A globular collection of promiscuously revolving particles inclines, if left to itself, to flatten down into a disc. The reason is this: In a system of the kind moment of momentum is invariable, while energy constantly diminishes. To render the contrast intelligible we have only to consider that moment of momentum is the algebraic sum of all the products of mass and motion in the aggregation, reduced to, or projected upon, its 'principal plane,' while energy is independent of the varied directions of velocity. Collisions consequently involve no diminution of moment of momentum, but combine with radiative waste to produce a steady loss of energy. Inevitably, then, the system will assume the form in which it possesses the minimum of energy that is consistent with the maintenance of its original momentum; and it is that of a disc extended in the principal plane. Retrograde movements will by the time this shape is definitively arrived at have become eliminated; the constituent particles circulate unanimously in one direction; and Sir Robert Ball adds that their circulation, owing to the more rapid rotation of the central mass, is along spiral paths.[93] They would accordingly present the twisted conformation so commonly observed in the heavens, and might even include subordinate centres of attraction, fitted to ripen and strengthen into a full-blown retinue of planets. Such are spiral nebulæ regarded in their direct mechanical aspect. Spherical nebulæ are their immediate progenitors; suns, with or without trains of dependent worlds, their lineal descendants.
Let us, however, consult some autographic records and weigh attentively what these peculiar objects tell us about themselves. We see at once that their curving lines, far from being laid down at the dictate of chance, follow a strictly defined plan. Spiral nebulæ are not formed like watch-springs by the windings of a single thread. They are always two-branched. From opposite extremities of an elongated nucleus issue a pair of nebulous arms, which enfold it in double convolutions. Their apparent superposition and interlacements occasion, in the Lyra nebula, the noted effect of a fringed and ruptured annulus, and it is of profound interest to perceive that even in gaseous masses the same constructive rule prevails as in the great Whirlpool in Canes Venatici.
Yet this circumstance is well-nigh irreconcilable with the hypothesis that an influx of material is in progress.[94] Falls due to gravity could not be limited to two narrow areas on the central body. Matter ejected from it might, on the other hand, quite conceivably follow this course. Interior strain could easily be supposed to cause yielding along a given diameter, and nowhere else. Solar disturbances partially and dimly illustrate such a kind of activity. Diametrically opposite prominences are not unknown. They indicate the action of an explosive force right across the solar globe. Similarly, the formation of a spiral nebula cannot be rightly apprehended otherwise than as the outcome of long-continued, oppositely-directed eruptions.
The history of the heavens involves the law of spirality. The scope of its dominion continually widens as research becomes intensified. The Huygenian 'portent' in the Sword of Orion now figures as merely the nucleus of the 'great winding Nebula' photographed by Professor W. H. Pickering in 1889. That the vast nebulosity encompassing the Pleiades is an analogous structure seems eminently probable, though the brilliancy of the enclosed stellar group obliterates most traces of its ground-plan. The magnitude of the mixed system, we are told by Professor Barnard,[95] who detected it in 1893 by means of a ten hours' exposure with the Willard lens, transcends our powers of realization. It covers 100 square degrees of the sky with intricate details. Again, some four minutes of arc to the north-west of the Ring in Lyra lies a small nebula discovered visually by Professor Barnard in 1893, and photographically resolved by Keeler into a delicate spiral. It is a two-branched, left-handed spiral, as the large adjacent object has also proved to be. One is, in fact, the miniature of the other, and they are now shown, by Professor Schaeberle's short-focus reflector, to be linked together by curving folds of nebulosity into a compound spiral system. The Dumb-bell is held, on the same authority, to be similarly conditioned, and the analogy frequently noted between its aspect and that of the Ring nebula has thus become incalculably widened in scale.
The galactic relations of the Magellanic Clouds are not easily defined. They are within the Milky Way, yet not of it. Enigmatical excrescences upon the universe, they suggest an origin from gigantic eddies in the onflowing current of sidereal arrangement. Their miscellaneous contents are, to all appearance, disposed along eddying lines. Mr. H. C. Russell's photographs[96] rendered this, in 1890, to some extent manifest, and their indications were ratified by the Arequipa plates, from the study of which Professor Pickering gained the conviction that the great Looped Nebula, 30 Doradûs, is the structural nucleus of the Nubecula Major. 'It seems,' he wrote,[97] 'to be the centre of a great spiral, and to bear the relation to the entire system that the nebula in Orion bears to the great spiral nebula which covers a large part of that constellation.'
On all sides, in the sidereal heavens, we can discern the signs of the working of a law of convolution. Sometimes they are patent to view; sometimes half submerged; but they can generally, with attention, be disentangled from overlying appearances. They are exhibited by stars no less than by nebulæ, as the late Dr. Roberts pointed out from convincing photographic evidence; the 'hairy' appendages of globular clusters betray them by their curvilinear forms; they meet us in every corner of the wide nebular realm. Many investigators recognise in the Milky Way itself the stamp of spirality. Stephen Alexander, of New Jersey,[98] regarded the majestic galactic arch as a four-branched spiral, resulting from catastrophic breaches in a primitive, equatorially loaded spheroid, the currents of matter ejected by which should, owing to their lower angular rotation, lag behind as they retreated from the nucleus, and thus flow along helicoidal lines. R. A. Proctor subsequently devised convoluted galactic streams, which, however, corresponded imperfectly with what the sky showed. And Dr. Easton[99] has designed by way of simple illustration an elaborate series of spires, originating possibly from a central galactic condensation, the projection of which upon the sphere may, he thinks, account for the known peculiarities of the Milky Way.
Our interior situation, nevertheless, makes it extremely difficult to determine the real relations in space of the star-streams circling around it. The observed facts are, perhaps, equally compatible with many other structural schemes besides those based on the idea of spirality; and it will be prudent to adopt none, for the present, with settled conviction. We can, however, gather one sufficiently definite piece of information regarding the history of the Cosmos. All the inmates of the heavens, stellar and nebular, represent quite evidently the débris of a primitive rotating spheroid. Its equator is still marked by the galactic annulus, its poles by a double canopy of white nebulæ. The gyrating movement which it once possessed as a whole doubtless survives in its parts, but ages must elapse before the fundamental sidereal drift can be elicited.
FOOTNOTES:
[85] Keeler, Lick Publications, vol. iii., p. 214.
[86] Rosse, Transactions Royal Dublin Society, vol. ii., p. 93.