NEBUL&, in astronomy. There are spots in the heavens, called Nebulae, some of which consist of clusters of telescopic stars, others appear as luminous spots, of different forms. The most considerable is one in the midway, between the two stars on the blade of Orion's sword, mark ed 6 by Bayer, discovered in the year .1656, by Huygens; it contains only seven stars, and the other part is a bright spot upon a dark ground, and appears like an opening into brighter regions beyond.
Dr. Halley, and others, have discovered nebula in different parts of the heavens. In the "Connoissance des Temps," for 1783, and 1784, there is a catalogue of 103 nebula, observed by Messier and Mechain. But to Dr. Herschel we are in debted forcatalogues of 2000 nebulz, and clusters of stars, which he himself has discovered. Some of them form a round compact system, others are more irregu lar, of various forms, and some are long and narrow. The globular system of stars appear thicker in the middle than they would do, if the stars were all at equal distances from each other; they are, there fore, condensed towards the centre. That stars should be thus accidentally dispos ed is too improbable a supposition to be admitted ; he supposes, therefore, that they are brought together by their mutual attractions, and that the gradual conden sation towards the centre is a proof of a central power of such a kind. Ile ob serves, also, that there are some addition al circumstances in the appearance of ex tended clusters and nebula, that very much favour the idea of a power lodged in the brightest part. For although the form of them be not globular, it is plant that there is a tendency to sphericity. As the stars in the same nebulz must be very nearly all at the same relative dis tances from us, and they appear nearly of the same size, their real magnitudes must be nearly equal. Granting, there fore, that these nebulz and clusters of stars are formed by mutual attraction, Dr. Herschel concludes that we may judge of their relative age by the disposition of their component parts, those being the oldest which are most compressed. He supposes, and indeed offers powerful ar guments to prove, that the milky way is the nebulz of which our sun is one of its components.
Dr. Herschel has also discovered other phenomena in the heavens, which be calls nebulous stars; that is, stars sur rounded with a faint luminous atmosphere of large extent. Those which have been thus styled by other astronomers, he says, ought not to have been so called, for on examination they have proved to be either mere clusters of stars, plainly to be dia. tinguished by his large telescopes, or such nebulous appearances as might be occasioned by a multitude of stars at a vast distance. The milky way consists entirely of stars; and he says, " I have been led on by degrees from the most evident congeries of stars, to other groups in which the lucid points were smaller, but still very plainly to be seen; and from them to such, wherein they could but barely be suspected, until I ar rived at last to spots in which no trace of a star was to be discerned. But then the
gradations to these latter were by such connected steps, as left no room kir doubt but that all these phenomena were equally occasioned by stars variously dis persed in the immense expanse of the universe." In the same paper is given an account of some nebulous stars, one of which is thus described : " Nov. 13, 1790. A most singular phenomenon! A star of the eighth magnitude, with a faint luminous atmos phere, of a circular form, and of about 3' in diameter. The star is perfectly in the centre, and the atmosphere is so diluted, faint, and equal throughout, that there can be no surmise of its consisting of stars, nor can there be a doubt of the eri.
dent connexion between the atmosphere and the star. Another star, not much less in brightness, and in the same field of view with the above, was perfectly free from any such appearance." Hence, Dr. Herschel draws the fbllowing consequen ces: Granting the connexion between the star and the surrounding nebulosity, if it consist of stars very remote, which gives the nebulous appearance, the cen tral star, which is visible, must be im mensely greater than the rest; or if the central star be no bigger than common, how extremely small and compressed must be those other luminous points which occasion the nebulosity. As, by the former supposition, the luminous central point must far exceed the standard of what we call a star ; so in the latter, the shining matter about the centre will be too small to come under the same deno mination; we, therefore, either have a central body which is not a star, or a star which is involved in a shining fluid, of a nature totally unknown to us. This last opinion Dr. Herschel adopts.
Light reflected from the star could not be seen at this distance. Besides, the outward parts are nearly as bright as those near the star. Moreover, a cluster of stars will not so completely account for the milkiness, or soft tint of the light of these nebula, as a self-luminous fluid. " What a field of novelty," says Dr. Herschel, "is here opened to our con ceptions. A shining fluid, of a brightness sufficient to reach us from the regions of a star of the 8th, 9th, 10th, 11th, 12th magnitude, and of an extent so consider able as to take up 3, 4, 5, or 6 minutes in diameter." He conjectures that this shining fluid may be composed of the light perpetually emitted from millions of stars. See Philos. Trans. vol. lxxxi. p. 1. on Nebulous Stars, properly so tailed.