418 
Conclusion 
[CH. XVII 
the present total mass of the star. Not only, then, are the masses of the stars 
gradually melting away into radiation, but in most stars the greater part of 
the mass has already so melted. 
The amount of mass which is left to a star provides a measure of the 
length of time during which it can continue to emit radiation. We find, for 
instance, that the sun possesses enough mass to continue to radiate at its 
present rate for 15 million million years. Actually the sun can look forward 
to a longer life than this, for as a star ages the rate at which it radiates 
away energy, and so the rate at which it spends its mass, continually 
diminishes. W'hen allowance is made for this senile tendency to parsimony, 
we find that stars such as our sun can continue to shine for some hundreds 
of millions of millions of years. 
389 . At the beginning of its life a star has a huge store of mass, the 
greater part of which is destined ultimately to be transformed into radiation. 
There is only one way known to physics in which such enormous amounts of 
mass can be stored, namely in the form of electrons and protons which are 
combined into atoms, although not necessarily atoms of terrestrially known 
types. And the radiation of the stars must be provided by the annihilation 
of these atoms. An investigation into the stability of the stars has shewn that 
for the stars to be stable structures, not liable suddenly to transform their whole 
mass explosively into radiation, their atoms must liberate energy spontaneously 
as the radioactive atoms do, the rate of liberation not depending to any great 
extent on the density or temperature. The relatively cool temperature of the 
earth’s surface proves that terrestrial atoms have no appreciable capacity for 
liberating energy, so that the atoms which liberate energy in the sun and 
stars must be of different type from terrestrial atoms. 
Thus the future radiation is stored in the form of electrons and protons in 
the star, and the process of liberation of energy must consist of an annihilation 
of matter, electrons and protons neutralising one another and setting free 
radiation of mass equivalent to that of the annihilated matter. Different 
types of matter must be liable to annihilation at different rates, so that it 
ought to be possible to estimate the age of a star either from the amount of 
matter left, or from the proportions in which the different types of matter 
occur. An important reservation must, however, be made. We have found 
reasons for thinking that atoms which are completely broken up into their con 
stituent electrons and protons are immune from annihilation. If so we cannot 
estimate the age of a star unless we know for how long and to what extent 
its atoms have been preserved from annihilation in this manner. 
For instance, if we suppose that the sun has always been a normal star 
radiating energy at the rate normally appropriate to its mass, calculation 
shews that it cannot have existed for more than 8 million million years. But 
this length of life can be extended indefinitely if we suppose part of it to