Athwart the False Cross, from δ Velorum to ι Carinae, a line passing
on leads to the round white spot which we found to be a star-cluster.
A little further in the same direction is a larger curved white patch,
bright enough to be visible, once it is familiar, even after the moon
has risen. This is the Great Nebula in Argo, the Keyhole Nebula, in
which Eta Argūs once blazed out. Even a binocular will divide it into
two parts separated by a chasm, and will show the pearly background
powdered over with many small stars.
But even the most powerful telescopes do not resolve this pale
background into stars, as they resolve the star-cluster just mentioned:
it remains a pearly mist, the brighter part strangely broken by dark
rifts, the fainter, beyond the chasm, a tangled skein of long cloudy
streaks reaching out into the darkness and gradually, irregularly,
fading away.
When Herschel found this background unresolvable into stars, he
concluded that it did not form part of the Milky Way, but was at
an immeasurable distance behind, so that here he was looking right
through the Galaxy at a still more distant region of stars, too distant
and faint for his telescope to distinguish them separately. But the
spectroscope has taught us that these cloud-like nebulae, though stars
are often mingled with them, are not formed of stars at all, but of
inchoate masses of faintly luminous gas; and they cluster so thickly in
the Milky Way, generally avoiding other parts of the sky, that it seems
evident that they lie in it and form part of it. They are also found in
great numbers in the Greater Magellanic Cloud.
If the days of the Herschel’s photography had not come to the aid
of astronomers, and Sir John speaks of the feeling of despair which
often almost overcame him when trying, night after night, to draw the
“endless details” of this nebula, so capricious in their forms are
its curving branches and the dark spaces between, so strangely does
its brightness vary in different regions, and so numerous are the
stars scattered over it. With extraordinary patience he succeeded in
cataloguing the positions of over 1200 of these. To compare the present
aspect of the stars with his catalogue would be a laborious task, but
might lead to results of great value.
The curious dark oval rift in the midst of the bright part, which
he compared with a keyhole, he found to be not entirely devoid of
light, a thin nebulous veil covering part of it; and many of the dark
lanes and holes which in small instruments look perfectly black, are
actually filled with faint stars and extremely faint nebulosity. The
whole region near the nebula is exceedingly rich in stars, and also in
star-clusters, as we have already seen. To quote Herschel once again:
“Nor is it easy for language to convey a full impression of the beauty
and sublimity of the spectacle it offers when viewed in a sweep,
ushered in as it is by so glorious and innumerable a procession of
stars, to which it forms a sort of climax, justifying expressions
which, though I find them written in my journal, in the excitement of
the moment, would be thought extravagant if conveyed to these pages. In
fact, it is impossible for anyone with the least spark of astronomical
enthusiasm about him to pass soberly in review, with a powerful
telescope and in a fine night, that portion of the southern sky ...
such are the variety and interest of the objects he will encounter,
and such the dazzling richness of the starry ground on which they are
presented to his gaze.”
In the constellation of the Sword-fish, on the edge of the Great Cloud
of Magellan, is another nebula, 30 Doradūs, the Great Looped Nebula,
which is even more marvellous in complexity of structure than the
Keyhole Nebula in Argo. No photograph can reproduce, and no words can
describe, the filmy appearance of these nebulae as seen in a telescope.
The Looped Nebula seems to consist entirely of strangely curved and
twisted streamers on a background of dark sky, with a few sparkling
stars of various brightness scattered over it. At the complicated
centre one of the loops forms a nearly perfect figure-of-eight, and
another takes the outline of an eye.
Brightest of all the large gaseous nebulae is the well-known Orion
Nebula, in the sword of the giant. A 3-inch telescope shows the main
features well, the dark bay running into its brightest region, the
row of three brilliant stars and the “trapezium” of four tiny ones
very close together, and the long outlying branches which have such
fantastic curves. Because of its comparative brightness, its entrancing
beauty, and its position where it can be seen from all latitudes, this
nebula has been studied more than any other. The first drawing of it
was made in 1656, the first photograph in 1880. It remains a baffling
mystery still, but a few facts have emerged.
Its distance is immeasurable: it has been guessed at a thousand
light-years. It must, therefore, be inconceivably vast in extent, but
it is probably excessively tenuous, like a comet’s tail, of which a
million miles contain a negligible amount of matter. It is almost
stationary in space, and a careful study of its form since 1758
proves that there has been no visible change, except perhaps in the
relative brightness of some of its parts. Yet a recent spectroscopic
investigation shows that movements are taking place in different
directions within the nebula, and a slow rotation of the whole mass, or
of its brightest portion, is suggested.
It is composed of faintly luminous gas, though whether it glows from
heat or from some other cause we do not know. Photographs of nebulae
are very misleading with regard to brightness: one must remember that
they have often been exposed for many hours. Helium, hydrogen, and
an unknown gas which we call nebulium are mingled together, but not
in equal quantities. In some of the fainter regions of the nebula,
especially on the south and west borders, hydrogen produces a great
deal of the light; in the brightest parts, near the trapezium, the glow
of nebulium is much more prominent.
It is scarcely doubtful that many of the stars which appear to be
involved in the nebula are physically connected with it, especially
since they are of a type frequently found near nebulae, viz. very blue
Orion-type stars with some of their hydrogen lines not dark but bright,
as in the nebula.
The southern hemisphere is rich in nebulae smaller but of the same kind
as these three magnificent objects, the Keyhole, the Looped, and the
Orion Nebulae—that is, large irregular masses of gas, often spangled
with stars—and each has some special beauty of its own; but for most of
them large telescopes are needed to grasp the faint details. There is
a nest of them in the northern part of Sagittarius: a cloudy streak
visible to the naked eye, a little north of the star γ Sagittarii,
represents three nebulae and clusters close together—M 8, M 20, and
M 21. The first is a wonderful combination of a bright scattered
star-cluster and a gaseous nebula, with dark rifts dividing the cloudy
structure. The second is the celebrated Trifid Nebula, less bright
and large, but with even more striking black lanes which split the
principal part into three almost separate portions. Many faint stars
are scattered over it, but as they are scarcely more numerous than in
the surrounding regions, most of them probably are not connected with
the nebula. M 21 is a star-cluster.
Near these, where Sagittarius borders on Aquila,[11] is a small but
very remarkable nebula, known from its shape as the Horseshoe or the
Omega Nebula (M 17). It has a curious mottled appearance, with bright
knots here and there.
[11] On Scutum in maps where this constellation is not included in
Aquila.
And a little further west, near together, are two wonderful nebulae
which surround the two stars Rho Ophiuchi and Nu Scorpii. Professor
Barnard, who has studied and taken exquisite photographs of many
nebulae, considers the first of these the finest in the sky, because
of its dark, winding lanes and the veiling of the stars in places by
partly transparent nebulous matter.