moonlight. 1 Fortu- 
 nately its appearance will not be spoiled by moon- 
 light in 1910. Its return in 1835 was computed 
 by Damoiseau, Lehmanii, Lubbock, Pontecoulant, 
 Rosenberger, and Stratford. Rosenberger pre- 
 dicted the perihelion passage for November 11, 
 1835 ; Stratford for November 15, and Lehmann for 
 November 26. This event actually occurred on 
 November 16 a remarkable fulfilment of an astro- 
 nomical prediction. It was first seen in August 
 " almost precisely in the spot in which Herschel 
 saw the planet Uranus." Admiral Smyth first saw 
 it on August 24 as " a nebulous blot of indistinct 
 form and misty appearance." On the 28th the 
 nucleus was visible, and very distinct on the 31st. 
 1 Philosophical Transactions, R.A.S., 1759, p. 95. 
 
 X 
 
306 STUDIES IN ASTRONOMY 
 
 On the 9th and 10th October an appearance of a 
 luminous brush or fan accompanied the nucleus. 
 On November 7, the head was seen by Struve to 
 pass over a star of the llth magnitude without 
 dimming its light in the least, and some other 
 similar cases were observed by the great Russian 
 astronomer. The comet passed through perihelion, 
 or nearest point to the sun, on November 16, after 
 which it passed into the southern hemisphere 
 and ceased to be visible in Europe. At first looking 
 like a round nebula, it began to develop a tail 
 011 October 2. This tail rather rapidly increased, 
 and was 4 or 5 long on October 5. It reached 
 its greatest length about 20 on October 15, 
 and then rapidly decreased. It also showed small 
 tails or jets turned towards the sun. In Febru- 
 ary, 1836, Sir John Herschel, observing it at the 
 Cape of Good Hope, said that the comet kept 
 him up " all night and every night," and he stated, 
 " It is altogether the most beautiful thing I ever 
 saw in a telescope. The most surprising thing 
 about it, however, is the enormous increase of its 
 dimensions within the last week, being now more 
 than triple the diameter which it had on the 20th 
 instant, when I first observed it. A few days ago 
 it threw out two feeble tails ; it has none now." * 
 
 At the next return, in 1910, the comet will be 
 very favourably situated for observation. Accord- 
 ing to present calculations, it will have, at the 
 1 Monthly Notices, R.A.S., 1836, p. 190. 
 
THE COMING COMET 307 
 
 end of October, 1909, the same theoretical bright- 
 ness as when it was last seen by Dr. Lamont, with 
 the Munich refractor, on May 17, 1836. At that 
 time its position will be near the star 130 Tauri. 
 Then, retrograding with a slow, southerly motion 
 in declination, it will pass through the constella- 
 tions Aries and Pisces in January, 1910. On June 
 12, the calculated position is close to the bright 
 star Capella, and 5 days later it will be on the 
 confines of the Lynx and Leo Minor. At this 
 period the comet will attain its least distance 
 from 'the earth about 23 millions of miles. It 
 will be most conspicuous during the first half of 
 June in the absence of the moon (full moon on 
 June 22, 1910). It will, of course, be closely 
 watched by all astronomers, and its light will be 
 examined with the spectroscope for the first time 
 in its history. 
 
 Halley's comet moves in a very elongated 
 ellipse, the eccentricity of the orbit being 0-967. 
 Its perihelion distance is about 54 millions of 
 miles, so that on its next return it will pass much 
 nearer to the earth than it does to the sun, and it 
 will probably be a pretty bright object. Its return 
 may be confidently expected. As the poet says 
 
 " The star will come. It dare not by one hour 
 Cheat Science, or falsify her calculation ; 
 Men will have passed, but, watchful in the tower, 
 Man shall remain in sleepless contemplation ; 
 And should all men have perished in their turn, 
 Truth in their place would watch that star's return." l 
 
 1 " Prudhomme." Translated by Arthur O'Shaughnessy. 
 
XXVI 
 
 Immensity and Minuteness 
 
 WE are accustomed to consider the numbers 
 dealt with in astronomy as vast and 
 wonderful. And so they are. Even 
 the nearest fixed star to the earth is placed at a 
 distance so great that it seems impossible for the 
 mind to imagine its reality. The distance of the 
 sun is very great when compared with the ter- 
 restrial distances with which we are familiar ; 
 and when we try to imagine that the distance of 
 Alpha Centauri is 271,000 times the sun's distance 
 from the earth, our mind fails to grasp the idea of 
 so vast a distance. The only way in which we 
 can hope to gain even a faint idea of this enor- 
 mous distance is to consider the time that light 
 takes to reach us from the nearest fixed star. 
 Coming from the sun in 8 minutes and 18 seconds, 
 light takes about 4^ years to reach us from Alpha 
 Centauri. And if this is the nearest of the stars, 
 how can we attempt to imagine the distance of 
 the farthest visible in the largest telescopes ? 
 
IMMENSITY AND MINUTENESS 309 
 
 But these marvels revealed to us by the tele- 
 scope are perhaps not more wonderful than the 
 facts disclosed by the microscope, and those 
 inferred by physicists with reference to the con- 
 stitution of matter. According to the molecular 
 theory of matter, all bodies solid, liquid, and 
 gaseous are composed of an enormous number of 
 molecules, all vibrating round a mean position. 
 Some have disputed this hypothesis, and contend 
 that matter may possibly be homogeneous and 
 continuous, and not composed of molecules or 
 atoms; but Cauchy has shown mathematically 
 that if matter were homogeneous, and not mole- 
 cular, there would be no dispersion of light 
 through a glass prism. The existence, therefore, 
 of the science of spectrum analysis seems to prove 
 conclusively that glass, at least, is molecular in 
 structure. And yet the molecules of which it is 
 composed are quite beyond the reach of our most 
 powerful microscopes. " Pine rulings on glass, 
 whose distance apart is less than half of the wave 
 length of light, are readily resolved with optical 
 distinctness by our modern microscopes, while the 
 intimate texture of the glass is apparently as far 
 removed from resolution as with the unarmed 
 eye." Professor Tynclall considered that the 
 world of molecules and atoms lies, " in all proba- 
 bility, vastly farther beyond the range of the 
 microscope than the range of the microscope at 
 its maximum lies beyond that of the unaided 
 
310 STUDIES IN ASTRONOMY 
 
 eye." This is like the close binary stars recently 
 discovered by the spectroscope, which are pro- 
 bably as far beyond the reach of our largest tele- 
 scopes as an ordinary telescopic double star is 
 beyond the reach of the naked eye. This mar- 
 vellous minuteness of the molecules of matter 
 seems as difficult to imagine as the vast distances 
 of the stars. 
 
 Cauchy concluded, from optical experiments, 
 that the constituent atoms of matter are so small 
 that 400 million go to an inch. Clausius and 
 Clark Maxwell found 500 millions from considera- 
 tions of gaseous phenomena. From electrical 
 experiments, Sir William Thomson (now Lord 
 Kelvin) found 700 millions to the inch. Perhaps 
 we may assume as a mean of these results that 
 500 millions of atoms placed in a straight line 
 would measure an inch. 
 
