CONSTITUTION OF THE STARS RUSSELL 149 



in which the brighter stars have an egregious preference, and it has 

 already been seen that, in these stages, great brightness means large 

 mass. 



A more searching test is found in the densities of stars of the vari- 

 ous sorts; for here we can make our comparison quantitative in- 

 stead of merely qualitative. The stars of increasing temperature 

 should have densities at which the simple gas laws can be trusted 

 to apply, at least approximately ; the dwarfs should be so dense that 

 we can be sure that these laws fail of application ; while the hottest 

 stars should have an intermediate density corresponding to the 

 region in which the gas laws are strikingly at work. From a general 

 knowledge of the properties of matter, we can say with certainty 

 that a density less than ten times that of air falls in the first class, 

 one greater than that of water in the second, while the " twilight 

 zone " between corresponds to densities in the neighborhood of one- 

 tenth to one-quarter that of water, and perhaps a little higher. Now 

 we have already seen that the redder giant stars are less dense than 

 air — the whiter ones being probably from ten to fifty times denser; 

 that the average density of the A stars is one-fifth that of the sun, 

 or one-third that of water, while their individual densities range 

 from about fifty times that of air to that of water, and that the 

 dwarf stars have densities running from about that of water up 

 to four or five times as much. The agreement is perfect throughout 

 and there can be no remaining doubt that the proposed physical 

 model represents what actually happens in the stars. 



This theory of stellar evolution was first propounded by Sir Nor- 

 man Lockyer, who outlines clearly the physical processes involved. 

 His criteria for distinguishing between stars of rising and falling 

 temperature were spectroscopic, and chosen in a rather arbitrary 

 way, with little explanation (though they were not very far from 

 anticipating Adams' later discovery), and his views failed of 

 general acceptance. It fell to the speaker's lot, some years later, to 

 revive the theory, and point out the importance of the absolute 

 magnitudes, which, indeed, furnish the key to the whole problem. 

 This invaluable aid was not available when Lockyer began his 

 work — for in those days little indeed was known of stellar paral- 

 laxes — so that it is not surprising that his individual assignments- 

 of stars to the classes of rising and falling temperature are often 

 erroneous. With the wealth of material now available, it is an easy 

 matter to point out stars in every successive stage of evolution, and 

 to assign the large majority of those for which we have data to 

 their place in its sequence. Mention should again be made, how- 

 ever, of the few, faint, but perplexing white stars of low luminosity. 



1454—25 11 



