The nebulæ are very far from being uniformly distributed in space. In some regions they are rare, while in others they are numerous and crowded together, forming many small, irregular groups, differing in size and in richness of aggregation. The grouping of the nebulæ does not occur at random in any part of the heavens, as might naturally be supposed, but, on the contrary, it is chiefly confined to certain regions. Outside of these regions nebulæ are rare and are separated from each other by immense intervals; so that these isolated objects appear as if they were lost wanderers from the great nebulous systems.
The regions where the nebulæ congregate in great number are very extensive, and in a general view there are two vast systems of nebular agglomeration, occupying almost opposite points of the heavens, whose centres are not very distant from the poles of the Milky-way. In the northern hemisphere, the nebulous system is much richer and more condensed than in the southern hemisphere. The northern nebulæ are principally contained in the constellations Ursa Minor and Major, in Draco, Canes Venatici, Bootes, Leo Major and Minor, Coma Berenices, and Virgo. In this region, which occupies about ⅛ of the whole surface of the heavens, ⅓ of the known nebulæ are assembled. The southern nebulæ are more evenly distributed and less numerous, with the exception of two comparatively small, but very remarkable centres of condensation which, together with many star-clusters, constitute the Magellanic clouds.
These two vast nebular groups are by no means regular in outline, and send various branches toward each other. They are separated by a wide and very irregular belt, comparatively free from nebulæ, which encircles the celestial sphere, and whose medial line approximately coincides with that of the galactic belt. The Milky-way, so rich in star-clusters, is very barren in nebulæ; but it is a very remarkable fact, nevertheless, that almost all the brightest, largest, and most complicated nebulæ of the heavens are situated either within it, or in its immediate vicinity. Such are the great nebulæ in Orion and Andromeda; the nebula of ζ Orionis; the Ring nebula in Lyra; the bifurcate nebula in Cygnus; the Dumb-bell nebula in Vulpecula; the Fan, Horse-shoe, Trifid and Winged nebulæ in Sagittarius; the great nebula around η Argus Navis, and the Crab nebula in Taurus.
Aside from the discovery of some of the largest nebulæ by different observers, and their subsequent arrangement in catalogues by Lacaille and Messier, very little had been done towards the study of these objects before 1779, when Sir W. Herschel began to observe them with the earnestness of purpose which was one of the distinctive points of the character of this great man. He successively published three catalogues in 1786, 1789, and 1802, in which the position of 2,500 nebulous objects was given. This number was more than doubled before 1864, when Sir John Herschel published his catalogue of 5,079 nebulæ and star-clusters. To this long list must be added several hundred similar objects, since discovered by D'Arrest, Stephan, Gould and others. But, as has been shown above, among the so-called nebulæ are many star-clusters which do not properly belong to the same class of objects, it being sometimes impossible in the present state of our knowledge to know whether a nebulous object belongs to one class or to the other.
The nebulæ exhibit a great variety of forms and appearances, and, in accordance with their most typical characters, they are usually divided into several classes, which are: the Nebulous stars, the Circular, or Planetary, the Elliptical, the Annular, the Spiral and Irregular nebulæ.
PLATE XV.—THE GREAT NEBULA IN ORION.
From a study made in the years 1875-76
The so-called nebulous stars consist of a faint nebulosity, usually circular, surrounding a bright and sharp star, which generally occupies its centre. The nebulosity surrounding these stars varies in brightness as well as in extent, and while, in general, its light gradually fades away, it sometimes terminates quite suddenly. Such nebulosities are usually brighter and more condensed towards the central star. The stars thus surrounded do not seem, however, to be distinguished from others by any additional peculiarity. Some nebulæ of this kind are round, with one star in the centre; others are oval and have two stars, one at each of their foci. The nebulous star, τ Orionis, represented at the upper part of Plate XV., above the great nebula, has a bright star at its centre and two smaller ones on the side. The association of double stars with nebulæ is very remarkable, and may in some cases indicate a mutual relation between them.