COSMOGONY AND STELLAR EVOLUTION. 



By J. H. Jeans, F. R. S. 



I. THE EVOLUTION OF GASEOUS MASSES. 



The progress of observational astronomy has made it abundantly 

 clear that astronomical formations fall into well-defined classes: 

 they are almost "manufactured articles " in the sense in which Clerk 

 Maxwell applied the phrase to atoms. Just as atoms of hydrogen or 

 calcium are believed to be of similar structure no matter where they 

 are found, so star-clusters, spiral nebulae, binaiw stars are seen to be 

 similar, although in a less degree, no matter in what part of the sky 

 they appear. The problem of cosmogony is to investigate the origins 

 of these comparatively uniform formations and the process of transi- 

 tion from one class to another. 



In attacking this problem the cosmogonist of to-day stands upon the 

 shoulders not only of previous cosmogonists, but also, what is of even 

 greater importance, upon the shoulders of the brilliant and indus- 

 trious astronomical observers of the past century. We shall find it 

 convenient to take as our starting point the most famous theory of 

 cosmogony ever propounded — the nebular hypothesis of Laplace — 

 and we shall examine to what extent it remains tenable in the light 

 of modern observational and theoretical research. 



Laplace's hypothesis referred primarily to the genesis of the solar 

 system, which he believed to have originated out of a hot nebulous 

 mass that shrank as it cooled. The nebula was supposed to be in 

 rotation, so that the principle of conservation of angular momentum 

 required that as the mass cooled its speed of rotation should increase. 

 It is well known that a mass either of gas or of liquid in rotation 

 can not rest in equilibrium in the spherical shape which would be 

 assumed in the absence of rotation. If the rotation is very slow the 

 equilibrium shape will be an oblate spheriod of small eccentricity. 

 As the rotation increases, the ellipticity will increase, but it is found 

 that the spheroidal shape is soon departed from. Laplace believed, as 

 a matter of conjecture rather than of reasoned proof, that with con- 



1 Lectures delivered at King's College, London, on May 3 and 10, 1921. Reprinted by 

 permission from Nature, June 30 and July 7, 1921. 



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