The constitution and evolution of the stars

106 Recent Advances in Stellar Astronomy
short run, we might find alternate over-production, lead-
ing to expansion of the star and cooling, and under-
production, permitting contraction and heating; and os-
cillations of just this sort appear to happen in the Cepheid
variables. Though the star may thus be kept shining for
a very long time, it cannot go on forever, for the store
of internal energy, however vast, must be finite, and will
gradually be used up. As this happens, the star will con-
tract, although very slowly, and ultimately pass through
the various giant and dwarf stages, in substantially the
manner which was described earlier.

Such a store of available energy will account for the
facts; but how shall we attempt to account for the store
of energy itself? One thing is clear at the start. The
only places small enough to contain so huge an accumula-
tion are the nuclei of the atoms. I say “small” advisedly,
for it is only when the constituent parts of which the
atoms are built come exceedingly close together that the
forces between them can become great enough to ac-
count for their possession of such an amount of energy.
Radio-active energy, which comes from atomic nuclei,
represents indeed one such gigantic store. But the amount
of energy which must once have been stored in each
gram of the Sun’s mass, to account for its past radia-
tion of heat, is even greater than that contained in uran-
ium. We cannot do more than guess where it may have
been hidden; but one very recent piece of work affords
a possible clue.

Aston, in one of the brilliant researches which we have
learned to associate with the Cavandish Laboratory at
Cambridge, has invented a beautiful apparatus which sorts
atoms, by giving them electrical charges and shooting them
through a vacuum under the influence of electric and mag-

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