Chapter XXXI: White Nebulæ

“White nebulæ”—so called by Professor Young[876]—are those giving continuous spectra. They are in an immense majority. They are reckoned by thousands, or tens of thousands, gaseous nebulæ by the score. True, very little progress has been made with their actual spectroscopic examination, the faintness of their rays forming, in general, an insuperable obstacle to their analysis; but their shape and aspect supply indications, rarely misleading, as to the quality of their light. That of elliptical and spiral nebulæ is, to the best of our knowledge, always continuous; and with these may be classed the round, centrally-condensed objects which abound in every nebular region of the sky. Several other varieties of this great sidereal family might be indicated, but they are by comparison scantily represented, and have been but little investigated. The paragon of white nebulæ is the grand ellipse in Andromeda. No other is visible to the naked eye; it should be, judging by appearances, much the nearest to the earth of the whole tribe; its structure is splendidly definite, and profoundly significant; its spectrum shows peculiarities challenging inquiries which must be long-continued and arduous, but promise results of far-reaching importance. In January 1899 Dr. Scheiner,[877] employing a small spectrograph in combination with a mirror of nearly thirteen inches aperture, and only forty inches focus—an apparatus specially adapted for dealing to advantage with objects of extended surface—obtained in seven hours a legible spectrograph of the nebula. The indications gathered from it were of a most surprising kind. Dark rays were perceived to interrupt the continuous light, and they seemed to agree with the Fraunhofer lines in the solar spectrum. The Andromeda nebula was accordingly inferred to be a genuine cluster of solar stars; but this conclusion is very far from being securely established. No bright lines could be made out in the Potsdam photograph, but many have been _seen_ at Tulse Hill.[878] On 13th November and 11th December 1897, when they were particularly distinct, approximate wave-lengths were assigned to six or seven, all of which fall near lines in the Wolf-Rayet stars. The reality of the coincidences cannot at present be pronounced upon; they are hinted at rather than asserted; but their verification would enforce an entire recasting of ideas as to the nature of white nebulæ.

A photograph of the Andromeda ellipse, taken by Dr. Roberts 10th October 1887, set the example, since extensively followed, of resolving into spirals, with the help of the camera, all sorts and conditions of nebulæ. It was, indeed, a memorable picture. The vast structure is shown in it and its successors[879] to be furrowed through and through by dark channels, or rather by a single continuous channel, winding in symmetrical convolutions in a left-handed direction from the compact nucleus outward to the dim, indefinite margin. Thus the nebula is not simply a concatenation of flat rings separated by vacant intervals; if it were, the problem of its construction would be less difficult; since the annular gaps might represent spaces cleared of their contents by exceptionally acute gravitational disturbance, while the ejection of matter along a spiral track belongs to a totally different order of phenomena, and implies the operation of laws scarcely yet brought within our ken.

The Andromeda nebula is presumably a round disc viewed obliquely. If so, the angle of its inclination is about 25°.[880] Remarkably enough, the nucleus does not share the elongation of the surrounding spires, as it should if it were no more than a flat condensation in their plane. Its outline is, on the contrary, circular,[881] and its true shape must be that of a globe. There is no probability that the innumerable stars strewing the formation have any physical connection with it. Two small nebulæ in its immediate neighbourhood, on the other hand, certainly belong to its system. The closer and brighter (M 32) was discovered by Le Gentil in 1749; the other, which is situated in a nearly opposite direction, by Caroline Herschel in 1783. Both can be seen with powerful telescopes to be included within the limits of the primary agglomeration.[882] Le Gentil’s nebula, indeed, appeared on a Meudon negative to lie as a condensed knot upon one of its external spires,[883] and the companion object doubtless owns a similar origin. The latter is an oval, apparently amorphous mass; its longer axis is inclined 60° to that of the great nebula. The two satellites may eventually yield signs of orbital revolution; or the whole disc perhaps rotates as one piece, and they along with it; we cannot attempt to decide which condition is the more likely to prevail. Perhaps neither to the exclusion of the other. It is conceivable that the more remote member of the system circulates independently, while the inner companion is borne onward with the general swirl.

