The Outline of Science, Volume 1

Why do we say "our universe"? Why not the universe? It is now believed by many of our most distinguished astronomers that our colossal family of stars is only one of many universes. By a universe an astronomer means any collection of stars which are close enough to control each other's movements by gravitation; and it is clear that there might be many universes, in this sense, separated from each other by profound abysses of space. Probably there are. For a long time we have been familiar with certain strange objects in the heavens which are called "spiral nebul " (Fig 4). We shall see at a later stage what a nebula is, and we shall see that some astronomers regard these spiral nebul as worlds "in the making." But some of the most eminent astronomers believe that they are separate universes—"island-universes" they call them—or great collections of millions of stars like our universe. There are certain peculiarities in the structure of the Milky Way which lead these astronomers to think that our universe may be[Pg 15] a spiral nebula, and that the other spiral nebul are "other universes." Photo: Harvard College Observatory. FIG. 2.—THE MILKY WAY Note the cloud-like effect. FIG. 3—THE MOON ENTERING THE SHADOW CAST BY THE EARTH The diagram shows the Moon partially eclipsed. From a photograph taken at the Yerkes Observatory FIG. 4.—THE GREAT NEBULA IN ANDROMEDA, MESSIER 31 Vast as is the Solar System, then, it is excessively minute in comparison with the Stellar System, the universe of the Stars, which is on a scale far transcending anything the human mind can apprehend.