ASTRONOMY 



mum temperature, the star, whose diameter has 

 now contracted to dimensions comparable with 

 that of the sun, starts on its downward course, 

 retracing the spectral series in the order B, A, F, 

 G, K, M. It loses more heat than it can gain by 

 contraction, and at length, after reaching the red 

 stage, passes below the limit of visibility. 



It is not pretended that this theory is correct 

 in all its features. It will certainly need revision 

 in the light of fuller knowledge. But at least it 

 affords a working hypothesis which appears not 

 to be in serious conflict with any facts at present 

 known. 



We may now go on to enquire what was the 

 star before it began its visible career. In what 

 state of aggregation or form did the matter of 

 which it was composed exist ? 



Ever since Laplace propounded the Nebular 

 Hypothesis of the origin of the Solar System 

 there has been a general acceptance of the view 

 that nebulas precede stars in the order of evolution. 

 It seems natural to assume that widely diffuse 

 aggregates of matter form systems dynamically 

 unstable, and that under the action of gravitation 

 they will tend to condense into more compact 

 and stable forms. It is true that mathematical 

 investigation has seriously challenged the com- 

 petence of the Nebular Hypothesis to account for 

 the origin of the Solar System, but the grounds 



