of the matter out of which stars
(_i.e._ suns) have been and are being evolved. The different types of
star spectra form such a complete and gradual sequence (from simple
spectra resembling those of nebulae onwards through types of gradually
increasing complexity) as to suggest that we have before us, written in
the cryptograms of these spectra, the complete story of the evolution of
suns from the inchoate nebula onwards to the most active sun (like our
own), and then downward to the almost heatless and invisible ball. The
period during which human life has existed upon our globe is probably
too short--even if our first parents had begun the work--to afford
observational proof of such a cycle of change in any particular star;
but the fact of such evolution, with the evidence before us, can hardly
be doubted."[34]

[32] The name Al gul, meaning the Demon, was what the old Arabian
astronomers called it, which looks very much as if they had already
noticed its rapid fluctuations in brightness.

[33] Mr. Gore thinks that the companion of Algol may be a star of the
sixth magnitude.

[34] Presidential Address to the British Association for the Advancement
of Science (Leicester, 1907), by Sir David Gill, K.C.B., LL.D., F.R.S.,
&c. &c.

[Illustration: PLATE XXIV. THE GREAT NEBULA IN THE CONSTELLATION OF
ORION

From a photograph taken at the Yerkes Observatory.

(Page 316)]

CHAPTER XXV

THE STELLAR UNIVERSE

The stars appear fairly evenly distributed all around us, except in one
portion of the sky where they seem very crowded, and so give one an
impression of being very distant. This portion, known as the Milky Way,
stretches, as we have already said, in the form of a broad band right
round the entire heavens. In those regions of the sky most distant from
the Milky Way the stars appear to be thinly sown, but become more and
more closely massed together as the Milky Way is approached.

This apparent distribution of the stars in space has given rise to a
theory which was much favoured by Sir William Herschel, and which is
usually credited to him, although it was really suggested by one Thomas
Wright of Durham in 1750; that is to say, some thirty years or more
before Herschel propounded it. According to this, which is known as the
"Disc" or "Grindstone" Theory, the stars are considered as arranged in
space somewhat in the form of a thick disc, or grindstone, close to the
_central_ parts of which our solar sy