 These atoms are, of course, quite beyond the 
 power of our microscopes, as I have said ; but let 
 us consider some living organisms which can be 
 seen with the microscope. Certain forms of infu- 
 soria are so minute that an individual specimen 
 can lie between two divisions of an inch divided 
 into 25,000 parts ! Taking the height of a man at 
 6 feet, or 2 yards, a length of an inch would be 
 for this microscopical creature equivalent to a 
 distance of 50,000 yards, or about 28 miles, for a 
 human being ; and to such an animalcula, a globe 
 of 23J feet in diameter would be as large as the 
 
IMMENSITY AND MINUTENESS 311 
 
 . whole earth is to us ! What would, then, repre- 
 sent the distance of the sun and of the nearest 
 fixed star to such a creature? Taking the sun's 
 distance at 92,800,000 miles, it would be repre- 
 sented by 3,270,000 inches, or over 51 miles, and 
 the distance of the nearest fixed star by about 
 14 millions of miles! So that for even these 
 microscopical quantities, the proportional dis- 
 tances of the stars would still be represented by 
 enormous numbers. For an atom of matter of the 
 500 millionth of an inch in diameter, one inch 
 would represent about 568,000 miles for a human 
 being, or more than double the distance of the 
 moon from the earth. So that on this scale the 
 sun's distance would be represented by 164 inches, 
 or 13 feet 8 inches, and the distance of the nearest 
 fixed star by about 700 miles ! Now, 700 miles is 
 3^0 of the moon's mean distance from the earth. 
 Perhaps the farthest visible star is not more than 
 340 times the distance of Alpha Centauri. If this 
 be so, we may say that the diameter of the sphere 
 containing the earth and moon the earth's 
 system, as it may be called bears the same pro- 
 portion to the diameter of the ultimate atom of 
 matter that the diameter of the visible universe 
 does to the height of a man. Although man's 
 physical stature is, of course, very small compared 
 with the extent of the visible universe, the ulti- 
 mate atom is equally small when compared with 
 the diameter of the lunar orbit. According to 
 
312 STUDIES IN ASTRONOMY 
 
 Dr. Johnstone Stoney, the number of atoms con- 
 tained in a cubic millimetre of solids and liquids 
 is something like 10 21 , that is, 1 followed by 21 
 cyphers ! How many atoms are contained in the 
 earth's mass? I leave this calculation to my 
 readers. 
 
XXVII 
 
 Light, Electricity, and the Ether 
 
 WHILE the illustrious Fresnel was prov- 
 ing, by experiments, that light was 
 due to the vibrations of an ethereal 
 medium which fills all space, the famous Ampere 
 was investigating the laws which ruled the action 
 of electrical currents, and thus founded the science 
 of electro-dynamics. The idea occurred to Ampere 
 that the ether of space which forms the medium 
 for the transmission of light might also serve for 
 the propagation of electricity, and this happy 
 idea has been confirmed by modern researches. 
 But the true relation between light and electricity 
 was first suggested by the late Professor Clark 
 Maxwell, and was developed in recent years by 
 Hertz, who was the real discoverer of the principles 
 of wireless telegraphy. 
 
 All bodies may be divided into two classes, 
 namely, conductors, which convey electrical cur- 
 rents, and insulators, or those which do not 
 conduct electricity. The latter are also called 
 dielectrics. The old electricians thought that all 
 
314 STUDIES IN ASTRONOMY 
 
 insulators were the same, and acted in the same 
 way in preventing the passage of the electrical 
 current ; but modern researches show that this is 
 not the case. If we consider light as an electric 
 phenomenon, we must conclude that it is propa- 
 gated through an insulating medium, for the 
 ether of space is certainly a dielectric. Maxwell's 
 researches tended to show that currents were 
 formed in dielectrics ; but before his time this was 
 not suspected. Maxwell, however, explained the 
 apparent anomaly by stating that dielectrics do 
 not prevent the passage of a current by means of 
 a greater resistance than conductors, but by 
 resistance of another kind. 
 
 According to Maxwell's views of the nature of 
 dielectrics, the difference in the modes of action 
 of the two bodies is somewhat similar to the dif- 
 ference between the action of a spring which we 
 try to compress and the motion of a body through 
 water or other resisting medium. The former 
 may be called elastic resistance, and the latter 
 viscous resistance. Dielectrics may, then, be com- 
 pared to elastic solids, and conductors to viscous 
 liquids. On this view, Maxwell supposed two 
 classes of currents, namely, currents of displace- 
 ment passing through dielectrics, and currents of 
 conduction traversing conductors. The former 
 are of short duration, but the latter continue as 
 long as the electromotive force remains in action. 
 The heating of a wire through which electrical 
 
LIGHT, ELECTRICITY, AND ETHER 315 
 
 currents are passing is thus explained by the 
 friction due to viscosity. 
 
 Electrical currents become perceptible in three 
 ways: (1) by their heating effects; (2) by their 
 action on magnets and currents ; and (3) by the 
 induced currents which they produce. According 
 to Maxwell's theory, the currents in dielectrics 
 should give rise to similar effects. Why, then, 
 are they not perceptible ? The reason is that they 
 are of small intensity and short duration. With 
 a very rapid alternation of currents, however, 
 their effects should become perceptible. 
 
 It is to this rapid alternation of currents that 
 according to Maxwell light waves are produced 
 in the ether ; and by induction, these waves travel 
 through space. The vibrations of sound are longi- 
 tudinal, but those of light are transversal, according 
 to the theories of both Fresnel and Maxwell. 
 
 These views, expressed many years ago, were, 
 of course, purely theoretical, and it was necessary 
 that they should be proved by experiment. 
 According to the old views, electrical induction 
 should be produced instantaneously ; but, accord- 
 ing to the new views, it should be produced with 
 a finite velocity, namely, with the velocity of 
 light. If such a velocity of propagation existed, 
 it was, of course, very difficult to determine it 
 experimentally, as the velocity at which light 
 travels renders it for short distances practi- 
 cally instantaneous. This difficulty was, however, 
 
316 STUDIES IN ASTRONOMY 
 
 overcome by the eminent German physicist, Hertz, 
 whose death at the early age of 37 has been 
 recently deplored. Hertz's method of proof rests 
 on the principle of the interferences of waves 
 of different phase. This principle applies to all 
 wave motion which is propagated with a finite 
 velocity. It should therefore be applicable to 
 electrical induction, and if, as was formerly sup- 
 posed, it is propagated instantaneously, there 
 would be no interference in the electrical waves ; 
 but if, on the contrary, it has a progressive 
 motion, like light, the interference might be made 
 perceptible by suitable experiments. 
 
 By some very ingenious experiments, Hertz has 
 shown that electrical waves travelling along a 
 wire can be reflected and refracted, like light, and 
 that interference effects are produced by reflec- 
 tion. He also showed that the velocity of propa- 
 gation through air is finite, and equal to that 
 along a wire ; but he did not succeed in measur- 
 ing the actual velocity. M. Blondlot has, how- 
 ever, recently measured the velocity of the 
 electrical disturbance along a wire, and finds it to 
 be about 186,000 miles a second, or practically the 
 same as that of light, thus proving the truth of 
 Maxwell's theory. It seems, then, very probable 
 that light and electricity are identical, or, at least, 
 that they are different manifestations of the same 
 phenomenon a phenomenon due to wave motion 
 in the ether of space. 
 
