CHAPTER XVII. OF CLUSTERS OF STARS AND NEBULE. GLOBULAR CLUSTERS. THEIR OF CLUSTERING GROUPS OF STARS. DISTRIBUTION OVER THE HEAVENS. THEIR IRREGULAR CLUSTERS. - THEORY OF THE FORMATION OF OF ELLIPTIC NEBULÆ, ANNULAR AND PLANETARY NEBULE. CLUSTERS BY NEBULOUS SUBSIDENCE. -THAT OF ANDROMEDA. - -DOUBLE NEBULE.- NEBULOUS STARS.-CONNEXION OF NEBULE WITH DOUBLE STARS. WHOLLY IRREGULAR. OF DISTRIBUTION MARKS THEM AS OUTLIERS OF THE GALAXY. INSULATED NEBULE OF FORMS NOT - NEBULÆ, AND NEBULOUS GROUP OF ORION SAGITTARIUS -OF OF CYGNUS. -THE MAGELLANIC CLOUDS. SINTHE ZODIACAL (864.) WHEN we cast our eyes over the concave of the heavens in a clear night, we do not fail to observe that here and there are groups of stars which seem to be compressed together in a more condensed manner than in the neighbouring parts, forming bright patches and clusters, which attract attention, as if they were there brought together by some general cause other than casual distribution. There is a group, called the Pleiades, in which six or seven stars may be noticed, if the eye be directed full upon it; and many more if the eye be turned carelessly aside, while the attention is kept directed upon the group. Telescopes show fifty or sixty It is a very remarkable fact, that the center of the visual area is far less sensible to feeble impressions of light, than the exterior portions of the retina. Few persons are aware of the extent to which this comparative insensibility extends, previous to trial. To estimate it, let the reader look alternately full at a star of the fifth magnitude, and beside it; or choose two, equally bright, and about 3° or 4° apart, and look full at one of them, the probability is, he will see only the other. The fact accounts for the multitude of stars with which we are impressed by a general view of the heavens; their paucity when we come to count them. large stars thus crowded together in a very moderate space, comparatively insulated from the rest of the heavens. The constellation called Coma Berenices is another such group, more diffused, and consisting on the whole of larger stars. (865.) In the constellation Cancer, there is a somewhat similar, but less definite, luminous spot, called Præsepe, or the bee-hive, which a very moderate telescope, - an ordinary night-glass for instance, -resolves entirely into stars. In the sword-handle of Perseus, also, is another such spot, crowded with stars, which requires rather a better telescope to resolve into individuals separated from each other. These are called clusters of stars; and, whatever be their nature, it is certain that other laws of aggregation subsist in these spots, than those which have determined the scattering of stars over the general surface of the sky. This conclusion is still more strongly pressed upon us, when we come to bring very powerful telescopes to bear on these and similar spots. There are a great number of objects which have been mistaken for comets, and, in fact, have very much the appearance of comets without tails: small round, or oval nebulous specks, which telescopes of moderate power only show as such. Messier has given, in the Connois. des Temps for 1784, a list of the places of 103 objects of this sort; which all those who search for comets ought to be familiar with, to avoid being misled by their similarity of appearance. That they are not, however, comets, their fixity sufficiently proves; and when we come to examine them with instruments of great power,—such as reflectors of eighteen inches, two feet, or more in aperture,any such idea is completely destroyed. They are then, for the most part, perceived to consist entirely 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. (See fig. 1. pl. II., which represents (somewhat rudely) the thirteenth nebula of Messier's list (described by him as nébuleuse sans étoiles), as seen in a reflector of 18 inches aperture and 20 feet focal length.) Many of them, indeed, are of an exactly round figure, and convey the complete idea of a globular space filled full of - stars, insulated in the heavens, and constituting in itself a family or society apart from the rest, and subject only to its own internal laws. It would be a vain task to attempt to count the stars in one of these globular clusters. They are not to be reckoned by hundreds; and on a rough calculation, grounded on the apparent intervals between them at the borders, and the angular diameter of the whole group, it would appear that many clusters of this description must contain, at least, five thousand stars, compacted and wedged together in a round space, whose angular diameter does not exceed eight or ten minutes; that is to say, in an area not more than a tenth part of that covered by the moon. (866.) Perhaps it may be thought to savour of the gigantesque to look upon the individuals of such a group as suns like our own, and their mutual distances as equal to those which separate our sun from the nearest fixed star: yet, when we consider that their united lustre affects the eye with a less impression of light than a star of the fourth magnitude, (for the largest of these