In its main lines the spectrum of a star corresponds to its colour, and
we may roughly group the stars into red, yellow, and white. This is also
the order of increasing temperature, the red stars being the coolest and
the white stars the hottest. We might therefore imagine that the white
stars are the youngest, and that as they grow older and cooler they
become yellowish, then red, and finally become invisible--just as a
cooling white-hot iron would do. But a very interesting recent research
shows that there are two kinds of red stars; some of them are amongst
the oldest stars and some are amongst the youngest. The 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.