460 ASTROPHYSICS 



panion. The subject is one of great difficulty and importance, and, 

 unfortunately, laboratory methods are on too small a scale of mass 

 and pressure to solve the problem. 



Up to the year 1800 only twelve variable stars were known. Chand- 

 ler's catalogue, 1 dated 1888, contains 225 entries. The remarkable 

 progress made by astronomical science in the past fifteen years is 

 fairly indicated by the fact that in this interval the number of known 

 variable stars increased from 225 to more than 1400. To Harvard 

 College Observatory belongs the great credit of discovering nearly 

 900 of these objects. 



In many respects variable stars constitute the most interesting 

 class of objects in the heavens. The tens of millions of ordinary stars 

 are undoubtedly growing older; and the tens of thousands of nebulae, 

 from which stars will eventually be formed by processes of condens- 

 ation, are undergoing transformation; but appreciable changes in the 

 ordinary stars and in the nebulae proceed with extreme deliberation, 

 and no permanent changes have yet been noted. Variable stars, on 

 the contrary, are changing before our eyes; and they repeat their 

 fluctuations continually. They present opportunities for discoveries 

 of the greatest interest in themselves, and of remarkable utility in the 

 study of the problem of stellar evolution. 



It is a conservative statement that in nineteen variable stars out 

 of twenty we have little idea as to the causes of variability. The 

 causes of the variations have been determined in the case of Algol a 

 and a few others of that class: large dark companions revolve around 

 these stars, and once in every revolution the companions pass between 

 us and the principal stars, thus preventing a portion of their light 

 from reaching us. In Zeta Geminorum 3 and three or four others of its 

 class the spectroscope has shown that massive dark companions are 

 close to, and rapidly revolving around, the principal stars. These 

 invisible companions produce disturbances in the extensive atmo- 

 spheres of the stars, and cause the observed variations in brightness, 

 but the nature of the disturbances is still a matter of conjecture. 

 Omicron Ceti 4 and other stars of its class have given no evidence of 

 companions. Brightness variations in them seem to be due to in- 

 ternal causes. Perhaps they have reached the age when solid crusts 

 attempt to form on their surfaces, just as one day a crust struggled 

 to form on the liquid earth. A crust formed one month may be 

 melted or sink to a lower level a few months later. Perhaps there 

 are "sun-spots" on these stars, in scale vastly more extensive and in 

 period shorter than those on our sun; but these suggested explana- 

 tions may be far from the truth. 



1 Astronomical Journal, vin, 81. * Astrophysical Journal, xin, 90. 



1 Clerke's System of the Stars, p. 128. * Lick Observatory Bulletin, no. 41. 



