144 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



of tho expansion of the sj^stcm of the galaxies which constitutes our 

 universe, as well as studies of the stabiHty of individual galaxies, make 

 it difficult to assign an age to the stars greater than 5,000 milUon 

 years. The radiation required for this period is amply provided for 

 by the transmutation hj^pothcsis, and the hundred fold greater 

 energy provided by the annihilation hypothesis would only be an 

 embarrassment. 



Both hypotheses were originHlly theoretical suggestions; but the 

 transmutation hypotlicsis can now claim a definite observational 

 basis. Take the sun, which we have found to be one third hydrogen 

 and two thirds not-hj'drogen. At 15 million degrees the hydrogen 

 is ionized and its nuclei — i. e., the protons — are traveling at average 

 speeds of 500 miles a second. We know that in the laboratory pro- 

 tons of this speed attack and enter the nuclei of other elements — the 

 not-hydrogcn — and bring about transmutations in tliem. We may 

 hope that in due time the physicists will be able to trace for us tho 

 whole sequence of changes direct and indirect which result, so that 

 we shall be able to find quantitatively the rate of disappearance of 

 free hydrogen under these conditions, and so find the amount of 

 subatomic energy of this kind liberated in the sun. If it is found 

 to agree with the sun's rate of radiation, we shall then have definite 

 proof that no other source — such as annihilation — is operative. We 

 are, of course, far from having the necessary knowledge at present. 

 It is comphcated by the fact that, although the protons enter atomic 

 nuclei and change the nuclei into new elements, in many cases the 

 new nucleus breaks down after a short time, a proton is shot out and 

 no permanent transmutation results. Such permanent transmutation 

 as is observed comes at the end of a chain of processes of which the 

 attack of the proton on the nucleus is the first. It is interesting to 

 notice that this was already foretold by the astronomical investiga- 

 tions which, as I have said, demand a time-lag between a stimulation 

 of the activity of the protons by rise of temperature and the corre- 

 sponding increase of output of subntomic energy. 



I have given you my unpression of the way in which this new knowl- 

 edge works in with, and so far as we can see, agrees with the existing 

 theory of stellar constitution, not because I lay stress on the rough 

 conclusions that can be drawn in the turmoil of new discovery — the 

 data available at present are far too scrappy — but because I want to 

 show how intensely important for astronomy is the work of atom- 

 splitting now in progress — so that we may look forward to great 

 developments in the future. 