LIGHT, ELECTRICITY, AND ETHER 317 
 
 The hypothesis that light is transmitted by 
 wave motion a theory now universally admitted 
 evidently necessitates the hypothesis of a 
 medium in which these waves are propagated 
 through space. Various views of the constitution 
 of this medium, known as the luminiferous ether, 
 have been advanced by eminent physicists. Some 
 of the properties attributed to this hypothetical 
 fluid are so anomalous that it is almost impossible 
 for the mind to conceive the existence of such a 
 medium. Sir John Herschel says, "Every phe- 
 nomenon of light points strongly to the concep- 
 tion of a solid rather than a fluid constitution of 
 the luminiferous ether, in the sense that none of 
 its elementary molecules are to be supposed capable 
 of interchanging places, or of bodily transfer to 
 any measurable distance from their special and 
 assigned localities in the universe." The famous 
 Dr. Young also says, "The luminiferous aether 
 pervading all space is not only highly elastic, but 
 absolutely solid." Now, as our finite minds can- 
 not grasp the idea of a solid which is impalpable 
 to the touch and invisible to our sight as the 
 ether evidently is any theory which would 
 relieve us from the necessity of imagining, or try- 
 ing to imagine, such an anomalous substance 
 should be very acceptable to our finite intelligence. 
 Such a theory was advanced a few years ago by 
 Professor de Volson Wood, and as his views, 
 Which are very carefully worked out, seem to be 
 
318 STUDIES IN ASTRONOMY 
 
 mathematically sound, some account of his hypo- 
 thesis of the constitution of the ether may prove 
 of interest to the general reader. 
 
 Professor Wood assumes that the ether is 
 gaseous in its nature, and, consequently, molecular 
 in structure, a conception which seems more pro- 
 bable than the hypothesis which ascribes to it the 
 properties of a solid. He starts with two assump- 
 tions, both of which are known to be true. These 
 are : (1) that light is transmitted through space 
 with a velocity of 186,300 miles per second ; and 
 (2) that the ether transmits 133 foot-pounds of 
 heat energy per second per square foot from the 
 sun to the earth. There is no doubt whatever as 
 to the velocity of light which has been determined 
 by various methods, all of which give results in 
 close agreement. With reference, however, to the 
 heat energy transmitted from the sun to the 
 earth, Herschel found 71 foot-pounds, and Sir 
 William Thompson (now Lord Kelvin) assumed 
 83*5 in his calculations ; but recent researches by 
 Professor Langley show that the real value is con- 
 siderably higher, and his results indicate the 
 number 133, the value adopted by Professor 
 Wood. I may here remark that any theory which 
 takes into account the limited velocity of light 
 for although very high, the velocity is evidently 
 limited commends itself at once to our favour- 
 able consideration. For no other theory of the 
 constitution of the ether attempts to explain, so 
 
LIGHT, ELECTRICITY, AND ETHER 319 
 
 far as I know, the limited velocity of light. Were 
 the ether an absolutely perfect fluid, we might 
 reasonably expect that the velocity of light would 
 be infinite, or, in other words, that its propaga- 
 tion through space would be, for all distances, 
 instantaneous. That this is not so, suggests that 
 the velocity is limited by the constitution of the 
 ether _in the same way that sound is limited in 
 velocity by the constitution of the earth's atmo- 
 sphere, or of the substance along which the sound 
 is conducted. 
 
 Starting with the above two assumptions, Pro- 
 fessor Wood computes from the known properties 
 and laws of gases that the density of the ether is 
 such that a weight of one cubic foot is the fraction 
 of a pound represented by 2 divided by 10 24 . 
 With this density, a cubic foot of the most perfect 
 vacuum which has yet been obtained by air- 
 pumps would contain "some 200 million million 
 times the quantity in a cubic foot of the aether." 
 In other words, " a quantity of the aether whose 
 volume equals that of the earth would weigh 
 about jo of a pound," or about f of an ounce. 
 Professor Wood also computes that the pressure 
 of the ether would also be very small about one 
 pound on a square mile. Far, therefore, from 
 being a solid, the ether is, on this theory, an 
 excessively attenuated gas, and such an hypo- 
 thesis certainly seems more plausible than the 
 anomalous theories which have been hitherto held. 
 
320 STUDIES IN ASTRONOMY 
 
 The chief objection which has been advanced 
 against a gaseous constitution of the ether is that 
 even with a highly rarefied gas a retarding in- 
 fluence would be produced on the motions of the 
 planets, which, in the course of time, would be 
 easily detected by astronomical observations. 
 But Professor Wood shows clearly that, with his 
 computed density of the ether, its resistance to 
 the motions of the planets and comets would be 
 absolutely insensible even in the course of ages. 
 
 According to the kinetic theory of gases, the 
 number of molecules even in a small volume of an 
 ordinary gas is enormous. According to Thomp- 
 son, the probable number in a cubic foot of air is 
 17 X 10 25 , an immensely large number. Even for 
 such a rarefied gas as the ether is supposed to be, 
 on Professor Wood's theory, the number of mole- 
 cules would be very great. He computes the 
 number at 10 16 . Large, however, as this number 
 is, the number given above for air is about 
 17,000 million times as large! From this result 
 it would seem that the law of Ampere and 
 Avogadro is not applicable to Professor Wood's 
 hypothetical medium. 
 
 Assuming that the earth's atmosphere is subject 
 to terrestrial attraction, and that it obeys the 
 well-known gaseous law of Boyle and Marriotte, 
 namely, that the density is proportional to the 
 pressure, we can find the law of the decrease of 
 density with distance from the earth, and hence 
 
LIGHT, ELECTRICITY, AND ETHER 321 
 
 the density at any given height above the earth's 
 surface. At a certain point the atmosphere will 
 become so rarefied that it would have the same 
 density and the same tension as the ether. Pro- 
 fessor Wood computes that this height is about 
 127 miles. This should be the extreme limit of 
 the earth's atmosphere. By another method, 
 however, he finds 169 miles as the extreme height 
 of the atmosphere. Both results are, however, 
 uncertain, for a uniform temperature is assumed 
 for the whole height ; and as we know that the 
 temperature diminishes as we ascend, the assump- 
 tion is incorrect. Assuming a probable law for 
 the decrease of temperature, and considering the 
 temperature observed by Glaisher in his famous 
 balloon ascents, he finds a height of 86 miles. 
 Under certain conditions, however, he finds that 
 the height might be increased to 110, and possibly 
 even to 120 miles. Observations of the height 
 at which meteors become visible indicate in some 
 cases a height of 100 miles or more, but the usual 
 height is between 70 and 80 miles. 
 
