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 nebulæ.
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 nebulæ, 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).