facts appear to
be that when a star is first formed it is not very hot. It is an immense
mass of diffuse gas glowing with a dull-red heat. It contracts under the
mutual gravitation of its particles, and as it does so it grows hotter.
It acquires a yellowish tinge. As it continues to contract it grows
hotter and hotter until its temperature reaches a maximum as a white
star. At this point the contraction process does not stop, but the
heating process does. Further contraction is now accompanied by cooling,
and the star goes through its colour changes again, but this time in the
inverse order. It contracts and cools to yellow and finally to red. But
when it again becomes a red star it is enormously denser and smaller
than when it began as a red star. Consequently the red stars are divided
into two classes called, appropriately, Giants and Dwarfs. This theory,
which we owe to an American astronomer, H. N. Russell, has been
successful in explaining a variety of phenomena, and there is
consequently good reason to suppose it to be true. But the question as
to how the red giant stars were formed has received less satisfactory
and precise answers.

The most commonly accepted theory is the nebular theory.

THE NEBULAR THEORY

Sec. 2

Nebulae are dim luminous cloud-like patches in the heavens, more like
wisps of smoke in some cases than anything else. Both photography and
the telescope show that they are very numerous, hundreds of thousands
being already known and the number being continually added to. They are
not small. Most of them are immensely large. Actual dimensions cannot be
given, because to estimate these we must first know definitely the
distance of the nebulae from the earth. The distances of some nebulae are
known approximately, and we can therefore form some idea of size in
these cases. The results are staggering. The mere visible surface of
some nebulae is so large that the whole stretch of the solar system would
be too small to form a convenient unit for measuring it. A ray of light
would require to travel for years to cross from side to side of such a
nebula. Its immensity is inconceivable to the human mind.

There appear to be two types of nebulae, and there is evidence suggesting
that the one type is only an earlier form of the other; but this again
we do not know.

The more primitive nebulae would seem to be composed of gas in an
extremely rarified form. It is difficult to convey an adequate idea of