 Although he considers the constitution of the 
 ether to be gaseous and molecular, Professor 
 Wood thinks "that the sether is a substance 
 entirely distinct from that of the atmosphere 
 that the former cannot be considered as the latter 
 greatly rarefied, as some have supposed." He 
 finds by computation that the density of the ether 
 at the surface of the sun and at an infinite distance 
 
 Y 
 
STUDIES IN ASTRONOMY 
 
 from that luminary, is sensibly the same, and 
 considers that, unlike the earth's atmosphere, 
 "the density and tension of the aether may be 
 considered uniform throughout space." It would 
 be impossible for a wave of light to be propagated 
 in air with the known velocity of light unless we 
 suppose the temperature of the air to be raised 
 enormously something like 400 billion degrees of 
 the Fahrenheit scale. Professor Wood also com- 
 putes the specific heat of the ether, and finds it 
 more than a billion times that of hydrogen, which 
 has the greatest specific heat of all known terres- 
 trial gases. He finds the ratio of elasticity of the 
 ether to its density to be very great, compared 
 with the same ratio in the case of air. His result 
 is 8 followed by 11 cyphers. He also shows that 
 the earth's attraction for the molecules of air 
 lying near the limit of the earth's atmosphere 
 " will exceed 500,000 the resistance of the aether ; 
 hence the molecules of air accompany the earth in 
 its orbit as certainly as does the moon, and are 
 far more rigidly bound to it than is its satellite." 
 
 A similar theory respecting the constitution of 
 the ether has been advanced by Mr. S. Tolver 
 Preston. He shows that the resistance offered by 
 the air to a body moving through it is due to the 
 comparatively slow motion of its molecules 
 about 1600 feet per second, or about that of a 
 rifle bullet and that consequently, even if its 
 density were as low at that of the ether, it would 
 
LIGHT, ELECTRICITY, AND ETHER 323 
 
 still offer great resistance to bodies moving with 
 planetary velocities. If we suppose the molecules 
 of the ether to be endowed with a very high 
 velocity, this resistance would vanish, as the 
 equilibrium of the medium would not then be 
 disturbed. He therefore concludes that the mole- 
 cules of the ether are extremely minute, and 
 moving with high velocities. Professor Wood 
 estimates the " mean square velocity " at 286,000 
 miles per second. Their minuteness " is absolutely 
 necessary to enable the ether to penetrate with 
 freedom the molecular interstices of matter." 
 Their high velocity is consistent with the hypo- 
 thesis of a large amount of energy being stored 
 up in the ether, for the energy of a moving body 
 varies as its mass multiplied by the square of its 
 velocity. A small body moving with a high 
 velocity may therefore possess more energy than 
 a much larger body moving with a small velocity. 
 As, according to the kinetic theory of gases, the 
 pressure exerted by a gas depends on the velocity 
 of its molecules, the ether may have a high 
 pressure without being dense or solid, as some 
 have supposed it to be. 
 
 The low density of the ether found by Professor 
 Wood has an important bearing on the question 
 of the supposed extinction of the light of very 
 distant stars by absorption hi the ether an idea 
 advocated by the elder Struve and other astro- 
 nomers. I have shown elsewhere that telescopic 
 
324 STUDIES IN ASTRONOMY 
 
 observations yield strong evidence against the 
 existence of any extinction of light, at least so far 
 as our largest telescopes are able to penetrate into 
 space. Let us see what effect the ether of Pro- 
 fessor Wood's theory would have on the light 
 of very distant stars. We can solve this problem 
 by comparing its effect with that of the earth's 
 atmosphere on the light of the stars. Although 
 the earth's atmosphere extends with constantly 
 diminishing density to a height of 100 miles or 
 more, its total effect may be assumed to be equal 
 to that of a homogeneous atmosphere of about 
 5 miles in height, and of a density equal to that 
 of the air at the surface of the earth. Now 
 measurements with photometers of the same star 
 at different altitudes above the horizon have 
 shown that the absorption of light by the earth's 
 atmosphere amounts in the case of a star in the 
 zenith to only about a quarter of a magnitude. 
 Assuming as we are justified in doing that the 
 absorption of light is proportional to the density 
 of the medium through which the light passes, 
 and taking the density of the ether as computed 
 by Professor Wood, I find that the thickness of 
 the ether which would absorb the same quantity 
 of light as the earth's atmosphere would be about 
 2 X 10 23 miles, or 2 followed by 23 cyphers an 
 enormous distance. Let us see what this implies. 
 Measures of parallax have shown that the average 
 parallax of stars of the 1st magnitude is about 
 
LIGHT, ELECTRICITY, AND ETHER 
 
 i~ of a second. Hence the parallax of stars of 
 the 16th magnitude about the faintest visible in 
 the great Lick telescope would be, if their faint- 
 ness is due to distance, about ToWo of a second. 
 This would indicate a distance of 2,062,650,000 
 times the sun's distance from the earth, or, hi 
 miles, nearly 2 followed by 17 cyphers. Hence it 
 follows that the thickness of ether necessary to 
 reduce the light of a star by only a quarter of a 
 magnitude would be about one million (10 6 ) tunes 
 the distance of stars of the 16th magnitude. 
 We may therefore conclude that, on Professor 
 Wood's theory of the constitution of the ether, 
 there would be no extinction of light due to the 
 ether alone, so far as the largest telescopes can 
 penetrate into the depths of space. So far as our 
 limited range of telescopic vision extends, we may 
 consider the ether as practically transparent, the 
 total loss of light being wholly due to our own 
 atmosphere. Of course there may possibly be 
 some extinction of light caused by meteoric dust 
 is space, but this hypothesis has nothing to do 
 with the ether or with the question of its con- 
 stitution which we have been here considering. 
 
 In a paper read before the American Association 
 for the Advancement of Science on August 13, 
 1898, Professor Brush announced the discovery 
 of a new gas, which he calls etherion. It has 
 "enormous heat-conducting capacity," and "its 
 mean molecular velocity is 100 times that of 
 
326 STUDIES IN ASTRONOMY 
 
 hydrogen," and Professor Brush is inclined to 
 believe that it is identical with the ether of space. 
 But I do not know whether this discovery has 
 been confirmed. 
 
APPENDIX 
 
 BINARY STABS. 
 NOTE A. 
 
 Star. 
 
 Period. 
 
 Semi-axis 
 major. 
 
 thetical 
 parallax. 
 
 Magni- 
 tude. 
 
 Spec- 
 trum. 
 
 Remarks. 
 
 
 years 
 
 sees. 
 
 sees. 
 
 
 
 
 Struve 3062 ... 
 
 104-61 
 
 1-3712 
 
 0-061 
 
 6-10 
 
 II. 
 
 
 f\ Cassiopeise... 
 
 195-76 
 
 8-2128 
 
 0-243 
 
 3-64 
 
 II. 
 
 
 7 Andromeda 
 
 54-0 
 
 0-3705 
 
 0-026 
 
 (5) 
 
 I. 
 
 Magnitude estimated 
 
 Sirius 
 
 52-20 
 
 8-0316 
 
 0-575 
 
 1-58 
 
 I. 
 
 
 9 Argus 
 
 22-0 
 
 0-6549 
 
 0-083 
 
 5-49 
 
 
 
 
 C Cancri 
 
 60-0 
 
 0-8579 
 
 0-056 
 
 4-71 
 
 II. 
 
