316 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
however, cannot be adjusted in such a way that the over-all abundance 
distribution would correspond to that of a thermodynamic equi- 
librium. It was therefore assumed that equilibrium considerations 
cannot be regarded as a useful way of obtaining reasonable approxi- 
mations. 
In the nonequilibrium theories it is hopefully assumed that a 
relatively simple type of kinetic process has led to the empirical 
abundance distribution. Two such theories have been attracting wide 
interest: the neutron buildup theory, proposed in 1948 by George 
Gamow, and the polyneutron fission theory by Mayer and Teller. 
According to the neutron buildup theory the heavier nuclei were 
formed by the addition of neutrons to very light nuclei and by sub- 
sequent beta decay into stable nuclear species. 
Many features, in particular the smoothness of the abundance lines 
(fig. 3) at higher mass numbers, show conclusively that such processes 
have indeed taken place. However, this theory cannot explain the 
abundance of the lighter elements, the excessive abundance of iron, 
and the existence of the light isotopes of many heavier elements. 
Similarly, the polyneutron fission theory predicts certain features 
in the abundance distribution but fails to approximate the over-all 
trend of the abundance data as a function of mass number. The 
theory leads to abundances of the heavy elements which are many 
thousand times too high. 
These and many other attempts have finally convinced scientists that 
it is impossible to explain the abundances of the elements and their 
isotopes as a product of one particular type of nuclear reaction. A 
group of scientists at the California Institute of Technology has found 
a surprisingly simple way out of this dilemma by considering solar 
and planetary matter as a mixture of the product of different types 
of nuclear reactions, in particular such reactions as can plausibly be 
assumed to occur in the interior of stars. 
Occasionally astronomers observe the sudden appearance of a 
bright new star, a so-called nova. The brightest of them, the super- 
novae, occur in our galaxies about once every 500 years. The super- 
novae, however, are bright enough to be observable in distant galaxies 
almost every year. The energy produced in a supernova outburst 
is equivalent to that of a hydrogen bomb of a size several times that 
of the sun. The debris of the stellar explosion is thrown out into 
space. 
One interesting observation points to a true similarity between man- 
made hydrogen bombs and supernova explosions. The astronomer 
Baade observed that the light intensity of some supernovae decreases 
in a regular way within 56 days to just one-half of its value. The 
debris from explosions of hydrogen bombs was found to contain the 
heavy isotope californium 254. This isotope has a natural fission 
