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THE SOURCE OF STELLAR ENERGY 
Subatomic Energy. 
204. Since we are limited to energy liberated in the deep interior of 
the star, extraneous sources of supply are ruled out, and it is scarcely 
possible to escape the conclusion that the supply of energy for future 
expenditure is already hidden in the star. Energy, however, cannot be 
successfully hidden; it betrays itself by its manifestation as mass. Energy 
and mass are equivalent, and we know the masses of the stars. 
This immediately sets an upper limit to the supply of energy available 
for radiation for all time (unless the star sweeps up further mass in its 
progress through space). The mass of the sun 1*985.10 33 gm. when ex 
pressed in energy units amounts to 1*7 85.10 54 ergs. This then is the total 
store. At the present rate of radiation it would last 15 billion years 
(1*5.10 13 ). If the whole of this supply is going to be used the sun in its 
later stages will be a star of smaller mass and eke out the supply by 
radiating less strongly. On the other hand, if the sun started as a star 
of infinitely large mass its present age must nevertheless be less than 10 13 
years owing to the greater rate of radiation for large masses. 
This time-scale is an upper limit because, although the energy is present 
in the star, we do not know how much of it is utilisable for the purposes of 
radiation. 
This store of energy is, with insignificant exception, energy of con 
stitution of the atoms and electrons or, as it is usually called, subatomic 
energy. The processes by which subatomic energy might be liberated are— 
I. (a) Breaking down of the more complex elements into simpler 
elements (radio-activity). 
(6) Building up of complex elements from simpler elements. 
II. Mutual cancellation of protons and electrons. 
It may seem anomalous that energy can be liberated both in the build 
ing up and in the breaking down of higher elements, but both cases can 
occur. Like chemical combination, the combination of protons and electrons 
in the nucleus is sometimes endothermic and sometimes exothermic. 
Breaking down of complex nuclei with liberation of energy is familiar in 
radio-active transformations. The only definitely known example of libera 
tion of energy in the building up of nuclei is in the formation of helium 
from hydrogen. The helium nucleus contains 4 protons (hydrogen 
nuclei) bound closely with 2 electrons; this gives it a net electric charge 
+ 2e in accordance with its atomic number Z = 2. The atomic weight 
4*00 comes from the mass of the protons, that of the electrons being 
insignificant, so that each proton is responsible for a mass 1*000. But the 
mass of the uncombined proton as it occurs in the hydrogen atom is 1*008. 
This difference is established by the chemically determined atomic weights