Far inferior to this “Ajax” among the nebulæ, although eminent among the “other Argives,” is a large lenticular object in Cetus (N.G.C. 252), noticed by Caroline Herschel in 1783. Sir John Herschel[884] considered its “streaky and knotty” texture to denote resolvability into stars; but it came out instead as a fine spiral in a photograph taken by Dr. Roberts 25th December 1899, a reproduction of which is given, by his kind permission, in Plate XX. The whorls are evidently much foreshortened. They are studded, as Dr. Roberts remarks,[885] “with numerous condensations of a stellar character,” while six ordinary stars are probably seen in projection upon them. Measures of their positions relative to each other and to exterior stars might serve, he adds, for the detection of any movements, rotational or translational, by which the nebula may be affected. Its considerable south latitude brings it within the spectrographic domain of the Cape Observatory, and the McClean apparatus might be competent to obtain an impression of a spectrum sure to prove interesting, if only it can be made distinctly visible.

A nebulous “ray” in Ursa Major (M 82) was described by Lord Rosse as “a most extraordinary object, at least ten minutes of arc in length, and crossed by several dark bands.”[886] These run obliquely to the axis, and give the nebula—as Mr. Ingall said—“a twisted appearance, like a distaff of flax.[887] It appears to possess two centres of condensation, which must lend no slight complexity to its internal economy. Each is perhaps the starting-point of a separate arrangement of luminous coils, but no fair view can be got of them; they are foreshortened into mere broken lines.” The nebula, Dr. Roberts explains,[888] is presented to us “in section, and the upper and lower surfaces are very rugged.” The divisions between the rings hence took shape in a negative, to which he gave three and a half hours’ exposure, 31st March 1889, as “rifts and attenuated places” not obviously fitting together into a harmonious plan. The profile of a corrugated disc is not an easily intelligible object, and that is all that can be seen of M 82. Perspective has done its utmost to disguise its true aspect. Turned edgewise towards the earth, it betrays only by its indentations and rugosities the effects of the ploughing action to which for ages it has been subjected. Swift’s “hairline nebulæ” belong to the same category. They show as bare streaks of nebulosity, bulging a little where the nuclei protrude. Presumably they are flat, circular surfaces, the planes of which coincide with the line of sight.

The “ray” in Ursa Major is not solitary. It is placed at a distance of only 42′ from a larger structure (M 81), evidently of the same general character. The two were photographed together by Dr. Roberts in 1889, and cannot be wholly disconnected. The primary—if we may call it so—resembles the great Andromeda nebula, and, like it, was resolved into a fine spiral. The spectra of both objects were found by Sir William Huggins to be continuous; but the significant details disguised by apparent continuity have still to be revealed.

PLATE XX.

Photograph of a Spiral Nebula in Cetus. Taken by Dr. Roberts, 25th
December 1899.
]

The essential formative law of white nebulæ is unmistakably that of spirality. This conviction, strongly upheld by the long series of the Crowborough pictures, was irrefutably established by Professor Keeler’s photographic survey with the Crossley reflector.[889] Owing to the strong light-collecting power of the instrument, the harvest of nebulæ garnered was so plentiful that the number within its reach over the whole heavens was estimated at no less than 120,000, and nearly all of these can be inferred, from the preliminary results obtained, to have a spiral shape. On the Lick plates, in fact, a small compact nebula, _not_ disposed in luminous coils, stood out as a rarity. All spindle-nebulæ were resolved into spirals viewed aslant, but into spirals of various degrees of complexity. Some consist merely of two curved branches, shaped like the letter S, and diverging oppositely from a nuclear condensation. An object of the kind situated in Pegasus (N.G.C. 7479) is reproduced from Professor Keeler’s photograph in Plate XXI. Subjoined are the drawings by J. Herschel, d’Arrest, Lord Rosse, and Tempel, with which Keeler compared the autograph picture. They make an instructive study. Herschel saw the object as a narrow spindle “extended between two stars,” d’Arrest as a lozenge; Lord Rosse perceived, in addition, a mass of spiral convolutions surrounding a faint star, while Tempel caught the double effect of a round attached to an elongated patch of luminosity, but failed to discern their true connection. At last on the Lick plates the object disclosed itself under an intelligible aspect. “A glance at the photograph,” Professor Keeler wrote,[890] “shows that the nebula is a two-branched, left-handed spiral, with a nucleus or condensation near the point of inflection. The preceding branch is strong and single, but the following branch is split into two, which cross where their curvature is greatest, at some distance from the centre of the spiral, and unite again at their extremities. This appearance in the components of the following branch, and the fact that the ends of both branches curve around so as to approach the centre more closely than do the intermediate parts, are doubtless effects of projection, the plane of the spiral lying obliquely to the line of sight.”

