NEAT

A term applied to cattle: neat's foot oil is the fat obtained by boiling calves' feet. NEBULiE. (Lat. nebula, a cloud.) in Astronomy, the name given, on account of their general cloudy appearance, to a very numerous cla of celestial objects, being, however, for by far the greater part, telescopic, and only visible in telescopes of considerable power. It is to Sir William Herschel that astronomy is indebted for the first examination and analysis of these remarkable objects. A few of them have indeed been known since the discovery of the telescope, and one or two of them are visible to the naked eye; but his powerful telescopes first disclosed the fact of their existence in immense numbers, and in all quarters of the heavens, not indeed distributed uniformly, but, generally speaking, with a marked preference to a broad zone, cro ing the milky-way nearly at right angles, and whose general direction is not very remote ft-om that of the hour circle of Oh and 12h. Nebulae are divided by Sir W. Herschel into the following cla es: — 1st, Clusters of stars, in which the stars are clearly distinguishable; 2d, Resolvable nebulae, or such as excite a suspicion that they consist of stars, and which any increase of the optical power of the telescope might be expected to resolve into distinct stars; 3d, Nebulae, properly so called, in which there is no appearance whatever of stars; 4th, Planetary nebula; 5th, Stellar nebula;; and, 6th, Nebulous stars. Clusters of Stars These are either globular or of an irregular figure, forming bright isolated patches, which attract attention, as if they were brought together by some general cause. The Pleiades is a cluster of this sort: the naked eye can distinctly perceive six or seven stars in It, and may catch occasional glimpses of a great many more; but the telescope shows fifty or sixty crowded together, in a very moderate space, and insulated from the rest of the heavens. A luminous spot, called Prcesepe, or the Beehive, in the constellation Cancer, is resolved entirely into stars by an ordinary telescope. In the sword817 NEBULA. handle of Perseus is another such spot, crowded with stars but not so easily resolved, there are a great number of le distmct nebulous specks of the same kind, which m ordinary telescopes have much the appearance of comets without tails, and have frequently been mistaken for such; when, however, they are examined with instruments of great power, such as reflectors of )g inches, 2 feet, or more m aperture, any such idea is completely destroyed. They are then, for the most part, perceived to consist entirelv of stars, crowded together so as to occupy almost a definite outline, and to run up to a blaze of light in the centre, where their condensation is usually the greatest. Many of them are of an exactly round figure. Others, again, are of an irregular form, and le definite in their outline, so that it is not easy to say where they terminate. In some of them the stars are nearly all of a size, in others extremely different; and it IS no uncommon thing to find a very red star, much brighter than the rest, occupying a conspicuous situation in the group. Sir W. Herschel regards these as globular clusters in a le advanced state of condensation; conceiving all such groups as approaching, by their mutual attraction, to the globular figure, and a embling themselves together from all the surrounding region. Resolvable nebulce are considered as objects of the same nature as the preceding; but as being either too remote, or consisting of stars too faint, to affect us by their individual light. They are universally round or oval; their irregularities of form being extinguished by the distance, and only the general figure of the condensed parts being discernible. In telescopes of insufficient optical power, all the great globular clusters exhibit themselves under this appearance. Nebulce, properly so called, present a great variety of appearances. One of the most remarkable is in the constellation Orion; and its appearance is very different from what might be supposed to arise from the aggregation of an immense collection of small stars. It is formed of little flocky ma es, like wisps of cloud; and such wisps seem to adhere to many small stars at its outskirts, and especially to one considerable star, which it envelopes with a nebulous atmosphere of considerable extent and singular figure. This nebula was discovered by Huygens in 1656, who gave figures representing its appearance in his telescope. On comparing these with its present appearance, several astronomers have concluded that it has undergone a perceptible change; but the evidence of such change is by no means to be relied on. There is a nebula in the constellation of Andromeda visible to the naked e^e, and i often mistaken fora comet. Its appearance is described by Simon Marius as that of a candle shining through horn. Its form is a pretty long oval, increasing by insensible gradations of brightne , at first very gradually, but at last more rapidly, up to a central point, which, though very much brighter than the rest, is yet evidently not stellar, but only nebulous matter in a high state of condensation. It has in it a few small stars; but they are obviously casual; and the nebula itself offers not the slightest appearance to give ground for a suspicion of its consisting of stars. It is nearly half a degree long, and fifteen or twenty minutes broad. Like that last described, a very numerous cla of nebulae are of a round or oval figure, increasing more or le in density towards the central point. In this respect, however, they differ extremely; in some the condensation being slight and gradual, in others great and sudden. They also present great diversity of appearance.in respect of deviation from the spherical form. Some are only slightly elliptic, others much extended in length; and in some the extension is so great as to give the nebula the character of a long, narrow, spindle shaped ray, tapering away at both ends to points. Some nebulae are annular; but these are among the rarest objects in the heavens. The most conspicuous is situated half way between the stars /3 and y Lyras, and may be seen with a telescope of moderate power. It is small, and particularly well defined, so as to have, in fact, much more the appearance of a flat oval solid ring than of a nebula. Planetary nebuUB have exactly the appearance of planets, — round or slightly oval disks, in some instances • quite sharply terminated, in others a little hazy at the borders, and of a light exactly equable, or only a little mottled, which, in some of them, approaches in vividne to that of actual planets. Whatever the nature of these objects may be, they must be of enormous magnitude. One in Aquarius presents a diameter of 20"; another, in Andromeda, has a visible disk of 12", perfectly defined and round. Granting them to be equally distant from us with the stars, their real dimensions must be such as would fill, on the lowest computation, the whole orbit of Uranus. Their intrinsic splendour must also be immeasurably in ferior to that of the sun's; for a circular portion of the sun's disk, subtending an angle of 20", would give ahght equal to 100 full moons, whereas the nebulae in question are hardly discernible with the naked eye. Stellar nebuUe are those in which the condensation of the nebulous matter towards the centre is great and sudden; so sudden, indeed, as to present the appearance 3G [s. 830]