The nebula has a nucleus with a star exactly in the center. This is very conspicuous on a photograph, but barely if at all visible with a 36-inch reflector.
Another curious class of nebulæ are designated as planetary, on account of their form. These consist of minute, round disks of light, having somewhat the appearance of a planet. The appellation was suggested by this appearance. These objects are for the most part faint and difficult.
It is impossible to estimate the number of nebulæ in the heavens. New ones have been from time to time discovered, located and described by many observers during the last thirty years. Among these Lewis Swift is worthy of special mention. On photographing the sky near the galactic pole with the Crossley reflector, Keeler found no less than seven of these objects in a space of about one-half a square degree. He therefore estimates the whole number in the heavens capable of being photographed at several hundred thousand. It may be assumed that only a moderate fraction of these are visible to the eye, even aided by the largest telescopes.
Among the most singular of these objects are large diffused nebulæ, sometimes extending through a region of several degrees. A number of these were discovered by Herschel. Barnard, W. H. Pickering and others have photographed these for us. One of the most remarkable of them winds around in the constellation Orion in such a way that at first sight one might be disposed to inquire whether the impression on the photographic plate might not have been the result of some defect in the apparatus or some reflection of the light of the neighboring stars, which is so apt to occur in these delicate photographic operations. But its existence happens to be completely confirmed by independent testimony.It was first detected by W. H. Pickering and afterwards independently by Barnard.
A curious fact connected with the distribution of nebulæ over the sky is that it is in a certain sense the reverse of that of the stars. The latter are, as we shall hereafter show in detail, vastly more numerous in the regions near the Milky Way and fewer in number near the poles of that belt. But the reverse is the case with the nebulæ proper. They are least numerous in the Milky Way and increase in number as we go from it in either direction. Precisely what this signifies one would not at the present time be able to say. Perhaps the most obvious suggestion would be that in these two opposite nebulous regions the nebulæ have not yet condensed into stars. This, however, would be a purely speculative explanation.
On the other hand, star-clusters are more numerous in the galactic region. This, however, is little more than saying that in the regions where the stars are so much more numerous than elsewhere many of
