There is, on the whole, a concurrence of evidence that gaseous nebulæ are at a very early stage of growth. They are the least elaborated of sidereal objects; they seem, many of them, barely to have crossed the threshold of creation. Yet their mutual relations in time are by no means obvious. They cannot easily be disposed in any kind of rational sequence. Each of the great nebulæ, at any rate, exhibits features and occupies a position shared by none of its fellows. The most discerning cosmologist cannot pretend to say that the Argo nebula, say, is of greater or less antiquity than the Orion or the 'America' nebula. They are individual growths, simultaneous, not successive. The line of development suggested by their relationships is rather towards the formation of star-clusters than of diverse nebular species. Thus, the Pleiades illustrate not improbably the future condition of the Orion nebula, the contained stars having gained predominance over their misty envelopments, though fragmentary swaddling-bands, later, presumably, to be shaken off, still adhere to many of them.
Planetary nebulæ have much more in common than irregular nebulæ, and their minor varieties might, with some plausibility, be associated with differences in relative age. They are marked chiefly by the character of the nuclear star which, in nearly all such objects, appears to act as the pivot of the surrounding vaporous structure. The supposition lies close at hand that it is designed as a provision for the nourishment of the star—that the star gains in mass and light at the expense of the nebula, which it is eventually destined wholly to absorb and supersede. On this view, planetaries like the green glow-lamp at the pole of the ecliptic (N.G.C. 6,543) should be regarded as the most advanced, while Webb's planetary in Cygnus (N.G.C. 7,027) would exemplify an inchoate condition. In the former the central star is of 9·6 magnitude, and sharply stellar; in the latter it is double and diffuse,[85] perhaps a wide binary system in embryo.
The question is, however, still open as to the real nature of the connection between planetaries and their central stars. The pabulum theory is a promising conjecture; but no facts with which we are acquainted stringently enforce it. Ideas on the subject will need complete revision if the traces of spirality noted from time to time in some of these peculiar objects prove to be of radical significance. The oculi, distinctive of the 'Owl nebula' (N.G.C. 3,587), as originally shown by the Parsonstown reflector, consisted of luminous traceries coiled round two interior stars,[86] but the appearance was either due to illusion, or became effaced by change, since the camera has refused to endorse it as genuine. The 'helical' planetary in Draco[87] is doubtless essentially a spiral conformation;[88] and Professor Schaeberle, by means of exposures with a 13-inch reflector of 20 inches focus, has compelled, not only the Ring nebula in Lyra,[89] but the Dumb-bell in Vulpecula, to betray the surprising secret of their whorled structure. Both these nebulæ give a spectrum of bright lines, and inventiveness is at a loss to devise means for building up gaseous materials into edifices of strongly characterized architectural forms. The materials, indeed, may not be wholly gaseous;[90] or we possibly see (as Professor Darwin long ago suggested) merely illuminated stream-lines of motion furrowing an obscure mass. But if this be indeed so, there is the further question to be asked: What direction does the motion take? Do the tides set inward or outward?
Our spontaneous impressions are all in favour of concentrative tendencies. We cannot easily shake off centripetal prejudices. Our lives are passed under a regimen of central attraction, and we naturally incline to universalize our experience. Herschel's scheme of sidereal evolution invites accordingly at first sight ready acceptance. Stars seem as if they could not act otherwise than as foci of condensation in nebulæ; the lucid stuff involving them must, apparently, with the lapse of ages, settle down towards their surfaces, and become absorbed into their substance. Such processes, indeed, belong, unless counteracted by different modes of action, to the inevitable order of nature; but these may, and probably do, exist. From sundry quarters the conviction is pressed upon us that cosmic bodies can drive out matter as well as draw it in. Repulsive forces insist upon recognition, and their effects become more palpable the more attentively they are considered. Under certain conditions they get the better of gravity; and stars may possibly, like cocoon-spinning insects, expend their organic energies in weaving themselves faintly lucent envelopes, the products of subtle and unaccountable activities.
The example of Nova Persei is fresh in every mind, but we make no pretension to decide the controversy it raised. A dogmatic pronouncement is unadvisable where the unknown elements of the question obscure and outweigh those that are known. A less slippery foundation for reasoning is afforded by the permanently visible spiral nebulæ, and features charged with an emphatic meaning have been revealed in them by photographic means.
Looking at the entire contents of the nebular heavens, we find the spiral type very largely predominant. It claims more specimens, and emerges more distinctly with each development of delineative power. Its chief prevalence is among 'white' nebulæ, showing continuous spectra.
They are vastly numerous. Gaseous nebulæ are reckoned by the score, white nebulæ by tens of thousands. Moreover, they collect near the poles of the Milky Way,[91] while the gaseous variety crowd towards its plane, both branches of the family thus manifesting galactic relationships, though of an opposite character. Now, these facts of distribution have some bearing on the question of relative age. There is, as already remarked, a consensus of opinion that objects showing a marked preference for the Milky Way are in a more primitive state than those withdrawn from it, and the inference is supported by the circumstance that nebulæ situated in high galactic latitudes shine with continuous light, those near the galactic equator with vivid lines. Yet it would be rash to assume that any individual nebula traverses these successive stages. The series would be satisfactorily established only if we could point to a number of intermediate instances, which seem to be almost wholly lacking. We cannot trace in nebular as we can in stellar growth the insensible gradations of progressive change. They are perhaps complicated in nebulæ by influences of a different kind from those which have gained the ascendancy in stars. Diffusive effects may in them be more conspicuous than concentrative effects;[92] or a balance may be temporarily struck between antagonistic tendencies.
Spiral conformation is the real crux of nebular cosmogony. The conditions from which it arises are met with only in the sidereal heavens, but are there widely prevalent. Though remote from our experience, they are fundamental in the realms of space. If we could define and comprehend them, we should be in a better position for determining the cosmical status of nebulæ.
The choice is open between two rival theories of nebulous spirals. The first is the more obvious, and readily falls in with admitted mechanical principles. Sir Robert Ball has adopted and ingeniously advocated this view.