N HERCULES

A wonderful cluster of stars. It has been estimated that the distance of
this cluster is such that it would take light more than 100,000 years to
reach us.]

THE STELLAR UNIVERSE

Sec. 1

The immensity of the Stellar Universe, as we have seen, is beyond our
apprehension. The sun is nothing more than a very ordinary star, perhaps
an insignificant one. There are stars enormously greater than the sun.
One such, Betelgeux, has recently been measured, and its diameter is
more than 300 times that of the sun.

The Evolution of Stars

The proof of the similarity between our sun and the stars has come to us
through the spectroscope. The elements that we find by its means in the
sun are also found in the same way in the stars. Matter, says the
spectroscope, is essentially the same everywhere, in the earth and the
sun, in the comet that visits us once in a thousand years, in the star
whose distance is incalculable, and in the great clouds of "fire-mist"
that we call nebulae.

In considering the evolution of the stars let us keep two points clearly
in mind. The starting-point, the nebula, is no figment of the scientific
imagination. Hundreds of thousands of nebulae, besides even vaster
irregular stretches of nebulous matter, exist in the heavens. But the
stages of the evolution of this stuff into stars are very largely a
matter of speculation. Possibly there is more than one line of
evolution, and the various theories may be reconciled. And this applies
also to the theories of the various stages through which the stars
themselves pass on their way to extinction.

The light of about a quarter of a million stars has been analysed in the
spectroscope, and it is found that they fall into about a dozen classes
which generally correspond to stages in their evolution (Fig. 21).

The Age of Stars

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