PROBLEMS OF MODERN SCIENCE 



in number, whose spectra contain bright lines, 

 and that these again developed into the B stars. 

 The close association of the nebulae with stars 

 seemed to lend support to this view. 



However, if Russell's theory of stellar evolution 

 is correct, it is difficult to find a place for them. 

 They certainly cannot come at the beginning or 

 end of the series, next to the M stars. 



Only about 150 planetaries are known, but if 

 they were the initial stages of stars we should expect 

 them to be much more numerous. 



These, and other considerations which I have 

 not time to mention, suggest that the gaseous 

 nebula are products of a side-path of evolution. 

 It is very significant that the nebulium lines 

 appear at a certain period in the history of new 

 stars, and there is little doubt that new stars are 

 due to the occurrence of some event of a cata- 

 strophic nature, probably a collision between two 

 masses of matter. In such a case temperature would 

 be raised rapidly to the point at which radiation 

 pressure becomes significant, and thus shells of 

 matter would be driven off giving rise to planetary 

 nebulas. Perhaps this may even be the origin of 

 the diffuse irregular nebulas. The appearances 

 which several of them present suggest rather 

 expulsion from, than attraction to, the stars with 

 which they are associated. 



The spiral nebulas are far more numerous than 

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