CHAPTER XXV.

GROUPS AND CLUSTERS OF SUNS.

We have been thinking a good deal about single stars and double stars, as seen from earth. Now we have to turn our attention to groups, clusters, _masses_ of stars, in the far regions of space.

Have you ever noticed on a winter night, when the sky was clear and dotted with twinkling stars, a band of faintly-glimmering light stretching across the heavens from one horizon to the other? The band is irregular in shape, sometimes broader, sometimes narrower; here more bright, there more dim. If you were in the Southern Hemisphere you would see the same soft belt of light passing all across the southern heavens. This band or belt is called the Milky Way.

But what is the Milky Way? It is made up of stars. So much we know. As the astronomer turns his telescope to the zone of faintly-gleaming light, he finds stars appearing behind stars in countless multitudes; and the stronger his telescope, the more the white light changes into distant stars.

Our sun we believe to be one of the stars of the Milky Way; merely one star among millions of stars; merely one golden grain among the millions of sparkling gold-dust grains scattered lavishly through creation. Scattered, not recklessly, not by chance, but placed, arranged, and guided each by its Maker’s upholding hand.

The Milky Way, or the Galaxy, as it has been called, has great interest for astronomers. Many have been the attempts made to discover its actual size, its real shape, how many stars it contains, how far it extends; but to all such questions the only safe answers to be returned are fenced around with “perhaps” and “may be.” There are many very remarkable clusters of stars to be seen in the heavens--some few visible as faint spots of light to the naked eye, though the greater number are only to be seen through a telescope. Either with the naked eye, or in telescopes of varying power, they show first as mere glimmers of light, which, viewed with a more powerful telescope, separate into clusters of distant stars.

[Illustration: A CLUSTER OF STARS IN CENTAURUS.]

The most common shape of these clusters is globular--to the eye appearing simply round. Stars gather densely near the center, and gradually open out to a thin scattering about the edge. Thousands of suns are often thus collected into one cluster.

The clusters are to be seen in all parts of the sky; but the greater number seem to be gathered into the space covered by the Milky Way and by the famous south Magellanic Clouds.

Some of them are beautifully colored; as, for instance, a cluster in Toucan, not visible from England, the center of which is rose-colored, bordered with white. No doubt it contains a large number of bright red suns, surrounded by a scattering of white suns.

It used to be supposed that many of these clusters were other vast gatherings or galaxies of stars, like the Milky Way, lying at enormous distances from us. This now seems unlikely. The present idea rather is, that each cluster, in place of being another “Milky Way” of millions of widely-scattered suns, is one great _star-system_, consisting indeed of thousands of suns, but all moving round one center. Probably most of these clusters are themselves a part of the collection of stars to which our sun belongs.

If this be so, and if worlds are traveling among the suns--as may well be, since they are doubtless quite far enough apart for each sun to have his own little or great system of planets--what sights must be seen by the inhabitants of such planets!

We do not indeed know the distance between the separate suns of a cluster, which may be far greater than appears to us. But if astronomers calculate rightly, they are near enough together to shed bright light on all sides of a planet revolving in their midst. The said planet might perhaps not have within view a single sun equal in apparent size to our sun as seen from earth; yet thousands of lesser suns, shining brightly in the firmament night and day, would cause a radiance which we never enjoy.

No, not _night_. In such a world there could be no night. Worlds in the midst of a star-cluster must be regions of perpetual day. No night, no starry heaven, no sunrise lights or sunset glories, no shadow mingling with sunshine, but one continual, ceaseless blaze of brightness. We can hardly picture, even in imagination, such a condition of things.

[Illustration: THE GREAT NEBULA IN ANDROMEDA, COMPARED WITH SIZE OF THE SOLAR SYSTEM.]

Besides star-clusters there are also nebulæ. The word _nebula_ comes from the Latin word for “cloud,” and the nebulæ are so named from their cloudlike appearance.

It is not easy to draw a line of clear division between nebulæ and distant star-clusters; for both have at first sight the same dim, white, cloudy look. In past days the star-clusters were included by astronomers under the general class of nebulæ. And as with the star-clusters, so with many of the nebulæ, the more powerful telescopes of modern days have shown them to be great clusters, or systems, or galaxies of stars, at vast distances from us.

It was supposed with the nebulæ, as with some of the star-clusters, that they were other “Milky Ways” of countless stars, far beyond the outside boundaries of our Milky Way. This may still be true of some or many nebulæ, but certainly not of all.

For there are different kinds of nebulæ. Some may, as just said, be vast gatherings of stars lying at distances beyond calculation, almost beyond imagination. Others appear rather to be clusters of stars, like those already described, probably situated in our own galaxy of stars. There is also a third kind of nebula. Many nebulæ, once supposed to be clusters of stars, having been lately examined by means of the spectroscope, are found to be enormous masses of glowing gas, and not solid bodies at all.

The number of nebulæ known amounts to many thousands. They are commonly divided into classes, according to their seeming shape. There are nebulæ of regular form, and nebulæ of irregular form. There are circular nebulæ, oval nebulæ, annular nebulæ, conical nebulæ, cometary nebulæ, spiral nebulæ, and nebulæ of every imaginable description. These shapes would no doubt entirely change, if we could see them nearer; and indeed, even in more powerful telescopes, they are often found to look quite different. While on the subject of clusters and nebulæ, mention should be made of the famous Magellanic Clouds in the southern heavens. Sometimes they are called the Cape Clouds. They differ from other nebulæ in many points, and more particularly in their apparent size. The Great Cloud is about two hundred times the size of the full moon, while the Small Cloud is about one-quarter as large. In appearance they are not unlike two patches of the Milky Way, separated and moved to a distance from the main stream.

These clouds are surrounded by a very barren portion of the heavens, containing few stars; but in themselves they are peculiarly rich. Seen through a powerful telescope, they are found to abound with stars. The Greater Cloud alone contains over six hundred from the seventh to the tenth magnitudes, countless tiny star-points of lesser magnitudes, star-clusters of all descriptions, and nearly three hundred nebulæ,--all crowded into this seemingly limited space.

Among many famous nebulæ, the one in Orion and the one in Andromeda may be particularly mentioned. The size of the latter, as compared with the size of our Solar System, is shown in the cut of this nebula. On the hypothesis of a complete resolvability into stars, the mind is lost in numbering the myriads of suns, the agglomerated individual lights of which produce these nebulous fringes of such different intensities. What must be the extent of this universe, of which each sun is no more than a grain of luminous dust!