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.