Lord Rosse’s star occupies the centre of the space fenced round by the preceding branch (that to the left). “It would be of great interest,” the Lick astronomer continued, “to know whether this singular position of the star is accidental, or whether the star and the nebula are physically connected, and if so, in what way the star was left in its present position during the process of contraction. On the first of these questions an investigation of the spectrum, which will be made in due time” (the time, alas! never came), “may throw light. Assuming for the present that the star is physically connected with the nebula, it seems to me possible that the proximity of this star may account for the unsymmetrical appearance of the spiral, which may be due to an actual difference in the dimensions of the two branches, or to their lying in differently inclined planes.”

The first nebula in which a spiral conformation was recognised is still unsurpassed as a specimen of its class. We are enabled, by Mr. W. E. Wilson’s kindness, to reproduce in Plate XXII. his fine picture of this stupendous object. The coils are left-handed; they follow, as they issue from the nucleus, the line of movement taken by the hands of a watch. Our view of them is straight and square; they can be little, if at all, foreshortened. Yet they do not wind symmetrically round their origin. Their flow is broken and distorted, like the current of a river by jutting rocks. The spiral is fundamentally double. Two main streams leave the nucleus at diametrically opposite points, and preserve their separate individuality until they melt away into the outer darkness. Their course seems to be prescribed essentially by the combination of an ejective with a rotatory velocity; disturbances, however, manifestly supervene. The branches divide and reunite; they are cloven and bossy; they swerve widely from the circular track. This is especially remarkable in the case of the longest and brightest arm, which stretches irregularly outward to join a secondary exterior nucleus. This circumstance alone suffices to prove that the diffusion of matter in this formation has been outward. The perturbing mass was undeniably there before the luminous stream which it diverted began to flow; and its flow was quite plainly towards it from within. Other indications of centrifugal action are visible. Mr. Wilson’s photograph shows “cometary tails curved like a plume away from the central nucleus,” attached to some of the denser knots on the convolutions of the spiral;[891] and these effects of apparent repulsion are likewise clearly legible on Lick and Crowborough plates of the same object.

PLATE XXI.

1. Photograph of a Spiral Nebula in Pegasus. Taken by the late
Professor Keeler.

2. Drawings of the same Nebula by Herschel, d’Arrest, Rosse, and
Tempel.
]

Indraughts or infalls from space are not here concerned, whereas, in Mr. T. C. Chamberlin’s words,[892] “the effects of explosive projection, combined with concurrent rotation, must obviously give rise to a spiral form.” Each such nebula (and there are tens of thousands of them) results, in his view, from the “approach without collision” of a roving star to a compact gaseous mass. Strained to the point of disruption by tidal influences, this embryo vortex would, at a given moment, project from both extremities of the ellipsoid into which it had become elongated, a stream of material curved into whorls through the continual slackening of its angular rate of rotation; and the double catastrophic outrush served to constitute a great system of shining spires, subsequently diversified by the supervening phenomena of minor outbreaks. This rationale has much to recommend it, and probably rests upon a substratum of truth; yet the events contemplated in it are on a small scale by comparison with the grandiose dimensions which we must ascribe to spiral nebulæ.

Lord Rosse described a nebula situated near the star 83 Ursæ Majoris (M 101 = N.G.C. 5457, 5458), as a large faintish spiral, with several arms and knots, at least 14′ across.[893] A four hours’ exposure at Lick brought into view a surprising wealth of intricate details. The groundwork of the structure agrees closely with that of the great spiral in the Hunting Dogs. It is composed of two main effusions, sweeping round from left to right. But they spread, and split, and ramify, drawn hither and thither by multiple attractions, while preserving in their complex interlacings, the whirling impress of their origin.

“Three-branched spirals” still survive here and there in catalogues. Such were supposed to be the delicate objects, M 99 in Virgo (N.G.C. 4254), and M 83 in the head of the Centaur. But the triplicate form ascribed to them was most likely of optical creation. There is no satisfactory evidence that it exists in nature. So far as we can judge, the spiral type originated, by fundamental necessity, through a double outflow, in contrary directions, from the parent mass. A mode of genesis is intimated which recalls, though distantly, the diametrically opposed eruptions not uncommonly witnessed on the sun. It may be added that no genuine spiral appears to be a simple watch-spring coil. This, to be sure, is, to some extent, a matter of definition. It depends upon what we agree to call a spiral nebula. Yet the difference will most likely prove to be radical between stars with curving trains, like Maia in the Pleiades, and those cosmic “whirlpools,” every trait of which testifies to the counterplay of multiple activities.

