Growth of Our Knowledge of the Nebuæ

Abstract

OUR present knowledge of those celestial bodies which we term nebulæ may be said to date from a paper by Sir William Herschel on nebulous stars, published in 1791 (Phil. Trans., vol. Ixxxi. p. 71). It is perfectly true that we have not here the first recorded observations of nebulas: several observers before Sir William Herschel, and Sir William Herschel himself, had previously referred to them. All observers previous to Sir William Herschel, among whom we may include Kepler, Tycho Brahé, Halley, and others, were of opinion that the nebulæ were composed of something differing entirely in its essence from stars. There was no question whatever of their being simply clusters of stars considerably removed. Tycho Brahé, in the record of his observations of the new star observed by him in Cassiopeia, suggested that it was in some way generated from an ethereal substance, and to him the Milky Way was composed of the same material. This ethereal substance was liable to dissipation by light and heat, and in this way he accounted for the ultimate disappearance of the star. Kepler shared this opinion, and it may be stated that it was generally accepted at the time that Sir William Herschel began his observations of I nebulæ about the year 1780. His first important paper, however, did not deal with these objects: it had reference to the motion of the sun in space (Phil. Trans., vol. Ixxiii., published in 1783). In this memoir he points out the universal sway of gravitation in the celestial spaces; and the infinite possibilities opened out by such an all-prevailing and pervading cause seem, although he does not state it in terms, to have led him to the conclusion that such ideas as Brahé's and Kepler's were invalid. His first real survey of the nebulæ appears in his paper of 1784 (Phil. Trans., vol. Ixxiii.) He began by observing those bodies which had already been recorded in the Conuaissance des Temps for 1783, and then those further afield; and it is not a little remarkable that in this first paper he describes almost every distinct form of nebulae which has been observed from that day to the years about 1846, when Lord Rosse brought a still more powerful instrument than Herscllel's largest to bear upon these objects. He noticed that in certain parts of the heavens there was a marked absence of stars, and that this was so invariably followed by the appearance of nebulæ on the confines of the empty region that he records in his memoir that after passing over one of them he was in the habit of giving the word to his assistant to “prepare for nebulæ.”This strengthened his view as to the power of gravitation, and as to nebulæ being masses of stars produced by it.