Chapter XL: The Nature of Nebulæ

The relations of white to green nebulæ are obscure. Unitive links between the two classes have yet to be established. In most respects they stand at present widely apart. They present a superficial likeness, but their dissimilarities seem to be radical. They are unconnected by any marked spectral affinities; they differ organically in structure; their distribution on the sphere is regulated by opposite principles. Hence their genealogical precedence remains unsettled. It would be rash to say that either family developed from the other, or even that they are collateral offshoots from a common stock. That a line of continuity will, sooner or later, become traceable is more than likely, but we must wait for the guidance of facts with regard to it; premature divinatory efforts are usually good for less than nothing.

As to the constitution of white nebulæ, we seem on the verge of knowing something definite. Premonitions of their being a species of fine-grained star-cluster have become audible. The subject, however, is not ripe for discussion. A comparatively advanced stage has, on the other hand, been reached by the problems connected with gaseous nebulæ, since their spectra let us, to a certain extent, into the secret of their composition. This, on the whole, seems to be remarkably uniform. Individual differences, it is true, both physical and chemical, distinguish the various members of the class; but they are of a subordinate kind. We may then safely attempt to generalise as regards a few of their more obvious properties. Three of these can be at once enumerated:—

(1) Gaseous nebulæ are almost perfectly transparent.[1171]

(2) They shine with extreme feebleness.

(3) Their mass is vanishingly small in proportion to their bulk.

We will take each point separately. That nebulæ offer no appreciable obstacle to the transmission of light is attested by the unaltered radiance of stars shining through them. No absorption that can possibly be due to the cosmic fog in which they are plunged to depths of many millions of miles, is traceable in the spectra of such objects as Θ Orionis, of ρ Ophiuchi, of Maia or Merope in the Pleiades. Similarly, the central stars of planetary nebulæ shine through an interposed medium, the extent of which is measured by the radius of each gaseous globe; and this, by a rough minimum estimate, can rarely be less, and must often be a great deal more than 50,000 to 60,000 million miles. Yet from the heart of these extraordinary formations the light of their nuclei comes to us, so far as it is possible to judge, absolutely intact. The impotence of comets for light-stoppage is thus vastly enhanced in nebulæ.

The feebleness of their luminosity is a matter of direct observation. A shining superficies, unlike a shining point, loses none of its lustre with increased distance. Its area of course diminishes according to the law of inverse squares, but every minute element of that area continues to radiate with the same intensity as before. The sun, for instance, is no less _bright_ as viewed from Neptune than when it crosses the meridian of Khartoum, but it is 900 times _smaller_. So with the nebulæ. They are really as faint as they appear. Using the best available data, Mr. Ranyard arrived at the conclusion that a planetary of the most vivid kind emits per square mile less than (1)/(22,000) millionth of the light sent abroad by the solar photosphere. This implies—adopting the result of Langley’s experiment at Pittsburg—that white-hot iron glows at least 4,000,000 times more powerfully than the bluish disc of the “Saturn” or the “Owl” nebula. Moreover, the differences in areal lustre between one nebula and another represent actual varieties of emissive strength. Remoteness has nothing to do with producing them. A _debilissima_—a “breath-stain” on the sky—may be as near to us as the great hiatus in the vault through which Huygens half-imagined the blazing of empyrean fires.

Finally, nebulæ being prodigiously voluminous and of apparently insignificant mass, must be of exceedingly low mean density. This fundamental fact was realised with uncompromising distinctness by Mr. Ranyard in 1892.[1172] Taking, for illustrative purposes, the Orion nebula to be a sphere 20′ in diameter, composed uniformly of materials 1,000,000 times rarer than atmospheric air at sea-level, he found that its mass would be such as to impart, to a body at a distance from its centre equal to that of α Centauri from ourselves, a circular velocity of 180, or a parabolic velocity of 255 miles per second. In the neighbourhood of the nebula, accordingly, there should be a marked prevalence of large proper motions. A star, travelling across the line of sight under the influence of its attraction at the rate of 100 miles a second, would, it was shown, have an annual displacement on the sphere of no less than 25·5″, and this independently of remoteness. For with the same angular dimensions, the solid contents of the nebula would increase as the cube of the distance assumed for it; while the seeming velocities of bodies in gravitational dependence upon it would undergo no change. This is rendered obvious by a moment’s consideration. For take any given star circulating round its centre of gravity at the rate, let us suppose, of 100 miles a second. And let us further suppose, to begin with, that the distance of the system is such that light would spend ten years on the journey thence to our eyes. Let us now double that distance and follow out the consequences. First, the nebula is eight times more massive than would have comported with the previous arrangement. Next, the revolving star is twice as far from it as before, since the apparent interval has not changed. Whence we easily gather, by the application of Kepler’s third law, that it now moves with double its previous speed. But its distance has, by hypothesis, also been doubled; consequently, its proper, or apparent motion remains just what it was. If, then, the stars about the Orion nebula were really in swift circulation, they should appear to be conspicuously progressive. This, however, is so far from being the case that the region is one of exceptional fixedness. The spectroscopic information at command is to a corresponding effect. The six brightest stars of the constellation, measured at Potsdam in 1892, proved to be all affected, in varying degrees, by the retreat of our system from that locality of the heavens, but gave no signs of travelling rapidly on their own account. The conclusion is inevitable that the Orion nebula—and it may be accepted as typical—contains inestimably less matter than should be comprised by it if its average density were that of a Crookes vacuum. Mr. Ranyard, indeed, assigned to it a consistence not exceeding (1)/(10,000) millionth that of air at standard pressure, which, he continued, “would about correspond to the mean density of the solar nebulous mass, supposing it to have been spherical when its radius was a little more than 107 astronomical units, or when the sun occupied a sphere with a radius of a little more than three and a half times the distance of Neptune.” The potential solar system in those days lay muffled in the haze of a small planetary nebula.

