W 313-316] Stellar Spectroscopy 403 



be powerful enough to detect the minute quantity involved, 

 the line will appear doubled, one component being due to 

 one star and one to the other. A periodic doubling of 

 this kind was detected at the end of 1889 by Professor 

 E. C. Pickering of Harvard in the case of Ursae, which 

 was thus for the first time shewn to be binary, and found 

 to have the remarkably short period of only 104 days. 

 This discovery was followed almost immediately by Pro- 

 fessor Vogel's detection of a periodical shift in the position 

 of the dark lines in the spectrum of the variable star Algol 

 (chapter xn., 266) ; but as in this case no doubling of the 

 lines can be seen, the inference is that the companion star 

 is nearly or quite dark, so that as the two revolve round 

 one another the spectrum of the bright star shifts in the 

 manner observed. Thus the eclipse-theory of Algol's 

 variability received a striking verification. 



A number of other cases of both classes of spectroscopic 

 binary stars (as they may conveniently be called) have 

 since been discovered. The upper part of fig. 103 shews 

 the doubling of ,one of the lines in the spectrum of the 

 double star /? Aurigae ; and the lower part shews the 

 corresponding part of the spectrum at a time when the line 

 appeared single. 



315. Variable stars of different kinds have received a 

 good deal of attention during this century, particularly 

 during the last few years. About 400 stars are now clearly 

 recognised as variable, while in a large number of other 

 cases variability of light has been suspected ; except, how- 

 ever,, in a few cases, like that of Algol, the causes of 

 variability are still extremely obscure. 



316. The study of the relative brightness of stars a 

 branch of astronomy now generally known as stellar photo- 

 metry has also been carried on extensively during the 

 century and has now been put on a scientific basis. The 

 traditional classification of stars into magnitudes, according 

 to their brightness, was almost wholly arbitrary, and 

 decidedly uncertain. As soon as exact quantitative com- 

 parisons of stars of different brightness began to be carried 

 out on a considerable scale, the need of a more precise 

 system of classification became felt. John Herschel was 

 one of the pioneers in this direction ; he suggested a scale 



