ABUNDANCE OF CHEMICAL ELEMENTS—SUESS 317 
half-life of just 56 days. The possibility has been widely discussed 
recently that the light emitted by the gas cloud resulting from the ex- 
plosion of a supernova is essentially supplied by energy from the 
breakup of californium 254 nuclei. 
If this hypothesis is true, then the occurrence of such a transuranium 
element as californium in supernovae proves that neutron buildup 
of heavy elements takes place during the explosion, necessarily lead- 
ing to the formation of all nuclear species predicted by the neutron 
buildup theory. 
There is a variety of other possibilities that can lead to the 
explosion of a star. Helium, continuously produced from hydrogen 
in most stars, can react at the high pressures of a stellar interior to 
form carbon 12, oxygen 16, and heavier elements. When the buildup 
of nuclei reaches iron, the star can become unstable and expel a large 
fraction of its mass into outer space. The debris of such stars will 
contain large amounts of iron. Other types of stellar explosions will 
result in the formation of the light elements preferentially. Certain 
types of stars have been observed to eject matter continuously. Vari- 
ous mechanisms of nuclear synthesis proposed in earlier theories 
may be realized in the interior of different types of stars. In this 
way it becomes understandable that these theories were capable of 
predicting the relative abundances of nucleids of a certain type and 
of a certain mass range, but always failed to account for all the 
empirical facts. 
The question of whether one can accept the hypothesis that the 
elements have formed in stars depends largely on quantitative consid- 
erations of the absolute amounts that can be produced in such a 
way during a reasonable time interval, and the amount necessary 
to explain the observed composition of the stars of our galaxies. 
One can estimate that at present only about one-half of the mass of 
our galaxies is concentrated in stars. The other half is present in the 
form of interstellar gas and dust. 
It can be assumed that about 7 billion years ago our galaxies had 
the form of a huge gas mass of pure hydrogen. Out of this gas mass, 
large hydrogen stars condensed. In their interior hydrogen was con- 
verted into helium and into heavier elements. The rate of such 
processes in large stars is greater than in small ones, so that the large 
stars become unstable relatively quickly. The matter they ejected 
contained heavier elements. New generations of stars accumulated 
continuously from interstellar matter. They contained increasing 
amounts of heavier elements. Our sun, the solar system, and the 
earth were formed about 4.5 billion years ago. New astronomical 
evidence indicates that stars younger than our sun contain a larger 
percentage of heavier elements. 
