Nebulæ

Nebulæ are cloudy patches of light in the heavens. Some, as those in Andromeda and Orion, are visible to the naked eye, but the greater number can only be seen in telescopes of considerable power. Before the invention of the Spectroscope (q.v.) they were for a time all considered to be star-clusters. This instrument has shown that these clusters, which appear as nebulae in small telescopes, are totally different in constitution from nebulae proper (see STARS). Halley in 1716 gave a small list of 6 nebulae, but the chief workers in this field are Messier, who in 1784 catalogued 103; Sir W. Herschel, who alone discovered more than 2500; and Sir J. Herschel, who added to them more than 2000. These lists, however, include many star-clusters. Nebulae proper have been classified as (1) annular, (2) elliptic, (3) spiral, (4) planetary, and (5) nebulous stars. These various forms do not indicate specific differences, but rather result from (1) the various presentation of their edges and surfaces to the spectator, and (2) their differently condensed material. For example, the elliptic nebula in Andromeda has been shown by photography to consist of a spiral, or a congeries of rings, exhibiting thus in itself the marks of the first three classes. Planetary nebulae show faint discs, often bluish in colour, whose structureless appearance is probably only due to our imperfect telescopes. The spectrum of nebulae consists usually of a few bright lines indicating their gaseous structure. To what gases these lines belong is still uncertain, although they have been attributed to nitrogen, hydrogen, and magnesium vapour. In a few cases, as in a small nebula in Taurus and the great one in Orion, their light certainly varies in intensity, and they are sometimes associated with variable or temporary stars. They form, according to the theory of Laplace and Sir W. Herschel, the earliest stage in the formation of stars and planets. Though certainly not yet proved, this nebular theory is very generally accepted, and has been greatly strengthened by Lockyer's recent spectroscopic researches.