There is a fundamental difference between the illumination of these
little rings that I have shown you and the great rings in the heavens.
I had to illuminate our smoke with the help of the electric light,
for, unless I had done so, you would not have been able to see them.
This white substance formed by the union of ammonia and hydrochloric
acid has, of course, no more light of its own than a piece of chalk;
it requires other light falling upon it to make it visible. Were the
ring nebula in Lyra composed of this material, we could not see it.
The sunlight which illuminates the planets might, of course, light up
such an object as the ring, if it were comparatively near us; but Lyra
is at such a stupendous distance that any light which the sun could
send out there would be just as feeble as the light we receive from a
fixed star. Should we be able to show our smoke-rings, for instance,
if, instead of having the electric light, I merely cut a hole in the
ceiling and allowed the feeble twinkle of a star in the Great Bear to
shine through? In a similar way the sunbeams would be utterly powerless
to effect any illumination of objects in these stellar distances. If
the sun were to be extinguished altogether, the calamity would no doubt
be a very dire one so far as we are concerned, but the effect on the
other celestial bodies (moon and planets excepted) would be of the
slightest possible description. All the stars of heaven would continue
to shine as before. Not a point in one of the constellations would be
altered, not a variation in the brightness, not a change in the hue of
any star could be noticed. The thousands of nebulæ and clusters would
be absolutely unaltered; in fact, the total extinction of the sun would
be hardly remarked in the newspapers published in the Pleiades or in
Orion. There might possibly be a little line somewhere in an odd corner
to the effect “Mr. So-and-So, our well-known astronomer, has noticed
that a tiny star, inconspicuous to the eye, and absolutely of no
importance whatever, has now become invisible.”
If, therefore, it be not the sun which lights up this nebula, where
else can be the source of its illumination? There can be no other star
in the neighborhood adequate to the purpose, for, of course, such an
object would be brilliant to us if it were large enough and bright
enough to impart sufficient illumination to the nebula. It would be
absurd to say that you could see a man’s face by the light of a candle
while the candle itself was too faint or too distant to be visible.
The actual facts are, of course, the other way; the candle might be
visible, when it was impossible to discern the face which it lighted.
Hence we learn that the ring nebula must shine by some light of its
own, and now we have to consider how it can be possible for such
material to be self-luminous. The light of a nebula does not seem to
be like flame; it can, perhaps, be better represented by the pretty
electrical experiment with Geissler’s tubes. These are glass vessels
of various shapes, and they are all very nearly empty, as you will
understand when I tell you the way in which they have been prepared. A
little gas was allowed into each tube, and then almost all the gas was
taken out again, so that only a mere trace was left. I pass a current
of electricity through these tubes, and now you see they are glowing
with beautiful colors. The different gases give out lights of different
hues, and the optician has exerted his skill so as to make the effect
as beautiful as possible. The electricity, in passing through these
tubes, heats the gas which they contain, and makes it glow; and just
as this gas can, when heated sufficiently, give out light, so does the
great nebula, which is a mass of gas poised in space, become visible in
virtue of the heat which it contains.
We are not left quite in doubt as to the constitution of these gaseous
nebulæ, for we can submit their light to the prism in the way I
explained when we were speaking of the stars. Distant though that ring
in Lyra may be, it is interesting to learn that the ingredients from
which it is made are not entirely different from substances we know
on our earth. The water in this glass, and every drop of water, is
formed by the union of two gases, of which one is hydrogen. This is an
extremely light material, as you see by a little balloon which ascends
so prettily when filled with it. Hydrogen also burns very readily,
though the flame is almost invisible. When I blow a jet of oxygen
through the hydrogen, I produce a little flame with a very intense
heat. For instance, I hold a steel pen in the flame, and it glows and
sputters, and falls down in white-hot drops. It is needless to say
that, as a constituent of water, hydrogen is one of the most important
elements on this earth. It is, therefore, of interest to learn that
hydrogen in some form or other is a constituent of the most distant
objects in space that the telescope has revealed.