clusters is barely visible to the naked eye,) the idea we are thus compelled to form of their distance from us may prepare us for almost any estimate of their dimensions. At all events, we can hardly look upon a group thus insulated, thus in seipso totus, teres, atque rotundus, as not forming a system of a peculiar and definite character. Their round figure clearly indicates the existence of some general bond of union in the nature of an attractive force; and, in many of them, there is an evident acceleration in the rate of condensation as we approach the center, which is not referable to a merely uniform distribution of equidistant stars through a globular space, but marks an intrinsic density in their state of aggregation, greater in the center than at the surface of the mass. It is difficult to form any conception of the dynamical state of such a system. On the one hand, without a rotatory motion and a centrifugal force, it is hardly possible not to regard them as in a state of progressive collapse. On the other, granting such a motion and such a force, we find it no less difficult to reconcile the apparent sphericity of their form with a rotation of the whole system round any single axis, without which internal collisions might at first sight appear to be inevitable. If we suppose a globular space filled with equal stars, uniformly dispersed through it, and very numerous, each of them attracting every other with a force inversely as the square of the distance, the resultant force by which any one of them (those at the surface alone excepted) will be urged, in virtue of their joint attractions, will be directed towards the common center of the sphere, and will be directly as the distance therefrom. This follows from what Newton has proved of the internal attraction of a homogeneous sphere. (See also note on Art. 735.) Now, under such a law of force, each particular star would describe a perfect ellipse about the common center of gravity as its center, and that, in whatever plane and whatever direction it might revolve. The condition, therefore, of a rotation of the cluster, as a mass, about a single axis would be unnecessary. Each ellipse, whatever might be the proportion of its axis, or the inclination of its plane to the others, would be invariable in every particular, and all would be described in one common period, so that at the end of every such period, or annus magnus of the system, every star of the cluster (except the superficial ones) would be exactly re-established in its original position, thence to set out afresh, and run the same unvarying round for an indefinite succession of ages. Supposing their motions, therefore, to be so adjusted at any one moment as that the orbits should not intersect each other, and so that the magnitude of each star, and the sphere of its more intense attraction, should bear but a small proportion to the distance separating the individuals, such a system, it is obvious, might subsist, and realize, in great measure, that abstract and ideal harmony, which Newton, in the 89th Proposition of the First Book of the Principia, has shown to characterize a law of force directly as the distance.* (867.) The following are the places, for 1830, of the principal of these remarkable objects, as specimens of their class: See also Quarterly Review, No. 94. p. 540. Of these, by far the most conspicuous and remarkable is w Centauri the fifth of the list in order of Right Ascension. It is visible to the naked eye as a dim round cometic object about equal to a star 4.5 m., though probably if concentered in a single point, the impression on the eye would be much greater. Viewed in a powerful telescope it appears as a globe of fully 20' in diameter, very gradually increasing in brightness to the center, and composed of innumerable stars of the 13th and 15th magnitudes (the former probably being two or more of the latter closely juxtaposed). The 11th in order of the list (R. A. 16h 35m) is also visible to the naked eye in very fine nights, between n and Herculis, and is a superb object in a large telescope. Both were discovered by Halley, the former in 1677, and the latter in 1714. (868.) It is to Sir William Herschel that we owe the most complete analysis of the great variety of those objects which are generally classed under the common head of Nebulæ, but which have been separated by him into-1st. Clusters of stars, in which the stars are clearly distinguishable; and these, again, into globular and irregular clusters; 2d. Resolvable nebulæ, or such as excite a suspicion that they consist of stars, and which any increase of the optical power of the telescope may be expected to resolve into distinct stars; 3d. Nebulæ, properly so called, in which there is no appearance whatever of stars; which, again, have been subdivided into subordinate classes, according to their brightness and size; 4th. Planetary nebulæ; 5th. Stellar nebulæ; and, 6th. Nebulous stars. The great power of his telescopes disclosed the existence of an immense number of these objects before unknown, and showed them to be distributed over the heavens, not by any |