ing the discoveries made by Lord Rosse's telescope, says they may be separated into three classes; those which are round, of nearly uniform brightness; those which are round, but appear to have one or more nuclei; and those which are extended in one direction, so as to become long stripes or rays.† It should be added, that although there can be no doubt as to the regular character of the forms assumed by distant groups, yet as wonderful changes are made in their appearance by higher optical powers, we are not at liberty to assume that we have ascertained their forms with perfect accuracy. Thus some of the nebula which presented FIG. 76. FIG. 75. Cluster in Hercules. FIG. 76. Annular Nebula in Lyra. + Transactions of Royal Irish Academy, 1847. the appearance of a spherical body to Sir John Herschel's eighteen-inch reflector, have been transformed by Lord Rosse's six-feet speculum into a luminous spiral of unequal convolutions, which are prolonged at both extremities into granular globules. "Almost every new observation appears to confirm the fact of that curious tendency to spiral arrangement in these nebulous masses, of which mention has been so frequently made."* Sir John Herschel discovers in these aggregations of stars the operations of physical laws. "Their round form clearly indicates the existence of some general bond of union of 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 centre 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 centre than at the surface of the mass." The same distinguished astronomer regards it as a general law in the constitution of extended nebulæ, that their interior or brighter strata are more nearly spherical than their exterior or fainter, their ellipticity diminishing as we proceed from without inwards, a character which he represents as favouring, though not conclusively, "the idea of rotation on an axis, in the manner of a body whose component parts have such an amount of mutual connexion as to admit of such a mode of rotation, and of the exertion of some degree of pressure one on another." Some of the late disclosures of Lord Rosse's telescope, in regard to the prevalence of the spiral form in nebular groups, may so far effect these speculations, but in doing so they open to our view a more wonderful harmony, the law of which has not been determined. * President's address to British Association, 1858. Observations at Cape, p. 8. The Milky Way, which spans our heavens so conspicuously, is not a cluster of stars, but a succession of clusters. Our sun is one of the stars composing this system, and is supposed to be placed not far from the centre, but nearer the one side than the other, and in one of the poorer or almost vacant parts of its general mass. Sir posed by certain speculators to consist of a sort of luminous matter, or star-dust, out of which worlds are being made even now by general law. This supposition has not been confirmed by later speculation. Within these few years, not a few of these nebula which were regarded as being most certainly luminous vapours, have been shewn to be stars. The magnificent telescope of Lord Rosse, not long after it began to be used, shewed that the great nebula in Orion, which was supposed to be one of the most unresolvable of them all, consisted of clusters of distinct stellar bodies. Since that time, nebula after nebula has been resolved by Lord Rosse's telescope, and another of less power but in a finer climate, at Cambridge, in the United States. In 1850, Sir J. Herschel was prepared to declare it as being almost certain, since Lord Rosse's telescope had resolved, or rendered resolvable, multitudes of nebulæ, that all the rest could be resolved by a farther increase of optical power, and the language might be made still stronger and more decisive, in consequence of what has been accomplished by that magnificent telescope since that date. The nebulæ may now be confidently regarded as clusters of stars, and give evidence of order, combination, and law in the extreme boundary of that sphere of immeasurable magnitude which constitutes the universe as knowable by us. It is worthy of being mentioned, as illustrative of order and law, that there are to be seen in the expanse of heaven, in many places two or more stars which are apparently near each other, and which have been shown to be mutually connected as part of one system. It not unfrequently happens that a centre of light, which appears as only one star to the naked eye, is turned into two or more stars by a telescope of very ordinary power. Sometimes the relation is merely optical, and not real, that is, stars at a great distance from each other may seem near, because though the one be far behind the other, they lie nearly in the same line of vision to the eye. But the number of double stars in the heavens, being about 6000 in all, is far too numerous to be referred to any such cause. Among these, according to a table published in 1849, 650 are known in which a change of position can be incontestably proved. Besides it has been ascertained in regard to considerably more than 100 double stars, that they revolve about each other in regular orbits. In some cases there is a smaller star joined to a large one, in other cases there are two or more stars of nearly equal size revolving round a common centre of gravity. The orbit in which these connected stars move is ascertained to be elliptical. These phenomena lead Sir J. Herschel unhesitatingly to declare the stars to be subject to the same dynamical laws, and obedient to the same power of gravitation, which govern our sys Humboldt's Cosmos, vol. iii. p. 280, Otte's Translation; additions being made every year by the labours of Argelander, Sturve, &c. FIG. 78. Binary star, that is, two stars revolving round a common centre. |