remain inextricably mixed up with the other or real portion. The systematic aberration, amounting at its maximum (for stars 90° from the solar apex) to about 5" displaces all the stars in great circles diverging from that apex through angles proportional to the sines of their respective distances from it. This displacement, however, is permanent, and therefore uncognizable by any phænomenon, so long as the solar motion. remains invariable; but should it, in the course of ages, alter its direction and velocity, both the direction and amount of the displacement in question would alter with it. The change, however, would become mixed up with other changes in the apparent proper motions of the stars, and it would seem hopeless to attempt disentangling them. (863.) A singular, and at first sight paradoxical effect of the progressive movement of light, combined with the proper motion of the stars, is, that it alters the apparent periodic time in which the individuals of a binary star circulate about each other. To make this apparent, suppose them to circulate round each other in a plane perpendicular to the visual ray in a period of 10,000 days. Then if both the sun and the center of gravity of the binary system remained fixed in space, the relative apparent situation of the stars would be exactly restored to its former state after the lapse of this interval, and if the angle of position were 0° at first, after 10,000 days it would again be so. But now suppose that the center of gravity of the star were in the act of receding in a direct line from the sun with a velocity of one-tenth part of the radius of the earth's orbit per diem. Then at the expiration of 10,000 days it would be more remote from us by 1000 such radii, a space which light would require 5.7 days to traverse. Although really, therefore, the stars would have arrived at the position 0° at the exact expiration of 10,000 days, it would require 57 days more for the notice. of that fact to reach our system. In other words, the period would appear to us to be 1005'7 days, since we could only conclude the period to be completed when to us as observers the original angle of position was again restored. A contrary motion would produce a contrary effect. • Astronomische Nachrichten, No. 520. by the Author. CHAPTER XVII. OF CLUSTERS OF STARS AND NEBULE. THEIR OF CLUSTERING GROUPS OF STARS.- – GLOBULAR CLUSTERS. STABILITY DYNAMICALLY POSSIBLE.- LIST OF THE MOST REMARK - THAT OF ANDROMEDA. — ANNULAR AND PLANETARY NEBULÆ. -DOUBLE NEBULE. -NEBULOUS STARS.-CONNEXION OF NEBULÆ INSULATED NEBULE OF FORMS NOT WITH DOUBLE STARS. THEIR LAW WHOLLY IRREGULAR. OF AMORPHOUS NEBULE. (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. |