The large irregular nebulae described in the last chapter are all more
or less mingled with stars, at least in appearance, and it has been
suggested that they are star-clusters in process of formation, with
larger and brighter masses of filmy nebulosity all about them than at
later stages, for long-exposure photographs reveal some exceedingly
faint nebulosities surrounding Kappa Crucis and the Pleiades and other
fully-developed star-clusters. But this can only be a guess until we
know more about the nature of nebulae. In some regions of the sky we
find vast spaces thinly veiled by nebulosity so faint and transparent
that it seems to have reached the very limit at which matter can exist
and be recognised as such. Thus in the constellation of Orion nearly
all the bright stars are connected together by the vast convolutions
of an exceedingly faint diffused nebula in spiral form, the innermost
curve of which ends in the Great Nebula of the Sword, and the whole
region within is filled with faint light.
Quite distinct from these nebulae are others of perfectly regular form,
very small, purely gaseous, without intermingling of any stars, but
usually with one bright star-like nucleus at the centre. One form is
the ring nebula, of which much the best known is that in the northern
constellation of the Lyre. There are, however, some in the south. In a
large telescope they appear like little golden wedding-rings against
the dark sky background.
Another regular form is the “planetary nebula,” so called because
they look much like planets in large telescopes, being perfectly
round or oval with a sharply-defined edge, and in several cases
there are handle-like appendages, which may possibly be encircling
rings, like the rings of Saturn. These nebulae shine with a peculiar
bluish-green light, the colour of the unknown gas nebulium, of which
they are chiefly composed. In Hydra, south of the star Mu, is one of
the brightest and largest, known as H 27—that is, No. 27 on William
Herschel’s list. It is elliptical and of a lovely bluish colour, with a
bright nucleus exactly in the centre.
By means of these sharply-defined central nuclei it has been found
possible to measure the approaching or receding movements of these
nebulae, and although the one just mentioned is receding from us with a
speed of only 3½ miles a second, their average speed is high, amounting
to 40 or 50 miles a second. One in Sagittarius is receding at more
than 80 miles a second, and another in Lupus attains a speed of over a
hundred.
These are movements comparable with those of stars, but the average
is higher than even for the most rapidly moving class of stars, the
red-solar and Antarians. May we, then, place the planetary nebulae
at the end of our star-series, since we saw that from the blue down
to the red the average movements became faster and faster, and may
we believe that all stars eventually become gaseous nebulae, as “new
stars” seem to do? But we saw that in spectrum these nebulae rather
resemble the stars at the other end of the series, the Wolf-Rayet,
which lead directly to the hottest and brightest of all, the Orion
stars. Planetary nebulae also resemble Wolf-Rayet, Orion, and Sirian
stars, and differ from solar and red stars in that they cluster near
the Milky Way, and are scarcely ever found far from it. Their place in
the universe cannot be established yet.
One more kind of nebula, the most numerous of all, remains to be
mentioned, the so-called “white nebulae,” which do not glow green
like many of the brighter planetaries, but shine with a white light
and have more or less star-like spectra, although not even the most
powerful telescopes can resolve the white cloudiness into stars. The
typical nebula of this class is the famous Andromeda Nebula, visible
to the naked eye in northern skies as a large oval spot shining softly
“like a candle shining through horn.” Photography first disclosed
the remarkable fact that it has the form of a great, closely-wound
spiral, and further research has shown that by far the greater number
of “white nebulae” have this form. There is a very fine one in
Aquarius,[12] which has been known since 1824, but visual observations
gave absolutely no idea of its true form. A photograph exposed for four
hours in September 1912 showed it clearly as about two turns of a great
spiral.
[12] N. G. C. 7293.
The distribution of this kind of nebula is quite different from that of
the gaseous nebulae, for, instead of clustering towards the Milky Way,
they avoid it, and especially the brightest region, where we saw that
the others most abound, viz. in Scorpio, Sagittarius, and Ophiuchus. On
the contrary, the largest number of these is found near the north pole
of the Galaxy—that is, as far removed from it as possible, in Virgo.
There is, however, no corresponding group about the south pole of the
Galaxy.
One investigator has found the distance of the Andromeda Nebula to be
twenty light-years, but the distance and the movements of this type
are difficult to discover. They are evidently very different from the
others, and quite as mysterious.