XIVNEBULAE
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 starsscattered 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 powerfultelescope 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 veryclose 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 arevery 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 streakvisible 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.
And a little further west, near together, are two wonderful nebulae which surround the two stars Rho Ophiuchi and Nu Scorpii. ProfessorBarnard, 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.