88 The Outline of Science 



a dozen classes which generally correspond to stages in their evo- 

 lution (Fig. 21). 



The Age of Stars 



In its main lines the spectrum of a star corresponds to its 

 colour, and we may roughly group the stars into red, yellow, and 

 white. This is also the order of increasing temperature, the red 

 stars being the coolest and the white stars the hottest. We might 

 therefore imagine that the white stars are the youngest, and that 

 as they grow older and cooler they become yellowish, then red, 

 and finally become invisible just as a cooling white-hot iron 

 would do. But a very interesting recent research shows that there 

 are two kinds of red stars; some of them are amongst the oldest 

 stars and some are amongst the youngest. The facts appear to 

 l>e that when a star is first formed it is not very hot. It is an 

 immense mass of diffuse gas glowing with a dull-red heat. It 

 contracts under the mutual gravitation of its particles, and as 

 it does so it grows hotter. It acquires a yellowish tinge. As it 

 continues to contract it grows hotter and hotter until its tem- 

 perature reaches a maximum as a white star. At this point the 

 contraction process does not stop, but the heating process does. 

 Further contraction is now accompanied by cooling, and the 

 star goes through its colour changes again, but this time in 

 the inverse order. It contracts and cools to yellow and finally 

 to red. But when it again becomes a red star it is enormously 

 r and smaller than when it began as a red star. Conse- 

 quently the red stars arc divided into two classes called, appro- 

 priately, (nants and Dwarfs. This theory, which we owe to an 

 American astronomer, H. N. Russell, has been successful in 

 rx plaining a variety of phenomena, and there is consequently 

 g 00 *! t(l Mippose it to be true. But the question as to how 



I frj.-mt stars were formed has received less satisfactory and 



tt ansu 



The most commonly accepted theory is the nebular theory. 



