Part 9

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.

XVI

THE CLOUDS OF MAGELLAN

One of the wonders which most attracted the attention of early explorers in the southern hemisphere, and roused as much interest as the Southern Cross, was the pair of faint clouds, looking like detached pieces of the Milky Way, which are seen in the neighbourhood of the South Pole. Marco Polo made a sketch of the Greater Cloud, which he describes wonderingly as “a star as big as a sack.”

Although some star-maps show short branches of Milky Way pointing towards the two Clouds, this is incorrect, and they are quite separate from it. Herschel was struck by their isolation, especially in the case of the Little Cloud, which he described as situated in a “most oppressively desolate desert,” its only neighbour being the globular cluster 47 Toucani, which is near, but separated by a perfectly black sky.

The Greater Cloud is much brighter to the naked eye than the Lesser, and it is much more complex and interesting in the telescope. It contains, moreover, the wonderful Looped Nebula, of which we have already spoken.

Both Clouds consist of gaseous nebulae and star-clusters on a background of vague nebulosity and crowds of almost indistinguishable stars. But the white nebulae shun the Clouds, just as they shun the Milky Way.

An immense number of variable stars have been discovered in the Clouds of Magellan, of the same type as those in globular clusters. Miss Leavitt of Harvard Observatory catalogued from photographs no less than 969 in the Lesser Cloud and 800 in the Greater. In the latter the greatest number of variables was found in a stream of faint stars which connects a group of star-clusters with the Looped Nebula, and others occur locally in certain parts of the Cloud, but few are in its northern region or in parts where many of the brighter stars congregate. All the variables are very faint, the usual minimum in both Clouds being about fourteenth magnitude, and the maximum seldom more than one magnitude brighter. A few in the Lesser Cloud have been found with periods unusually long for this “cluster type” of variables, amounting to 32, 66, and even 127 days. These longer periods seem to belong to somewhat brighter stars, but they are quite as exact as the usual period of a few days or a single day.

XVII

THE MILKY WAY

Like a great river returning into itself, the Galaxy encircles the starry heavens, and those who know only its northern course have no idea of its brilliance and wonderful complexity in its brightest part.

Its light is soft, milky, and almost uniform, between Cygnus and Sirius, but when it enters Argo it becomes extremely broad, and spreads out like a river on a flat marshy plain, in many twisting channels with spaces between. Where Canopus shines on the bank there is a narrow winding ford right across its whole breadth, as if a path had been made by the crossing of a star.

After this it suddenly becomes extremely narrow, but so bright that all the light which was shining in the broad channel seems to be condensed in this narrow bed. In the brightest, richest part the Great Nebula of Argo is easily distinguished by the naked eye. Contrasting with this and other bright condensations are black gaps, the largest and blackest of which is the well-known Coal-Sack near the Southern Cross.

Photographed at Hanover, Cape Colony,

by Bailey and Schultz]

The river now divides. One short stream, which goes north from Centaur towards Antares, is faint and soon lost; but another northern stream is so bright and so persistent that from Centaur to Cygnus we may say that the Galaxy flows in a double current. This northern portion forms first the smoke of the Altar on which the Centaur is about to offer the Beast, then passes through the Scorpion into the Serpent-Holder, and here, between η Ophiuchi and Corona Australis, the double stream has its greatest width. The northern division soon grows dim and seems to die out, but begins again near β Ophiuchi, and, curving through a little group of stars, passes through the head of the Eagle and forms an oval lagoon in the Swan.

The southern stream passes through the Scorpion’s Tail into Sagittarius, then through the Eagle and the Arrow till it flows close beside the northern stream in the Swan, and finally rejoins it in a bright patch round α Cygni. Except just here it is much brighter than the northern stream, and its structure is even fuller of wonderful detail than in Argo. In Sagittarius it consists of great rounded patches with dark spaces between. The brightest of these contains the star γ Sagittarii; then follows a remarkable region of small patches and streaks, the portion passing through Sagittarius and Aquila being thickly studded with nebulae. This is followed by another bright patch, rivalling that round γ Sagittarii, which involves the stars λ and 6 Aquilae.

This ends the most brilliant and wonderful part of the Milky Way. When well seen, as we see it in the south, it recalls Herschel’s words, written at the Cape when it came into view in his telescope:

“The real Milky Way is just come on in great semi-nebulous masses, running into one another, heaps on heaps.” And again: “The Milky Way is like sand, not strewed evenly as with a sieve, but as if flung down by handfuls, and both hands at once.”

What is it? The ancients thought it the pathway of departed spirits, or fiery exhalations from the earth imprisoned in the skies, or a former road of the sun through the stars. But Democritus and some other inquiring Greeks believed it to be the shining of multitudes of stars too faint and too close together to be seen separately, and we know this to be the truth. We know also, from simply counting the stars in different regions of the sky, that their numbers increase regularly as we go from north or south towards the Milky Way, and stars of all magnitudes are most abundant within its course. We saw also that star-clusters and certain kinds of nebulae frequent it, while other kinds avoid it, and that blue and white stars are the most abundant near it, and tend to move through space in planes parallel with it, while the redder stars are scattered and move about in all directions.

Facts like these lead astronomers to believe that the Milky Way has a definite relation with all the visible universe, that even the most distant nebula is not an outlying universe apart from ours, but all are parts of one vast stellar system.

It is possible that the Milky Way, which we see as a great circle, double in one part, is really an immense spiral, and that we are nearest one curve of it, the great southern division which looks so bright. It may be that the spiral nebulae, vast though they are in terms of earthly measurement, are tiny models of one tremendous spiral which enfolds the universe with its coils.

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