If our Sun were moved to where Arcturus is we should see him only as a very dim star indeed. But Arcturus is one of the most brilliant stars in our sky. This seems to show that he must be an enormous Sun: a very giant among giants; so huge that our great Sun would perhaps be but as a little ball by his side. Capella, one of our most beautiful northern stars, is believed to be another giant Sun. Our Sun, at the distance of Capella, would be only just visible without a telescope, while Capella is almost, if not quite, as bright as Arcturus. Since the radiance of Capella is certainly not caused by nearness it is most likely caused by great size. So, although Sirius may be to us “the monarch of the starry skies,” he is monarch only in appearance. He is brightest because he is one of the nearer stars, not because he is really one of the very largest. Arcturus, Capella, and others also, are believed far to surpass him in size. In this little book I must not even try to tell you many of the wonders of the starry heavens. If you wish to learn more you will by-and-by read in other books about the many-colored suns which are seen in telescopes, and the pairs of suns which journey through the skies in company.[3] [3] See “Sun, Moon and Stars,” pp. 283-286. You will read also about the curious changeable stars, which get bright and dim by turns; and about the extraordinary New Stars, which sometimes appear and last for a while, and then vanish again.[4] [4] Ibid. pp. 279-282. I am only going to tell you now a very little about Star-Clusters and Nebulæ. A Star-Cluster is just what its name says it is--a Cluster of Stars very near together. Near, as seen by us at this distance; not always really near. A great many star-clusters are known, and some can be seen without a telescope, while others are mere specks even in a fairly good telescope. In some clusters only about one or two hundred stars are seen. In others we find a countless multitude of stars--thousands of suns seemingly packed together in a mass. The _packed_ look comes from great distance. If we were near enough we should see the suns of such a cluster to be well apart--perhaps even very widely separated. You know how the trees of a forest, which close at hand stand apart, seem in the distance to shrink close together. That is how the stars do. [Illustration: _The Nebula in Andromeda._] One very important cluster you may easily see any clear winter evening--the cluster of the Pleiades. Most people can make out five or six dim stars; and through a mere opera-glass a hundred may be counted. The word “nebula” means “a cloud.” _Nebulæ_ is the plural, meaning _clouds_. But the Nebulæ are not fleeting and watery clouds, like our Earth-clouds. They are pale patches of light in the sky, fixed as the stars themselves--in one spot century after century.