I9I2] AND OTHER CHARACTERISTICS OF STARS. 579 



The highest temperature will be attained at a density for which 

 the departures from the gas laws are already considerable, but prob- 

 ably long before the density becomes as great as that of water. 



The density of the stars of classes B and A (which all lines of 

 evidence show to be the hottest) is actually found to average about 

 one fifth that of water, that is, of just the order of magnitude pre- 

 dicted by this theory. It appears therefore to be a good working 

 hypothesis that the giant and dwarf stars represent different stages 

 in stellar evolution, the former, of great brightness and low density, 

 being stars effectively young, growing hotter and whiter; while the 

 latter, of small brightness and high density, are relatively old stars, 

 past their prime, and growing colder and redder. The stars of class 

 B, and probably many of those of class A as well, are in the prime 

 of life, and form the connecting link between the two kinds of 

 red stars. 



Princeton University Observatory, 

 May 25, 19 1 2. 



