182 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
until the modern science of nuclear physics was developed. Now ex- 
planations may be sought over a wide range of nuclear reactions giv- 
ing various lifetimes to stars up to the limit set by Einstein’s famous 
formula, 
energy =mass X (velocity of light)? 
For instance, if the whole of the sun’s mass were transformed directly 
into energy, the sun could radiate at the present rate for a million 
million years. But, if nuclear reactions supplied the energy, the 
possible lifetime (with the present rate of radiation) would be reduced 
according to the particular reaction involved. 
The nuclear reactions, in general, produce the transmutation of 
elements—the old dream of the alchemists. The most plausible of 
the current theories concerning the source of the sun’s energy, pro- 
posed by H. Bethe, is based on the carbon cycle in which, because of 
the presence of carbon at temperatures found in the sun, hydrogen 
nuclei may combine to form helium, releasing energy in the process. 
One test of the theory is furnished by a comparison of the relative 
abundance of the different isotopes of carbon actually observed in the 
sun with the relative abundances involved in the carbon cycle. 
In a vaguely analogous way, it is possible to speculate on the build- 
ing up of all elements from the primitive hydrogen atoms, and these 
speculations may be guided by the observed relative abundances of 
elements in stars of widely different physical characteristics. 
Thus the data on abundances, derived from large-scale spectra, 
bear directly on all theories concerning the source of stellar energy, 
the origin of chemical elements, the past history of the universe, and 
its future evolution. 
The third unique field of investigation for the 200-inch is cos- 
mology—the structure and behavior of the universe as a whole. As- 
tronomers hope that the observable region of space—the region that 
can be observed with telescopes—is a fair sample of the universe, and 
they attempt to infer the nature of the universe from the observed 
characteristics of the sample. The 200-inch, because of its great 
light-gathering power, should penetrate into space about twice as far 
as the 100-inch, and consequently will permit us to explore a volume 
of space about eight times that now available. The probability that 
the observable region may be a fair sample of the universe will thus 
be greatly increased. 
It was the 100-inch that opened this new field and prepared the 
way for the new telescope. The picture developed rather suddenly 
during the 1920’s. The sun with its family of planets seems isolated 
and lonely in space, but we know that it is merely one of the stars— 
one of several thousand million stars which, together, form the stellar 
system. This system is a swarm of stars which drifts through space 
