202 



The Evolution of Gaseous Masses 



[CH. VIII 



two classes Russell's giant and dwarf stars. In each class the absolute mag- 

 nitudes are found to cluster fairly closely about a maximum of frequency, the 

 absolute magnitudes of these maxima being as follows : 



201. It has now become clear that theory and observation agree in telling 

 precisely the same story, and that this confirms Russell's theoiy entirely. 

 This being so we are compelled to adopt Russell's theory of the order of 

 stellar evolution throughout the remainder of this book. Indeed in a book 

 such as the present, which attempts to explain observation in terms of theory, 

 it would be quite impossible to adopt the hypothesis that a mass of gas starts 

 its life hot and gets continually cooler. If we attempted to adopt this 

 hypothesis we should find that theory and observation would be pulling in 

 different directions at every stage of our investigation. 



At the same time we are not compelled altogether to abandon the older 

 view of stellar evolution. Our theoretical results have applied to a mass of 

 gas starting from a condition of extreme diffusion, not necessarily to an actual 

 star ; they have traced out the path along which the evolution of a star must 

 progress but nothing has shewn that a star may not be born half-way along 

 this path. So far as our theoretical results go, it is still quite possible for 

 every star in the universe to have been born as a 5-type star ; so far as the 

 observations of Russell and of Adams and Joy go, it is still quite possible for 

 ninety-nine per cent, of the stars in the universe to have been born as 

 J9 type stars. We shall return to these questions in the next chapter. 



As regards the main evolutionary problem, we have learnt that a mass of 

 gas, at least so long as it is of low density, will increase in density and also in 

 temperature. The earliest matter is that of lowest density. We have accord- 

 ingly been led by a theoretical path to the view which we described as the 

 " Theory of nebulous origin " in our Introductory Chapter, but the theory 

 must now be supposed to refer to the gas out of which the star originates 

 rather than to the star itself in the form of a star. 