Yet it is impossible to conceive of nebulæ as formed simply of matter in an aerial condition. They are no mere vague effusions. They possess definite and characteristic structure. Lord Rosse,[1173] indeed, thought sharpness of contours distinctive of the gaseous kind. Mr. Maunder[1174] speaks of their “strange and complicate shapes, showing here and there strongly-marked outlines”; and he adverts to the difficulty of explaining this peculiarity in vast, uncontrolled extensions of rarefied gas. The abolition of this incongruity was one of the strong points of Sir Norman Lockyer’s “meteoritic hypothesis” of nebular constitution. The spectroscope, it is true, pronounced against it; nebular chemistry has very little in common with the chemistry of “uranoliths”; yet amid much that was precarious or unsound, the valuable idea was introduced that a proportion of solid matter must enter into the composition of nebulæ. Its condition and distribution, however, remain unknown. It would seem to be devoid of light, for the faint continuous spectrum accompanying the nebular bright lines would be displayed even by a homogeneous gaseous mass, unless its radiations were of purely superficial origin.[1175] But here we meet the unresolved enigma of nebular luminosity. How do they shine? Is it through the direct agency of heat? Experimental evidence does not countenance this view. In the laboratory, hydrogen and helium can be induced to give out their characteristic rays only under the stress of electrical excitement. The concomitant high temperature might—as Sir William Huggins pointed out in 1891[1176]—prevail only along the path of the discharge, while the surrounding gases remained cool, producing inequalities in heat-distribution similar to those believed to exist in vacuum tubes. If, on the other hand, nebular emissions were of simply thermal production, the whole radiating mass should be at nearly the same pitch of incandescence; for if the temperature were lower in some than in other of its parts, absorption lines or reversals would betray the fact, and the nebular spectrum bears no legible marks of selective light-stoppage. Yet it is eminently improbable that formations so circumstanced are, in their entirety, excessively hot. Their solid inclusions should, if they were, glow powerfully, and strong continuous radiance would replace the dim band, grey through faintness, actually seen. On the supposition, however, that electrical discharges cause the glow of nebulæ, their average temperature, judging by Dr. Scheiner’s experimental results, might approximate to absolute zero.

The word “temperature,” indeed, when applied to matter in the last stage of attenuation, has an extremely dubious meaning. Taking the kinetic theory of gases to represent the literal truth, we find the effects of heating upon them to be twofold, namely, increase (1) of translatory, (2) of internal energy, the latter being perhaps a consequence of the former. Since, then, their constituent particles travel faster when heat is applied, they come into mutual collision more frequently and more violently. That is to say, if their number per unit volume remains the same. Let, however, the density of the gas be reduced, other things being unchanged, what will ensue? There will be fewer encounters in equal times, but they will _not_ be less violent, since the mean rate of the molecules has undergone no alteration. Hence, apart from one qualifying circumstance, the same species of vibration should be set going as when the gas was denser and equally hot. The qualifying circumstance is this. During the lengthened intervals between the collisions, the molecules are radiating—that is, imparting energy to the ether. If their free paths were sufficiently protracted, they would—if that be possible—lose all they possess, and need to be fully restocked at each encounter. The comparative isolation of the molecules would accordingly result in their being mostly dark, and what we must call cold; so that the extreme subtlety of matter seems incompatible with a high temperature in any intelligible sense of the term. But there is more. Reduction of pressure would have for its immediate result a general diminution of luminosity. In all probability, it would also produce a selective effect, for if, as seems likely, certain modes of molecular vibration are more persistent than others, they would, with the progress of cooling after each impact, tend to predominate, and the balance of intensity among the spectral lines of a glowing gas would thus be sensibly altered. Here, then, an explanation might be sought of the spectral anomalies of hydrogen in nebulæ. Nevertheless, it does not run quite smoothly, since up to the limit of possible rarefaction in vacuum tubes the crimson line of hydrogen gains consistently in strength.[1177] A corresponding difficulty presents itself in connection with the helium spectrum.[1178] “Green tubes” are at a higher degree of exhaustion than “yellow.” Yet in nebulæ the green ray is invisible; nebular helium radiates only D_{3} with a few members of the same “set.” By means of artificial exhaustion, accordingly, the nebular spectrum cannot apparently be imitated. The only experimental clue to its origin, in fact, is in Professor J. J. Thomson’s observation (already mentioned) that F and C are of interchangeable intensity at the negative and positive poles of a hydrogen tube.

To recapitulate. Two fundamental problems regarding gaseous nebulæ press for solution—one connected with their structural forms, the other with the nature of their light. Do they continuously fill the spaces they appear to occupy, or should we figure them to ourselves as collections of discrete bodies comparatively wide apart? In the former case the dark gaps and chasms which form one of their leading features should be ascribed, not to the absence of matter, but to defect of shining power, illuminative, not architectural contours being disclosed by them. In the latter, chiaroscuro effects would be reliable indices to the distribution of material.

Secondly, we are confronted with the mystery of their shining. Why and how are they lucent? Are they bright as a consequence of thermal or electrical stimulation, or is their radiance some undefined species of luminescence or phosphorescence? The answer must be given in view of the three following peculiarities of their spectra: (1) The leading hydrogen line in them is F, not C. (2) Helium shines in them as if in a “yellow” tube. (3) Reversals are absent; lines of absorption make no assured appearance, either as superimposed upon, or as subjacent to lines of emission. None of these points are easy to decide. Their consideration involves doubts and queries of an abstruse nature; but it will not therefore be neglected. Indeed, such a region of inquiry as is here presented, where we feel that at every step the Unknown may merge into the Unknowable, has a particular and an illimitable fascination.