 
 Strove 3121 ... 
 
 34-0 
 
 0-6692 
 
 0-063 
 
 7-26 
 
 II. 
 
 
 w Leonis 
 
 116-20 
 
 0-88241 
 
 0-037 
 
 5-55 
 
 II. 
 
 
 < Ursaa Maj.... 
 
 97-0 
 
 0-3440 
 
 0-016 
 
 4-54 
 
 II. 
 
 
 i , ... 
 
 60-0 
 
 2-508 
 
 0-163 
 
 3-86 
 
 II. 
 
 
 05234 
 
 77-0 
 
 0-3467 
 
 0-019 
 
 6-99 
 
 II. 
 
 
 02235 
 
 80-0 
 
 0-8690 
 
 0-047 
 
 5-56 
 
 II. 
 
 
 7 Centauri ... 
 
 88-0 
 
 1-0232 
 
 0-051 
 
 2-38 
 
 I. 
 
 
 7 Virginia ... 
 
 194-0 
 
 3-989 
 
 0-119 
 
 2-91 
 
 II. 
 
 F 
 
 42 Comae 
 
 25-556 
 
 0-6416 
 
 0-074 
 
 4-38 
 
 II. 
 
 
 02269 
 
 48-8 
 
 0-3248 
 
 0-024 
 
 6-75 
 
 I. 
 
 
 25 Can. Venat. 
 
 184-0 
 
 1-1307 
 
 0-035 
 
 5-00 
 
 I. 
 
 
 a Centauri ... 
 
 8110 
 
 17-70 
 
 0-944 
 
 0-06 
 
 II. 
 
 
 02 285 
 
 76-67 
 
 0-3975 
 
 0-022 
 
 7-24 
 
 
 
 
 Bootis 
 
 128-0 
 
 5-5578 
 
 0-218 
 
 4-64 
 
 II. 
 
 
 77 Cor. Bor. ... 
 
 41-60 
 
 0-9165 
 
 0-076 
 
 4-98 
 
 II. 
 
 
 h - BoGtis 
 
 219-42 
 
 1-2679 
 
 0-034 
 
 (6-5) 
 
 I. 
 
 Magnitude estimated 
 
 02 298 
 
 52-0 
 
 0-7989 
 
 0-057 
 
 6-80 
 
 II. 
 
 
 7 Cor. Bor. ... 
 
 73-0 
 
 0-7357 
 
 0-042 
 
 3-93 
 
 II 
 
 
 Scorpii 
 
 104-0 
 
 1-3612 
 
 0-061 
 
 4-16 
 
 II. 
 
 F8G 
 
 a COT. Bor. ... 
 
 370-0 
 
 3-8187 
 
 0-074 
 
 5-29 
 
 II. 
 
 
 (Herculis ... 
 
 35-0 
 
 1-4321 
 
 0-134 
 
 3-00 
 
 II. 
 
 
APPENDIX 
 
 BINARY STARS (continued). 
 
 Star. 
 
 Period. 
 
 Semi-axis 
 major. 
 
 Hypo- 
 thetical 
 parallax. 
 
 Magni- 
 tude. 
 
 Spec- 
 trum. 
 
 Remarks. 
 
 
 years 
 
 sees. 
 
 sees. 
 
 
 
 
 /3416 
 
 33-0 
 
 1-2212 
 
 0-118 
 
 5-85 
 
 II. 
 
 
 22173 
 
 46-0 
 
 1-1428 
 
 0-089 
 
 
 
 II. 
 
 
 fj. 1 Herculis ... 
 
 45-0 
 
 1-390 
 
 0-110 
 
 (9-4) 
 
 
 
 Magnitude estimated 
 
 T Ophiuchi ... 
 
 230-0 
 
 1-2495 
 
 0-033 
 
 4-88 
 
 II. 
 
 F 
 
 70 Ophiuchi... 
 
 88-3954 
 
 4-548 
 
 0-229 
 
 4-07 
 
 II. 
 
 Computed mass of 
 
 
 
 
 
 
 
 system equals 6 -368 
 
 
 
 
 
 
 
 times the sun's mass 
 
 99 Herculis ... 
 
 54-5 
 
 1-014 
 
 0-070 
 
 5-36 
 
 II. 
 
 Mass of system nearly 
 
 
 
 
 
 
 
 equals sun's mass 
 
 CSagittarii ... 
 
 18-85 
 
 0-686 
 
 0-097 
 
 2-71 
 
 I. 
 
 Star 1-75 magnitude 
 
 
 
 
 
 
 (A2F) 
 
 brighter than sun 
 
 7 Cor. Aust. ... 
 
 152-7 
 
 2-453 
 
 0-085 
 
 4-26 
 
 II. 
 
 
 )3 Delphini ... 
 
 27-66 
 
 0-6724 
 
 0-073 
 
 3-72 
 
 II. 
 
 
 4 Aquarii 
 
 129-0 
 
 0-732 
 
 0-028 
 
 6-03 
 
 II. 
 
 
 8 Equulei 
 
 11-45 
 
 0-452 
 
 0-089 
 
 4-61 
 
 II. 
 
 
 K Pegasi 
 
 11-42 
 
 0-4216 
 
 0-083 
 
 4-27 
 
 II. 
 
 
 SoPegasi ... 
 
 24-0 
 
 0-8904 
 
 0-107 
 
 5-83 
 
 III. 
 
 
 883 
 
 5-5 
 
 0-621 
 
 0-1993 
 
 (7-8) 
 
 
 
 Magnitude estimated 
 
 NOTE B. 
 
 The area of the whole sky is 41,253 square degrees, or 
 41253 x (3600) 2 = 534,638,880,000 square seconds. 
 
 Hence 
 
 = 5345 square seconds for each star. 
 
 Now, supposing each star to stand at the centre of a small 
 square, we have the side of this square, or the distance between 
 the two stars = V5346 = 73 seconds. Or, generally, if N be 
 the total number of stars in the sky, we have 
 
 Distance apart = A/ 
 
 534,638,880,000 
 
 N 
 
 for an equal distribution of stars. 
 
 Of course, the stars are not equally distributed, but the above 
 gives their average distance apart. 
 