“Cometary nebulæ” are not very rare, and they present aspects of considerable variety. The nuclei are not always stellar, nor are the appendages attached to them in all cases inflected. A few have been photographed. Thus an object (N.G.C. 1999) 50′ south of ι Orionis was noticed by Lord Rosse as resembling “a comet coiled into a ring nebula,”[894] and appeared under the same form on a plate exposed by Dr. Common with his three-foot (now the Crossley) reflector in February 1883.[895] Its spectrum has not, that we are aware of, been examined. “Reaping-hook” shapes also occur. West of the Argo nebula, a falcated and forked tail, 10′ long, was observed by Sir John Herschel to issue from a granulated, perhaps a double nucleus[896] (N.G.C. 3199). The inner edge is sharp, but it fades gradually outward. Cometary, too, is N.G.C. 520. It has an indistinct nucleus and a bifid train.[897] A nebulous hyperbola with a star near the vertex (N.G.C. 2366) is met with in Camelopardalis;[898] and fan-shaped appendages to stellar condensations are a recognised variety of the species. A pair of these singular objects were photographed by Professor Barnard, 2nd February 1894,[899] in the immediate neighbourhood of the bright-line star γ Cassiopeiæ. His sketch, showing their positions with regard to it, is copied in Plate XXIII. Fig. 1. These do not seem to be casual. The opening out of the two fans straight away from the star suggests an express plan of orientation. Each nebula is about 15′ in diameter. They are “excessively faint and dilute,” and almost elude visual observation. In Professor Barnard’s opinion, they would never have been detected otherwise than by chemical means.[900] Yet they photograph easily enough; and if this actinic quality denotes, as seems probable, a gaseous constitution, mention of them should, properly speaking, be postponed to a later chapter. Their spectral classification, however, is likely, for some time to come, to remain matter of conjecture. Dr. Roberts obtained, with ninety minutes’ exposure on 25th October 1895, an excellent photograph of the twin fan nebulæ, in which traces of a luminous connection are apparent.[901]

PLATE XXII.

Photograph of Whirlpool Nebula (M 51). Taken by Mr. W. E. Wilson, 6th
March 1897.
]

“Rifted” nebulæ must be classed as a variety of the elliptical sort. They appear either as cloven discs—N.G.C. 5128 in Centaur is an example—or as parallel rays, such as a bifid streak in Leo (N.G.C. 3628). A probably analogous structure in Andromeda was photographed by Dr. Roberts in 1891.[902] It came out immensely elongated, and with just such a “chink in the middle” as had been seen by Sir John Herschel, whose opinion that the nebula was the foreshortened representative of a thin, flat ring of enormous dimensions thus received strong confirmation. Yet the added light-power of the Parsonstown mirror had extended Herschel’s “chink” into a channel, running from end to end of the formation. Or rather the retinal impression afforded by it had been misinterpreted in this sense; for that it was a misinterpretation the camera incontrovertibly asserts. The error may serve as an illustration of Professor Keeler’s remark that “the most obvious tendency of the draughtsman is to prolong a line or curve beyond the point at which it actually stops.”[903] A suspicion even arises that other telescopic presentments of rays split throughout their length are similarly misleading, and that they are, in fact, like the glimmering oval in Andromeda, rings thrown into perspective. Otherwise why should the dark rifts always coincide in direction with the major axes of such formations? If these are really circular discs, they might as well run across as along them; but they never do. We must then choose between two inferences. Either the nebulæ are in fact, and not merely by optical projection, elliptical, in which case a longitudinal line of cleavage would be intelligible, or they are luminous rings viewed very obliquely. It must indeed be admitted that the strong development of nuclear condensations in some rifted nebulæ appears almost to exclude the latter alternative. Dr. Roberts’s photographs, for instance, of N.G.C. 4565 in Coma Berenices, and of N.G.C. 4594 in Virgo,[904] exhibit an arrangement of parts insistently demanding a different explanation. What seems certain is that no single principle is valid all round. Modifications must be introduced to meet the exigencies of nature’s endless variety.

