Fig. 7.—A Corner of the Constellation of Gemini, seen through the telescope.
Theophilus. The Infinite is beginning again. Let me shut my eyes.
The Author. Wait, I have not said all, I have only begun. I am coming to the nebulæ. Here, indeed, you may expect to grow giddy. The telescope has dispersed all the theories on which the different explanations of the nebulæ were built, and has shown us that they are collections of stars, which, in consequence of their excessive number, and their closeness to each other, appear to form a whole, a single vague and continuous brightness. But, when their dimensions and distances are amplified by the telescope, this diffused light transforms itself into a brilliant point, analogous to that presented by the sky, tapestried with small stars, in the same telescope. These nebulæ are groups of enormous numbers of stars, and even their nearness to each other is only in appearance. They are, in reality, separated by enormous distances, and it must not be supposed that they are all in the same plane; they belong, on the contrary, to very unequal depths in space, and it is only an optical effect which gathers them together on the field of the telescope in the same apparent plane.
The nebula of the Centaur is one of the most wonderful. To the naked eye it is but a dimly-lighted point in the sky; but, looked at through a good telescope, it takes the aspect represented by figure 8.
Fig. 8.—The Nebula of the Centaur.
On examination of this figure, it will be seen that a nebula is not the result of a collection of stars simply spread out upon a level in space, but of that of an assemblage of stars all placed at unequal distances, and forming almost a sphere. In fact the stars are crowded towards the centre, and are, on the contrary, more and more distant from one another as the outer edge is approached. If a spherical assemblage of stars were observed from a distance, it would present a similar aspect. This leads us to believe that the nebula of the Centaur, like the greater number of agglomerations of this kind, is spherical.
Is it possible to reckon the stars which form a nebula? Only approximately. Arago estimates the number of stars which form a nebula no larger than the tenth part of the apparent disc of the moon, at twenty thousand, at least. This result may give us an idea of the swarms of suns contained in the nebulæ, for these stellar masses are very numerous in the sky. In the depths of the nebulæ there are luminous points whose nature is as yet unrevealed by the telescope, which cannot be resolved into stars; but analogy leads us to believe that they are other and still more distant nebulæ, which, by reason of their apparent littleness, elude the scope of our instruments. But the time will come, when, thanks to the perfection which our telescopes shall have attained, this theory will be confirmed, and we shall thus see deeper and farther into immensity.
The stars which form the nebulæ are sometimes grouped so as to form regular shapes, spheres, or more or less lengthened ellipses. Sometimes the sphere is hollow in the centre, and so forms a ring. Nothing more varied, nothing more strange can be imagined than the forms of those nebulæ which have hitherto been examined, and which already number more than a million, of which no two are precisely alike. Certain nebulæ seem to be double, or joined. Others are lengthened out, like serpents, as in that of the Shield of Sobieski, represented in figure 9.