276 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1931 



time. The burning of hydrogen and oxygen would not last the sun 

 more than one-tenth of the lifetime of our earth. Radium has been 

 suggested. If the sun were made of pure radium, it would give out 

 as much heat as the sun has given out since the earth was started, but 

 it would be very unequally distributed over this period of time. For 

 2,000 years the sun would shine with a furious heat and then rapidly 

 cool and become invisible. 



Only two possibilities remain. The first is that matter itself is 

 being transformed into radiant energy deep in the stars. If this is 

 the source of the sun's heat, we can calculate on the theory of rela- 

 tivity that the sun is consuming every three hours as much matter 

 as there is in the bulk of Mount Wilson. And yet the sun is so large 

 that it could well stand this loss and go on shining for several million 

 million years to come. 



The other possibility is that the stars were once composed entirely 

 of hydrogen and that the atoms of hydrogen are uniting to build up 

 the heavier atoms of other elements. In the process of becoming thus 

 tightly packed, a small but definite fraction of the mass must be lost, 

 and its equivalent must appear as energy. If this is the source of 

 stellar energy, the life of a star is 100 times shorter than if there 

 were complete annihilation of matter, and every two minutes a mass 

 of hydrogen equivalent to the bulk of Mount Wilson is built up in 

 the sun into atoms of more complex elements. 



