The Milky Way 



supply a basis for the solution of the enigma 

 presented by Peregrina; for if we succeed in 

 explaining the nature of the periodical stars 

 we shall also understand the nature of the 

 temporary stars, on the assumption that their 

 period is longer than the duration of our observa- 

 tions. It is also possible that the complicated 

 course observed in the case of some periods 

 may be due to perturbations caused by the 

 influence of neighbouring stars. 



The periodicity of one of these variable 

 stars follows a law of extreme simplicity. This 

 star is Algol in the constellation Perseus. Algol 

 never goes out. During two and a half days 

 its brightness remains constant and of the 

 second magnitude, then it varies during nine 

 hours, with a minimum lasting eighteen minutes, 

 during which its brightness decreases to that 

 of a star of the fourth magnitude, after which 

 Algol resumes its original brightness, being 

 thus characterised by a constant brightness 

 interrupted by partial eclipses. Many are the 

 explanations advanced to account for this 

 phenomenon. Already in 1667 Bouillaud 



maintained that the surface of the stars is not 



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