Part 8

Near β Aquarii there shines with the light of a sixth-magnitude star another “magnificent ball of stars” which has been compared to “a heap of fine sand.” It is named 2 M Aquarii.

Over seventy of these tightly packed balls of stars are known, even counting only the brightest, and their distribution is rather curious. A large number (about twenty) occur in the Clouds of Magellan, and more than half of the seventy are in the Milky Way, not scattered evenly along its course, but almost if not entirely confined to its southern part, and chiefly gathered in a great group in its brightest portion, where it passes through Sagittarius, Ophiuchus, and Aquila. Here they are mingled with—or perhaps projected against—numerous stars of the same magnitudes; but many balls are also found outside the Milky Way, widely scattered, and in these parts of the sky there are relatively few of the faint-magnitude stars which compose all the globular clusters. 47 Toucani, for instance, though it is near the small Magellanic Cloud, stands quite apart from it, isolated in a black sky.

We do not know the distances of any of these balls of stars. Those which have been examined spectroscopically shine like Canopus—that is, they are of a type intermediate between Sirius and our sun—but the chief light comes, of course, from the brighter stars, and it may be that the fainter stars mingled with them belong to a different type.

A remarkable fact lately discovered is that many globular clusters—but not all—contain a large number of variable stars. These vary in light in a period of about a day and have a range of about one magnitude. They are not of the Algol type, nor quite of the usual “short-period” types, and it is not yet clear what is the cause of variation, though it seems probable that “cluster-variables” are double stars.

XIV

NEBULAE

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, 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.

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.

OTHER TYPES OF NEBULAE

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, 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.