§ 2
Nebulæ 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 nebulæ from the earth. The distances of some nebulæ 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 nebulæ 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 nebulæ, 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 nebulæ would seem to be composed of gas in an
extremely rarified form. It is difficult to convey an adequate idea of
the rarity of nebular gases. The residual gases in a vacuum tube are
dense by comparison. A cubic inch of air at ordinary pressure would
contain more matter than is contained in millions of cubic inches of the
gases of nebulæ. The light of even the faintest stars does not seem to
be dimmed by passing through a gaseous nebula, although we cannot be
sure on this point. The most remarkable physical fact about these gases
is that they are luminous. Whence they derive their luminosity we do not
know. It hardly seems possible to believe that extremely thin gases
exposed to the terrific cold of space can be so hot as to be luminous
and can retain their heat and their luminosity indefinitely. A cold
luminosity due to electrification, like that of the aurora borealis,
would seem to fit the case better.
Now the nebular theory is that out of great "fire-mists," such as we
have described, stars are born. We do not know whether gravitation is
the only or even the main force at work in a nebula, but it is supposed
that under the action of gravity the far-flung "fire-mists" would begin
to condense round centres of greatest density, heat being evolved in the
process. Of course the condensation would be enormously slow, although
the sudden irruption of a swarm of meteors or some solid body might
hasten matters greatly by providing large, ready-made centres of
condensation.