§ 3
Now let us return to our consideration of the sun.
To us on the earth the most patent and most astonishing fact about the
sun is its tremendous energy. Heat and light in amazing quantities pour
from it without ceasing.
Where does this energy come from? Enormous jets of red glowing gases can
be seen shooting outwards from the sun, like flames from a fire, for
thousands of miles. Does this argue fire, as we know fire on the earth?
On this point the scientist is sure. The sun is not burning, and
combustion is not the source of its heat. Combustion is a chemical
reaction between atoms. The conditions that make it possible are known
and the results are predictable and measurable. But no chemical reaction
of the nature of combustion as we know it will explain the sun's energy,
nor indeed will any ordinary chemical reaction of any kind. If the sun
were composed of combustible material throughout and the conditions of
combustion as we understand them were always present, the sun would burn
itself out in some thousands of years, with marked changes in its heat
and light production as the process advanced. There is no evidence of
such changes. There is, instead, strong evidence that the sun has been
emitting light and heat in prodigious quantities, not for thousands, but
for millions of years. Every addition to our knowledge that throws light
on the sun's age seems to make for increase rather than decrease of its
years. This makes the wonder of its energy greater.
And we cannot avoid the issue of the source of the energy by saying
merely that the sun is gradually radiating away an energy that
originated in some unknown manner, away back at the beginning of things.
Reliable calculations show that the years required for the mere cooling
of a globe like the sun could not possibly run to millions. In other
words, the sun's energy must be subject to continuous and more or less
steady renewal. However it may have acquired its enormous energy in the
past, it must have some source of energy in the present.
The best explanation that we have to-day of this continuous accretion of
energy is that it is due to shrinkage of the sun's bulk under the force
of gravity. Gravity is one of the most mysterious forces of nature, but
it is an obvious fact that bodies behave as if they attracted one
another, and Newton worked out the law of this attraction. We may say,
without trying to go too deeply into things, that every particle of
matter attracts every other throughout the universe. If the diameter of
the sun were to shrink by one mile all round, this would mean that all
the millions of tons in the outer one-mile thickness would have a
straight drop of one mile towards the centre. And that is not all,
because obviously the layers below this outer mile would also drop
inwards, each to a less degree than the one above it. What a tremendous
movement of matter, however slowly it might take place! And what a
tremendous energy would be involved! Astronomers calculate that the
above shrinkage of one mile all round would require fifty years for its
completion, assuming, reasonably, that there is close and continuous
relationship between loss of heat by radiation and shrinkage. Even if
this were true we need not feel over-anxious on this theory; before the
sun became too cold to support life many millions of years would be
required.
It was suggested at one time that falls of meteoric matter into the sun
would account for the sun's heat. This position is hardly tenable now.
The mere bulk of the meteoric matter required by the hypothesis, apart
from other reasons, is against it. There is undoubtedly an enormous
amount of meteoric matter moving about within the bounds of the solar
system, but most of it seems to be following definite routes round the
sun like the planets. The stray erratic quantities destined to meet
their doom by collision with the sun can hardly be sufficient to account
for the sun's heat.
Recent study of radio-active bodies has suggested another factor that
may be working powerfully along with the force of gravitation to
maintain the sun's store of heat. In radio-active bodies certain atoms
seem to be undergoing disintegration. These atoms appear to be splitting
up into very minute and primitive constituents. But since matter may be
split up into such constituents, may it not be built up from them?
The question is whether these "radio-active" elements are undergoing
disintegration, or formation, in the sun. If they are undergoing
disintegration--and the sun itself is undoubtedly radio-active--then we
have another source of heat for the sun that will last indefinitely.