regular in outline, and a large proportion containing condensed or stellar nuclei near their centers, they were called planetaries by Herschel, because, though faint, they present discs somewhat as the planets do, when viewed under low power.
There are several scores of so-called stellar nebulæ. In moderate-sized telescopes most of them look like ordinary stars. In large telescopes many of them are hazy, but some are as well defined as stars. The spectroscope shows that all are true nebulæ. If they were much closer to us we should doubtless see them as planetary nebulæ.
A few other interesting objects are known as ring nebulæ, the most noted case being the ring nebula in Lyra.
Among the remarkable facts of the stellar universe are these: the large irregular nebulæ, the ring nebulæ, the planetary nebulæ, and the stellar[1] nebulæ, with relatively rare exceptions, are in or very close to the Milky Way: and, on the contrary, the spirals in or near the Milky Way are of negligible number. The first group are without question an integral part of our stellar system. The spirals, seem not to be closely connected with our stellar system, yet their very avoidance of the Milky Way shows that they bear some intimate relationship to it. There is no occasion for surprise that a small group of special objects should be in the Milky Way structure; but that the scores of thousands, and perhaps hundreds of thousands, of spirals, should abhor the Milky Way is a fact which immediately arrests our attention and calls for explanation. Moore has suggested that their absence from the Milky Way may be apparent and not real: that any absorbing or obstructing medium in the Milky Way structure might prevent the light of the spirals from reaching us, especially if the spirals are extremely distant. If the light from very distant nebulæ is absorbed or obstructed, as a function of the angular distance from the galaxy, the nebulæ near the poles of the galaxy, other things being equal, should on the average be intrinsically brighter than the nebulæ in or near the Milky Way. Secondly, if such an effect exists, long-exposure photographs on regions near the galaxy should record nebulæ in numbers more nearly equal to those recorded by short exposures near the poles of the galaxy. An examination of existing Crossley reflector photographs has led to negative results on this question, and we must assume that the spiral nebulæ really avoid the Milky Way.
The question of the distances of the spiral nebulæ has long been held in mind. The evidence, to which we shall refer later, is to the effect that they are very far away, and accordingly that they are of enormously
- ↑ The terms irregular, ring, planetary and stellar are intended merely to differentiate these objects as to their appearance in the telescope or on the photographic plate. They do not in themselves indicate differences in constitution or physical condition. The ring, planetary and stellar nebulae have a great many characteristics in common.
