14 THE EVOLUTION OP THE UNIVERSE 



whatever allowance we make for higher temperature^ 

 they must be far larger. We cannot measure the size 

 of stars, but, when the distance is known, we can 

 measure their brilliance. Professor Pickering has 

 recently calculated that the star Rigel, in the con- 

 stellation Orion, gives out 87,000 times as much light 

 as our sun; and our sun has a temperature of 7,000° C. 

 at its metallic surface, and possibly 1,000,000° C. in 

 its interior. The star Canopus is said to be equal in 

 light-power to 50,000 suns like ours. 



Here we have a first suggestion of evolution in our 

 universe. The colours of the stars may have various 

 causes, but in the main they are determined by 

 temperature. Some are blue, some white, some yellow 

 (like our sun), and some red. That sounds like 

 globes of metal cooling down ; and the astronomer now 

 has instruments by which he can fully confirm this 

 first impression. The stars are masses of white-hot 

 metal, surrounded by flaming gas, which rise up to a 

 certain highest point of temperature (probably about 

 30,000° C. at the surface) and gradually cool down 

 until they cease to give out light. By an instrument 

 called the spectroscope — the most wonderful instru- 

 ment ever invented by man — we can analyse their 

 light and trace the different stages of their cooling.^ 



^ I wish to be brief and simple here, but the reader who would 

 like to know more about this fascinating branch of the science of 

 evolution will find the material of this chapter, fully and clearly 

 developed, in my recent work, The End of the World (Watts; 6s. 

 6d. net ; with many illustrations) , 