INDEX 
 
 Achernar, 35, 40, 45, 179, 180, 
 
 212 
 
 Adams, 63, 123, 290, 300 
 Airy, Sir George, 54, 57 
 Aitken, Professor, 289 
 Alcyone, 68-73 
 Aldebaran, 35, 41, 48, 57, 93, 107- 
 
 109, 162, 172, 212, 270 
 Algol, 118, 119, 128 
 
 variables, 128, 129, 162, 
 
 177 
 Al-Sufi, 48, 59, 69, 79, 178-186, 
 
 227 
 
 Altair, 35, 162, 163, 270 
 Ampere, 314 
 Anderson, Dr., 180, 275, 295, 297, 
 
 299 
 
 Andromedse, A, 126, 127 
 Andromeda nebula, 150, 152, 156, 
 
 157, 160, 174, 227 
 Antares, 35 
 Antoniadi, 300 
 Arcturus, 35, 42, 48-50, 162, 163, 
 
 212, 270 
 Arago, 43 
 Argelander, 53 
 Argils, 77, 284 
 Aristarchus, 3 
 Apex, solar, 53-57, 168, 170 
 Aurigse, 0, 120, 121, 127, 291 
 
 , 291, 292 
 Auwers, 94 
 Auzot, 18 
 
 Bailey, Mrs., 132 
 
 Professor, 71, 79, 132, 
 
 276, 277 
 Baily, 180 
 
 Ball, Sir Robert S., 43 
 Barnard, Professor, 137, 220, 243, 
 
 247, 248, 249, 273, 278, 299 
 Bayeux tapestry, 304 
 Behrmann, 262 
 Bellatrix, 163, 164 
 Belopolsky, 272, 273 
 Betelgeuse, 35, 162, 163, 212, 270 
 Binary stars, 103, 104 
 Birt, 11 
 
 Bischoffsheim Observatory, 28 
 " Blaze Star," 281 
 Blondlot, 316 
 Bompas, 56 
 Bond, 11, 24, 86, 87 
 Borelli, 17 
 Boss, 54 
 
 Bradley, 18, 38, 40, 51, 60, 62, 170 
 Bredikhin, 12 
 Brightest stars, 35, 212 
 Brinkley, 42 
 Briinnow, 95, 105 
 Brush, Professor, 325 
 Brussels Observatory, 254, 265, 
 
 266 
 
 Buckingham, 26 
 Buffham, 11, 78, 221 
 Burnham, 29, 65, 76, 102, 107, 
 
 112, 119 
 
330 
 
 INDEX 
 
 Cacciatori, 42 
 
 Calandrelli, 42 
 
 Calver, 21 
 
 Campani, 18, 19 
 
 Campbell, Professor, 123, 128, 
 
 271, 272, 283, 300 
 Canis Majoris, ft, 185, 186 
 Canopus, 35, 44, 96, 97, 212, 270 
 Cape Observatory, 25, 26 
 Capella, 35, 48, 124, 125, 162, 
 
 163, 200, 212, 270, 271, 272 
 Carrington, 68 
 Cassini, 10, 38, 48, 49, 60 
 Cassiopeise, ft, 93, 94, 290 
 
 7,293 
 
 6,50 
 
 /u, 50 
 
 r,, 103 
 
 Castor 
 
 35,122,127,128,163,272 
 
 Catalogues of nebulae, 188-192, 
 
 chaps, xix., xx. 
 Cauchoix, 23 
 Cauchy, 309, 310 
 Celoria, 172 
 Centauri, o, 5, 6, 35, 43, 50, 157, 
 
 175, 208, 212, 269, 309, 311 
 Centauri, ft, 212 
 
 o>, 79, 81-83, 132, 137, 
 
 197, 210, 276, 277 
 Centauri, x , 182, 183 
 Cerulli, 24 
 Chacornac, 234 
 Chamberlin Observatory, 25 
 Chandler, 119 
 Chase, 302 
 
 Chicago Observatory, 107 
 Clark, Alvan, 26, 27, 28, 30 
 Clausius, 310 
 
 Clusters, globular, chap. viii. 
 irregular, chap. xx. 
 Colours of stars, 161 
 Comet, Halley's, chap. xxv. 
 Comets, telescopic, 192 
 " Cometic nebulae," 199 
 Coming comet, chap. xxv. 
 Common, Dr., 21 
 
 Comstock, 96, 106 
 Conductors, 313, 314 
 Cooper, 23 
 
 Copeland, Professor, 295 
 Copernicus, 33, 48 
 Coronas Borealis, a, 289 
 Crab nebulae, 215, 216 
 Crabtree, 34 
 Croll, Dr., 152, 154, 155 
 Crossley reflector, 223, 278 
 Currents, electrical, 313-315 
 Cygni, a, 270 
 
 5, 103 
 
 61, 43, 253, 273 
 
 Damoiseau, 305 
 
 Dark bodies, 152, 154, 155, 157 
 
 "Darkness behind the stars," 
 
 chap. xiii. 
 Darquier, 246 
 D'Arrest, 189, 225 
 Darwin, Professor, 148 
 Dawes, 24, 61 
 Dearborn Observatory, 25 
 De Ball, 54 
 Delphini, W., 274 
 Dembowski, 65 
 Denebola (ft Leonis), 179 
 Denning, 68 
 
 Density of Algol variables, 129 
 Density of the ether, 319 
 Dielectrics, 313 
 Distances of stars, chap. iv. 
 Distance of nebulae, 197, 198. 
 
 200 
 
 Doberck, Dr., 64 
 Dollond, 22 
 Donatis comet, 257 
 Dorpat Observatory, 63 
 Draconis, (26), 104 
 Draper Catalogue, 170, 173 
 Dumb-bell nebula, 202, 225, 226, 
 
 289 
 
IN 7 DEX 
 
 331 
 
 B 
 
 Edinburgh Observatory, 26 
 Elkin, Dr., 41, 42, 94, 125, 250, 
 
 272, 274 
 Equulei, 5, 288 
 Eridani, a, 35, 40, 45, 179, 180, 
 
 212 
 Eridani, 6, 179, 180, 181 
 
 40(o 2 ), 112 
 Eros, 287 
 Eschatos, 180 
 Espin, 295 
 Ether, 317-326 
 Etna Observatory, 26 
 Extinction of light, 323-325 
 
 Feldhausen, 208 
 Flammarion, 11, 63, 267, 300 
 Flamsteed, 38, 49, 305 
 Fleming, Mrs., 275, 278-280 
 Fomalhaut, 35 
 Fresnel, 313, 315 
 Frost, 290 
 
 Galaxy. See Milky Way 
 Galileo, 12, 17, 38 
 Gaseous nebulae, 193-196 
 Gauges, star, 131 
 Geminorum, y, 185 
 77, 104 
 
 Genesis, 138 
 Gill, Sir David, 42, 44, 45, 79, 
 
 96, 97, 119, 122, 269, 284 
 Glaisher, 321 
 Globular clusters, chaps, viii., 
 
 xx. 
 
 Goodsell Observatory, 25 
 Gorton, 10 
 Gould, Dr., 172 
 Greenwich Observatory, 27 
 Gregory, 19 
 
 Groombridge 1830, 49 
 Grover, 11 
 
 Grubb, Sir Howard, 26, 27, 158 
 Gruis, a, 45 
 
 Hale, Professor, 164 
 Hall, Chester More, 22 
 
 Professor, 27, 41, 65, 112, 
 
 248 
 Halley, 48, 49, 64, 179, 214, 220, 
 
 chap. xxv. 
 
