180 



ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1920. 

 W. S. Adams's results up to Nov. 8, 1918. 



In addition to this confirmation by new observations we have had 

 an independent confirmation by the brilliant theoretical work of 

 Professor Eddington, who has attacked the problem of the life of a 

 star mathematically. He supposed a mass of gas first of all to be 

 simply under the action of its own gravity. It will consequently 

 contract, and owing to the contraction will rise in temperature ; but 

 Professor Eddington soon found that this simple hypothesis would 

 not answer — it led him to impossible results. Clearly something else 

 besides gravity must be at work, and he was driven to the further 

 hypothesis that the radiation-pressure inside the star played an 

 important part in its history. Radiation-pressure (or if we like 

 to call it so, light-pressure) is what makes the tail of a comet. As a 

 comet approaches the sun it begins to feel the effects of the fierce 

 light, which is known to be able to drive away very small particles 

 from the head of the comet, much as we can blow away chaff from 

 wheat. In consequence of this action the small dust-like particles 

 which may exist in the head are believed to be driven outward to 

 form the tail. But this force is not merely in existence on the 

 outside of the sun; it permeates its whole body. A particle inside 

 the sun is of course receiving radiation-pressure from all its sur- 

 roundings, but the pressure will naturally be greater on the hotter 



