ASTRONOMICAL PROGRESS IN 1900-1001. 



39 



will be until 1930, astronomers are waiting with 

 commendable patience for its next ' nearest ap- 

 proach, when the grand problem will be solved. 



The twentieth century was inaugurated by as- 

 tronomical incidents and an amount of discussion 

 rarely if ever equaled. Allusion is made to the 

 variation in the light of the asteroid Eros, and the 

 ; sudden outburst of a new first-magnitude star in 

 the constellation Perseus. The variability of 

 many stars and comets is well established; but 

 that a dark planet which shines by reflected sun- 

 light should vary in brilliance is a mystery too 

 deep for solution. More than 4G5 little planets 

 have been discovered revolving round the Sun be- 

 tween Mars and Jupiter, and Eros is the only one 

 in which variation in light has been certainly de- 

 tected, except their periodical variation by change 

 in distance from the Earth and the Sun. When 

 the novelty was first announced, it was ascribed 

 to chance, or more probably to error of observa- 

 tion. Three different theories have been advanced 

 to account for it, viz., (1) that the little planet 

 is double, the components revolving round each 

 other parallel to our line of sight, as do many of 

 the double stars that alternately occult each 

 other; (2) that two asteroids may have col- 

 lided and adhered to each other, forming an ob- 

 ject resembling a dumb-bell. If this object ro- 

 tated in 5h 16m, the light changes could be ex- 

 plained; (3) this supposes that the little planet, 

 only about 18 miles in diameter, has on its surface 

 two bright and two dark spots, at or near its 

 equator, its rotation on its axis presenting to the 

 Earth alternately its bright and dark spots. The 

 first and second theories are the only ones that 

 appear tenable. The most plausible, and the one 

 that explains what is observed, is that periodi- 

 cally it is occulted by something that cuts off a 

 portion of its light, the phenomenon being visible 

 from the Earth only when the rotatory motion 

 of the occulting object is parallel, or nearly so, 

 to our line of sight. This not only explains the 

 cause of the variation, but also its beginning and 

 cessation. If, as Prof. Pickering remarks, " the 

 variation is caused by its rotation, it is possible, 

 from measures of its light, to determine the time 

 of rotation and the direction of its axis in space. 

 The fact that its successive maxima and minima 

 are of unequal intensity, and that the intervals 

 between them are of variable length, would seem 

 to discountenance the first hypothesis that the 

 two sides are unequally dark. The variability of 

 the light of the planet is shown by the trails on 

 plates taken in 1893 and 1894, and particularly so 

 on those taken in 1896. In February, 1901, the 

 range of variability amounted to two magnitudes, 

 and on May 6 it was inappreciable." Prof. Wen- 

 dell, of Harvard Observatory, argues that " if the 

 variations were caused by markings on the sur- 

 face, it could scarcely have sunk to zero so sud- 

 denly." In 1903 the asteroid will again be in op- 

 position and situated as when its light mutations 

 were first observed. 



Double, Triple, and Binary Stars. To the 

 naked eye all the stars are single, but when ex- 

 amined with modern telescopes several thousand 

 are found to be double or triple. It is possible for 

 a star to appear double when one component hap- 

 pens to be almost exactly behind the other. If 

 no motion of revolution around each other is de- 

 tected after years of observation, they are called 

 " optically double stars." But such instances are 

 not common. If a motion of revolution of the 

 pair is detected, they are called " physically 

 double," or " binaries," and of these there are two 

 kinds, telescopic and spectroscopic. Several thou- 

 sand telescopic binaries have been discovered. 



In Vol. I (1900) of the publications of the 

 Yerkes Observatory, at Williams Bay, Wis., is a 

 catalogue of 1,290 double stars discovered and 

 micrometrically measured by the greatest living 

 double-star discoverer, Prof. Sherburne W. Burn- 

 ham, F. R. A. S., whose work was recognized by 

 the Royal Astronomical Society of England when 

 it awarded him its gold medal in 1894. 



In a catalogue recently issued, 2,000 are pub- 

 lished, all southern pairs. Prof. Aitkin, of Lick 

 Observatory, Mount Hamilton, Cal., publishes a 

 list of 02 pairs lately discovered with the 12-inch 

 telescope at that observatory, all having been re- 

 observed with the 36-inch, and compared with 

 Burnham's list. The list is a continuation of a 

 previous list of double stars discovered there. 



Generally the telescopic binaries are of long 

 periods, but how long never has teen ascertained. 

 One of long period is Castor, generally considered 

 to be at least one thousand years. Both com- 

 ponents are self-luminous suns like ours. At 

 least 30 telescopic binaries are known to have 

 periods of less than one hundred years. The five 

 shortest are Kappa Pegasi, 11.12 years; Delta 

 Equulei, 11.43 years; Xi Sagittarii, 18.85 years; 

 Rho Argus, 22 years; and 85 Pegasi, 24 years. 

 One of the most interesting of the double stars is 

 Sirius, the dog-star, remarkable as having been 

 pronounced a binary years before it was discov- 

 ered to be one, by its vibratory motion a strik- 

 ing instance of the refinements of modern astro- 

 nomical observations. A very interesting triple 

 star is Gamma Andromeda, a bright star that has 

 a double companion revolving around it; period 

 unknown. 



Until recent times there was no way of ascer- 

 taining whether there might not be others, too 

 close to be divided by the telescope. The prin- 

 ciples involved in the phenomena observed in the 

 resolution of spectroscopic binaries needs some 

 preliminary explanation. Should a spectroscope 

 be pointed to a star, it gives a spectrum resem- 

 bling a piece of a rainbow, crossed by many dark 

 lines. If a photograph of the lines be taken, and 

 after a time another, and the lines do not agree 

 exactly, it indicates that they are formed from 

 two stars instead of from one only. It also indi- 

 cates that the stars are revolving around each 

 other in a plane parallel to the line of sight. This 

 is called a spectroscopic binary. Of course, when 

 one is approaching our solar system, the other will 

 be receding from it. The waves of light from the 

 receding star being longer, its lines are all moved 

 slightly toward the red end of the spectrum, while 

 the lines from the approaching star are displaced 

 a like amount toward the violet, causing them al- 

 ternately to appear narrow, broad, and double 

 at equal periods of time, which, when ascertained, 

 gives the period of their revolution around each 

 other. They are too near to be divided by any 

 telescope, hence the periods of spectroscopic binary 

 stars are much shorter than those visually seen 

 by the telescope. This department of astronomy 

 is not new. In 1889 Miss Maury, of Harvard Col- 

 lege Observatory, while examining some Harvard 

 celestial photographs, found that the lines in the 

 spectrum of Zeta Ursa Majoris close up and sepa- 

 rate once in fifty-two days, thus indicating that 

 a complete revolution is made in one hundred and 

 four days. Prof. Campbell, director of Lick Ob- 

 servatory, announces that he has found, by spec- 

 troscopic methods, that the pole-star (Alpha Ursa 

 Minoris) is a spectroscopic trinary, consisting of 

 three suns belonging to a single system, which 

 revolve round each other, the brightest of which 

 is, as everybody knows, visible to the naked eye. 

 What causes this star to be the most wonderful 