 Halley's comet, chap. xxv. 
 Halsted Observatory, 26 
 Harkness, 7, 8 
 Hartmann, 285 
 Hartwig, 302 
 Harvard Observatory, 24, 26, 66, 
 
 217, 275, 296, 297, 300 
 Henderson, 44, 63 
 Henry, 26, 28 
 Hercules cluster, 220, 293 
 Herculis, /i, 101 
 
 (99), 101, 103 
 (109), 186 
 
 Herschel, Miss Caroline, 190, 204 
 Sir William, 19,38,53, 
 
 56, 57, 60, 76, 77, 107, 110, 111, 
 
 112, 131, 159, 172, 175, chaps. 
 
 xviii., xx., 247, 248 
 Herschel, Sir John, 61-63, 77, 79, 
 
 80, 113, chaps, xviii., xix., xx., 
 
 247, 248, 306, 317 
 Hertz, 316 
 
 Hevelius, 18, 224, 305 
 Hind, 63 
 Hinks, 302 
 Hipparchus, 178, 180 
 Holden, Professor, 222 
 Hooke, 38 
 Horrocks, 34 
 
 Houzeau, 180-186, chap. xxii. 
 Huggins, Sir William, 158-160, 
 
 162, 163, 173, 227, 234, 241, 277 
 Hussey, 288 
 Huygens, 18, 32, 33 
 
332 
 
 INDEX 
 
 Hyades, 172, 173 
 
 Hydri, 0, 96 
 
 Hyperion, 24 
 
 Hypothesis, nebular, chap, xiv., 
 
 199, 227 
 Hypothetical parallax, 98, 99 
 
 Induction, electrical, 315 
 Innes, B. T. A., 50, 270, 284 
 Insulators, 313 
 Irregular clusters, chap. xx. 
 nebulae, chap. xx. 
 
 Jacob, 63 
 
 Jansen, 17 
 
 Johnson, 123 
 
 Jupiter, 2, 4, 6, chap, ii., 29, 89 
 
 Kant, 140 
 
 Kapteyn, Professor, 95, 1G6, 170- 
 
 175, 270, 283, 301 
 Keats, 297 
 Keeler, Professor, 55, 150, 156, 
 
 249, 278, 285 
 Kelvin, Lord, 148, 318 
 Kempf, 274 
 
 Kepler, 32, 48, 281, 304 
 Kirch, 220 
 Klein, 287 
 Klinkerfues, 122 
 Kostinsky, 290, 300 
 
 Lacaille 9352, 49 
 La Hire, 305 
 Lalande 21185, 49, 305 
 Lambert, 52 
 
 Lamont, 307 
 Lane, 163, 178 
 Langley, 318 
 Laplace, 140-149 
 Lassell, 20, 222, 225, 239 
 Leavenworth, 27 
 Lehmann, 305 
 Le Gentil, 224, 228 
 Lepaute, Madame, 305 
 Leonis, 0, 179 
 7, 273 
 Leporis, ft, 104 
 Librae, 8, 289 
 K, 181 
 (37), 181 
 Lick Observatory, 7, 13, 21, 28, 
 
 29, 37, 107, 150, 271, 278 
 Light, velocity of, 318 
 Limited number of visible stars, 
 
 132-137 
 
 Lipperscheim, 17 
 Lockyer, Sir Norman, 173, 281 
 
 Dr., 275, 276 
 Lohse, 12, 281 
 
 Long-period variables, 164, 165 
 Longomontanus, 304 
 Lowell Observatory, 26 
 Lubbock, 305 
 Lyra, 47, 54, 56 
 
 ,, ring nebula in, chap. xxi. 
 Lyrse, o, 35, 42, 98, 162, 163, 171, 
 
 200, 212 
 
 M 
 
 Madler, 63 
 
 Magellanic clouds, 80, 210 
 Magnitudes, star, 35, 36, 212 
 Main, 68, 70, 73, 74 
 Manila Observatory, 25 
 Markree Observatory, 23 
 Masses of stars, chaps, x., xi. 
 Maraldi, 77 
 Mars, 2, 4, 27, 88 
 Markwick, Col., 285 
 Martin, 21 
 Maxwell, Clark, 310, 313-315 
 
INDEX 
 
 Mayer, Tobias, 52, 60 
 
 McCormick, 27 
 
 Mercury, 2, 4, 254 
 
 Merope, 70, 73 
 
 Merz and Mahler, 24, 25, 26 
 
 Messier, 75, 76, 77, 78, chap. xx. 
 
 Messier's nebulae, chap. xx. 
 
 Meudon Observatory, 26, 28 
 
 Meteoritic hypothesis, 173 
 
 Michell, 41, 70 
 
 Milan Observatory, 25 
 
 Milky nebulosity, 194-198, 203 
 
 Milky Way, 56, 57, 131-133, 171, 
 
 172, 175, 176, 198, 213, 263, 
 
 264, 286, 294 
 Miller, Dr., 158 
 Minor planets, 145 
 Mira Ceti, 293 
 Miraldi, 222 
 Mizar, 122 
 Molecules, size of, 310 ; number 
 
 of, 320 
 
 Molyneux, 38, 40 
 Monck, 87, 269, 281 
 Mostlin, 68 
 
 Motions, proper, 48-52, 167-172 
 Miiller, 274 
 Munckley, 305 
 
 N 
 
 Nearest star, 43, 44 
 Nebulae, chaps, xviii., xix., xx. 
 Nebulae, Messier's, chap. xx. 
 Nebular hypothesis, chap, xiv., 
 
 199, 227 
 
 Nebular spectrum, 159, 160 
 Nebulium, 277 
 Nebulous stars, 193-196 
 Neptune, 2, 4, 5, 148, 255 
 Newall, 27 
 Newcomb, Professor, 54, 72, 87, 
 
 285 
 
 Newkirk, Dr., 249, 250 
 Newton, Sir Isaac, 19, 21, 281 
 New stars, 193, 290, chap, xxiv., 
 
 278-283 
 
 New star in Perseus, 152, chap. 
 
 xxiv. 
 
 Nichol, Dr., 221 
 Noble, Capt., 11 
 Northumberland, Duke of, 23, 
 
 209 
 
 Nova Persei, 152, chap. xxiv. 
 Nubeculae, 206 
 
 Number of stars, 130-137, 289 
 Nyren, 290 
 
 O 
 
 " One sun by day," 91 
 
 Ophiuchi, T, 100 
 ' (70), 253 
 
 Orionis, o, 35, 162, 163, 212 
 7], 123, 124, 289 
 7 , 163, 164 (Bellatrix) 
 
 Orion nebula, 24, 151, 160, 196, 
 198, 199, 206, 210 
 
 " Orion stars," 161, 162, 164, 277 
 
 Owl nebula, 244 
 
 Palitzch, 305 
 Palmer, H. K., 76, 221 
 Parallax, solar, 287 
 
 of stars, 38, 39-45, 174 
 " hypothetical," 98, 99 
 Paris Observatory, 26, 48, 71 
 Pegasi, T?, 123 
 
 K, 126 
 
 (34), 104 
 
 ,, 276 
 Persei, Nova, 152, chap. xxiv. 
 
