46 THE FITNESS OF THE ENVIRONMENT 



are present in the sun, and very often, at 

 least, in the stars. Exceptions may arise, but 

 probably they will hardly suffice to invalidate 

 the rule. 



The large, dark, dense bodies which are 

 directly known to us are the planets and 

 their satellites. There are, however, many in- 

 dications that the heavens are occupied by 

 great numbers of "dead" suns, incrusted and 

 therefore no longer luminous. Such appear 

 to be the only conclusions which can be drawn 

 from a study of the energetics of solar evolu- 

 tion, for sooner or later a sun must cool from 

 loss of energy until at length a crust forms, and, 

 barring catastrophe, it must then endure for- 

 ever. Moreover, as we have seen, the varying 

 aspects of the stars seem to disclose suns in 

 all stages of such a process. 



More nearly direct is the evidence furnished 

 by study of variable stars of the Algol type. 

 Algol itself Q8 Persei) is a star of second 

 magnitude with a period of 2 days, 20 hours, 

 48 minutes, 53.8 seconds. During each period 



hitze fliichtigen Stoffe mussten in ihr vorkommen. Eine 

 entsprechende Beschaffenheit muss heute noch die Atmosphare 

 der Sonne besitzen." — G. Kirchhoff, — " Untersuchungen 

 iiber das Sonnenspektrum und die Spektren der Chemischen 

 Elemente." Abhandlungen der Koniglichen Akademie der 

 Wissenschaften zu Berlin, 1861. Zweite, durch einen An- 

 hang vermehrte Ausgabe. Berlin, 1862. 



