108 Recent Advances in Stellar Astronomy
protons would be greater than that of the alpha particle
by one part in 130. (The electrons weigh next to noth-
ing.) This seems to spoil the explanation altogether,
but an escape is found in that great resolver of otherwise
intractable difficulties, the Principle of Relativity. Accord-
ing to this, all energy has mass, and all mass is equiva-
lent to energy. The loss of mass in the formation of
the alpha particle would mean that, in forming it, energy
would be Fiberated, which would have to be put back into
it again in order to separate the parts. The calculated
amount of energy is so enormously great that it is not
at all surprising that the alpha particle is so stable.
Even in the collisions with other atomic nuclei which
shatter the latter into fragments, the forces (which can
be roughly calculated) are not nearly strong enough to
disintegrate it.
We may now suppose that, in the interior of the stars,
and by some process the details of which are still quite
unknown, the atoms of hydrogen are taken apart, and
the pieces—protons and electrons—built up into the nuclei
of heavier atoms, with just enough electrons left over
to build the outer parts of these. We cannot be sure, of
course, that such a thing actually happens; but, if it does,
the energy liberated will suffice for the present demands
of astrophysics. If the Sun, for example, was originally
all hydrogen, which was transformed in this fashion into
other elements, the energy which would be set free as a
by-product would keep it shining at the present rate for
about 120 billions of years.
Such is our present conception of the stars, their dis-
tance, their age, their nature, and their life-history. In
the grandeur of its sweep in space and time, and the
beauty and simplicity of the relations which it discloses