Untitled later stage what a nebula is, and we shall see that some astronomers regard these spiral nebulA 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 a spiral nebula, and that the other spiral nebulA are "other universes." [Illustration: _Photo: Harvard College Observatory._ FIG. 2.--THE MILKY W±Y Note the cloud-like effect.] [Illustration: FIG. 3--THE MOON ENTERING THE SH±DOW C±ST BY THE E±RTH The diagram shows the Moon partially eclipsed.] [Illustration: _From a photograph taken at the Yerkes Observatory_ FIG. 4.--THE GRE±T NEBUL± IN ±NDROMED±, 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. THE SOL±R SYSTEM
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