The Milky Way 



which separates them. It is even possible to 

 calculate the masses and the densities of the 

 two stars so that, thanks to Pickering's hypothesis, 

 the binary system of Algol would be nearly as 

 well defined as our own solar system. 



These inferences lacked the confirmation 

 of experiment, but not for long. Vogel observed 

 the spectrum of Algol at the moment when, 

 according to Pickering's hypothesis, the velocity 

 in the line of sight is greatest that is to say, 

 a quarter of a period before and after occulta- 

 tion. He was able to observe a noticeable 

 displacement of the spectral lines, which deviated 

 towards the red a quarter period before the 

 minimum, and towards the violet a quarter 

 period after. Thus, during its presumed period 

 of rotation, Algol has a variable velocity in a 

 direction sometimes toward us, sometimes away 

 from us. It is thus clearly established that 

 this bright star revolves around a star invisible 

 to us, and Pickering's theory is established 

 among the accepted truths of science. 



All the variable stars present problems 

 analogous with the one just discussed. The 



most frequent case consists in a continuous 



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