ASTRO-PHYSICS 285 



than enough to compensate for that lost by 

 radiation. This reasoning was appHed to the 

 sun, and estimates of the sun's Hfe as a useful 

 radiating system were made by Lord Kelvin and 

 others. But the past history of the sun was, 

 on these calculations, far too short to admit of 

 the periods required by the geologist and the 

 biologist for the formation of the earth's crust 

 and the evolution of species thereon. 



It was thought at first that the phenomena 

 of radio-activity would throw new light on this 

 problem. If but two or three parts in a million 

 of the sun's mass consist of radium, the present 

 rate of heat emission would be maintained. The 

 prevalence of helium suggests the occurrence of 

 radio-active processes, during which, as we 

 know, helium may be formed. But the balance 

 of evidence seems against the idea that enough 

 radio-active substance exists in the sun to explain 

 his output of heat. 



Indeed, no explanation yet suggested by 

 known processes is adequate. We are forced 

 to believe that some change, never seen in our 

 laboratories, must be going on inside the sun. 

 With the glimpses we have now had into the 

 wonders of atomic structure such an idea need 

 not surprise us. The temperature within the sun 

 must be far higher than the mere 6000° of the 

 radiating layer, and may be of the order of a 

 million degrees. At such temperatures phe- 

 nomena quite unknown to us may appear — the 

 disintegration of atoms stable on earth, or 

 possibly the direct conversion of electronic 

 matter into radiant energy by collision. This 

 is perhaps only vain speculation, put forward to 



