154 | ANNUAL REPORT SMITHSONIAN INSTITUTION, 1946: 
an electron and a proton in which both vanish, only to reappear as a 
radiation of fr equency 2 2.31028. There could be no doubt that such a 
process occurring within the deep interior of stars would certainly 
provide a potent source of energy. Chief drawback of the annihilation 
process was the total absence of observational evidence to support it. 
A less energetic but more likely reaction was the formation of heavier 
elements from hydrogen. If, as was then supposed, a helium nucleus 
consisted of four protons and two electrons, its atomic weight should 
be four times that of hydrogen, or 4X 1.008=4.032.7. On the contrary, 
accurate measurements made by Aston in 1920 with the mass spectro- 
graph revealed that the atomic weight of helium is 4.002. ‘Thus 0.030 
mass units had presumably been fiber rated as energy in the atom-build- 
ing process. 
While the transmutation of hydrogen seemed fairly plausible and 
was capable of furnishing the minimum amount of energy that the 
cosmologists demanded for their various hypotheses, yet Eddington, 
summarizing the state of affairs in 1926, found the outlook anything 
but bright. Attempts to formulate a theory of stellar evolution based 
upon the transmutation of hydrogen were invariably strained. Every 
argument led to a deadlock. “Unfortunately, the facts as yet do not 
fall into satisfactory order,” he regretfully admitted, “and we are still 
groping for a clue.” § 
For another 10 years astronomers continued to grope for the essen- 
tial clue before it gradually became apparent. In 1919, Rutherford, 
by bombarding nitrogen with alpha particles, had obtained oxygen of 
atomic weight 17—the first case of the artificial transmutation of the 
elements. A whole new field of research was opened, which atomic 
physicists immediately invaded with great exultation. Exciting dis- 
coveries and epoch-making developments followed with breath-taking 
speed : the positron, the neutron, artificial radioactivity, the cyclotron, 
together with the production of a host of strange particles by nuclear 
disintegration unknown a few years before. And since large quantities 
of energy were released in many of these disintegration oxperiment 
astronomers began to eye them with covetous eyes. 
By 1939 both our experimental and theoretical knowledge a 
progressed to such an extent that it was possible to decide which 
reactions might be responsible for the production of energy in stars 
similar tothesun.® First, all nuclear reactions that might conceivably 
contribute to generating energy in the sun were written down. Then 
T Now, the helium nucleus is believed composed of two protons of weight 2 1.00758 and 
two neutrons of weight 2X 1.00893, so that the weight of the separate parts should be 
4.03302. The latest value for the weight of the helium nucleus is 4.00280, giving a differ- 
ence of 0.038022. 
8 Internal Constitution of the Stars, p. 297. 
®°H. A. Bethe, Phys. Rev., Vol. 55, p. 434, 1939. 