 o, 124, 183, 184, 289 
 Perrine, 299, 301 
 Perrotin, 286, 287 
 Peters, 53 
 
 Photographs, star, 132-137 
 Piazzi, 41, 42 
 Pickering, Professor, 68, 71, 79, 
 
 131, 132, 162, 171, 180, 181, 
 
 186, 288, 289, 299, 302, 303 
 
334 
 
 INDEX 
 
 Pictor, 50 
 
 Piscis Australis, , 182 
 
 T?, 182 
 
 Piscium, 7 (6), 181 
 Plassmann, 296 
 Planetary atmospheres, 11 
 nebulse, 192, 193 
 Pleiades, chap, vii., 173 
 Pleione, 68-70 
 Pogson, 165 
 Pole Star, 42, 123, 272 
 Pollux, 35, 270 
 Pontecoulant, 305 
 Porrima, 60 
 Porro, 26 
 Postvarta, 60 
 
 Potsdam Observatory, 28, 55 
 Prsesepe, 231 
 Preston, Tolver, 322 
 Pritchard, Professor, 42, 43, 94, 
 
 120, 290 
 Proctor, 173 
 Procyon, 35, 42, 45, 46, 94, 96, 
 
 97, 103, 104, 105, 116, 163, 212, 
 
 270, 273, 274 
 
 Proper motions, 48-52, 167-172 
 Ptolemy, 47, 48, 49, 69, 178-186 
 Pulkowa Observatory, 65, 74 
 
 Quetelet, 260 
 
 Q 
 
 B 
 
 Rancken, 54 
 
 Reflected light, 113-116 
 
 Regulus, 35, 109, 110, 163, 270 
 
 Riccioli, 47 
 
 Richer, 48 
 
 Rigel, 35, 44, 111, 112, 162, 163, 
 
 212, 269, 277 
 Ring nebula, 156, 234, chap, xxi., 
 
 289 
 
 Ritchey, 21, 301 
 Roberts, Dr. A. W., 129, 285 
 
 Roberts, Dr. Isaac, 73, 76, 133- 
 137, 150, 157, 191, 202, chap. xx. 
 
 Roche, 145, 146, 147, 155 
 
 Rogovsky, 11 
 
 Rosenberger, 305 
 
 Rosse, Earl of, 20, 77, 149, 194, 
 203, chap, xx., 248 
 
 " Runaway stars," 270, 283 
 
 Ruskin, 156 
 
 Russell, Professor, 128, 129 
 
 Rutherford, 159 
 
 S 
 
 Satellites, stellar, chap. xi. 
 Saturn, 2, 4, 20, 89, 90, 155 
 Schaeberle, 104, 156, 157, 226, 
 
 234, 278, 293 
 Schemer, 12, 150, 249 
 Schiaparelli, 65 
 Schmidt, 275 
 Schuster, 292 
 Schwab, 284, 294 
 Scorpii, |, 100 
 Secchi, 159, 219, 234, 248 
 Secular variation of stars, chap. 
 
 xvii. 
 See, Dr., 64, 65, 96, 98, 99, 100, 
 
 102, 104, 118, 119, 274 
 Seeliger, 286, 302 
 Serpentis, o-, 184, 185 
 Shdanow, 41 
 Sidereal system, 192 
 Simon, 147, 148 
 Sirius, 25, 32, 35, 41, 42, 45, 46, 
 
 48, 83, 97, 103-105, 113-116, 
 
 162, 163, 171, 174, 212 
 Smyth, Admiral, 63, 64, 189, 190, 
 
 chap, xx., 305 
 Solar apex, 53-57, 168, 170 
 
 cluster, 172 
 
 parallax, 287 
 South, 61 
 Southern Cross, 44 
 Spectra of stars, 91, 92, 160-162, 
 
 171-174 
 
INDEX 
 
 Spectroscopic binaries, chap, xii., 
 
 271, 272, 289 
 Spencer, Herbert, 139 
 Spica, 121, 122, 124, 127, 162 
 Spiral nebulae, 149, 150, 155-157, 
 
 chap. xx. 
 
 Stars, age of, 161-164 
 Star colours, 161 
 Starlight, 285 
 Star magnitudes, 35, 36 
 
 maps, 262, 263 
 
 photographs, 132-137 
 Stebbins, 293, 300 
 Steinheil, 87 
 
 Stellar evolution, chap, xv., 292 
 ,, satellites, chap. xi. 
 spectra, chap, x., 160- 
 
 162, 171-174 
 
 Stellar universe, 135-137 
 Stratford, 305 
 Struve, L., 50 
 
 Otto, 23, 41, 53, 54, 64, 65, 
 
 104 
 
 Struve, W., 61, 175, 306 
 Stumpe, O., 54 
 Sun's motion in space, chap, v., 
 
 168, 169, 283 
 
 Suns of space, chap, x., 175 
 Sun's stellar magnitude, chap. 
 
 ix., 294 
 
 " Sweeps," 187, 209 
 Swift, Professor, 191 
 
 T 
 
 Tauri, a, 35, 41, 48, 57, 93, 107- 
 
 109, 162, 172, 212 
 Tauri, , 185 
 Telescopic comets, 192 
 Tempel, 72 
 Temporary stars, 152, 193, 240, 
 
 chap. xxiv. 
 Terby, 11 
 Tikhoff, 121, 291 
 Tisserand, 56 
 Trifid nebula, 223 
 Trouvelot, 222, 223 
 
 Tucanae, 96 
 
 Tully, 208 
 
 Turner, Professor, 287 
 
 "Twin suns," 103 
 
 Tycho Brahe, 33, 34, 49, 281 
 
 Tyndall, Professor, 309 
 
 U 
 
 Ubaghs, 54 
 
 Ulugh Beigh, 179 
 
 Uranus, 2, 4, 148 
 
 Ursse Majoris, a, 104, 287 
 C, 122 
 6, 95, 104 
 i, 104, 113 
 
 Variable stars, 164, 165, 177 
 Variation, secular, chap. xvii. 
 Vega, 35, 42, 98, 162, 163, 171, 
 
 200, 212, 250, 270, 273 
 Venus, 110 
 Velocity of light, 318 
 stars, 271 
 ,, sun's motion, 55, 56 
 Virginis, a (Spica), 121, 122, 124, 
 
 127, 162 
 Virginis, 7, chap, vi., 211, 272, 
 
 273 
 Visible stars, limited number of, 
 
 132-137 
 
 Visible universe, chap. xvi. 
 Vogel, 248, 281, 292 
 Von Gothard, 300 
 
 W 
 
 Wagner, 42, 94 
 Warner, 25, 30 
 Washburn Observatory, 24 
 Washington Observatory, 41 
 Webb, 71, 223, 224, 240, 243 
 WeUs, Miss, 275 
 
336 
 
 INDEX 
 
 Williams, Stanley, 182, 296 
 Wilson, Dr. W. E., Preface, 76, 
 
 223, 301 
 Winlock, 110 
 Winnecke, 43, 273 
 Wolf, 149 
 
 Max, 25, 278, 284, 300, 301 
 Wolf-Bayet stars, 161 
 Wood, Professor, 317-325 
 Wright, 91, 300 
 
 Yerkes Observatory, 21, 30, 273 
 Young, Professor, 87 
 
 Zodiacal light, 196, 253 
 Zollner, 86, 89, 281 
 Zwack, Father, 302 
 
 THE END 
 
 FEINTED BY WILLIAM CLOWES AND SONS, LIMITED, LONDON AND BECCLES. 
 
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