The great majority of white nebulæ might be called globular clusters in disguise. They present a round surface, condensed centrally by gradations testifying to their true spherical form. The only obvious distinction between them and “balls of stars” is that they are irresolvable by any telescopic powers that can be brought to bear upon them. And the suggestion lies close at hand that this quality depends wholly upon distance—that round nebulæ are neither more nor less than remote globular clusters. Yet it cannot be adopted without hesitation. The space-relations of the two classes of object are very different. Clusters frequent the Milky Way; white nebulæ avoid it. The discrepancy, it is true, may be capable of reconcilement, but by a somewhat elaborate artifice of speculation. Nor is there any immediate prospect of solving the difficulty by the aid of the spectroscope. We are unacquainted at present with any criterion for distinguishing continuous nebular light from that of compressed clusters. One may eventually be found, but its application must always be a matter of extreme delicacy.

PLATE XXIII.

1. Fan Nebulæ near γ Cassiopeiæ (Barnard).
2. Drawing of Struve’s Planetary with Spectrum (Keeler).
3. Drawing of Webb’s Planetary with Spectrum (Keeler).
4. Drawing of Annulated Planetary in Andromeda with Spectrum (Keeler).
]

Professor Max Wolf’s explorations of the heavens show them to be strewn with an incredible number of small faint nebulæ.[905] Directly visible only by elusive glimpses, they come out individually distinct and measurable on sensitive plates; and the Heidelberg observer has already laid his plans for the construction of a photographic catalogue of nebulæ, likely to be at least twenty times more voluminous than the most exhaustive visual enumeration. In certain regions he indeed found that only two per cent of the dim objects delineated on his plates had been previously recorded. The newly discovered crowd vary greatly in shape. Some are round and compact; many more are round and diffuse; there are spirals among them, and spindles, and draped or arched formations. A surprising number are marked “planetary,” and are hence, presumably, gaseous. The physical nature of the rest is inferable only when the definiteness of their shapes prescribe their arrangement in some established category.

Thus we are able to assert confidently that those disposed along coiling lines or projected into rays and ellipses, give continuous light; but if we attempt to go further, and obtain a clear conception as to how the light originated, embarrassments beset our path. Let us confront them fairly. The only white nebula of which the spectrum has been observed to any purpose is the great elliptical spiral in Andromeda. It almost certainly includes lines or bands of absorption; it is probably marked by traits of emission as well. Plausibility is thus lent to the opinion that the nebula is a genuine cluster of stars amalgamated by distance into a soft haze. The haze, however, shines very dimly; its lustre is almost evanescent comparatively to that of the sun. If, then, its component particles are true suns, they must be inordinately far apart. For the sake of giving some precision to our ideas on the subject, we will attempt to illustrate this numerically. If we assume the central parts of the nebula to possess ¹⁄₂₀₀th the intrinsic lustre of the full moon, or (what comes to the same) ¹⁄₁₂₄ millionth that of the sun, while consisting of scattered globes of solar brilliancy, it follows that this also is the proportion between the total bright area covered by their discs and the dark area of vacancy, the dimness of the nebula measuring the spread of the interspace. Hence the component stars, taking each to be half a million of miles in diameter, should be separated from its next neighbour by an interval of more than 5000 millions of miles, as seen projected upon a plane perpendicular to our line of vision. Their real distances, since they are presented to us slantwise, would of course be very much greater; but with them we are not just at present concerned. We must next try to form an estimate of how close together these sun-like bodies should appear to be in order to produce the observed effect of a smooth luminous surface. If the gaps amounted to one-tenth of a second, the nebula would certainly, with the powerful and perfect telescopes now in use, show symptoms of resolvability. Yet none appear. The fog does not even tend to condense into droplets, and the temporary star of 1885 stood out, to the last hour of its visibility, by contrasted light-quality from the soft surrounding glow. Allowing, then, that the linear intervals of 5000 millions of miles between the constituent bodies of the Andromeda nebula are represented by optical intervals of ¹⁄₂₀th of a second, we arrive at a parallax for that vast structure of less than ¹⁄₁₀₀₀″. In other words, its rays spend about 3300 years in travelling to the earth. At this distance, the stars we have supposed aggregated in it would appear of fifteenth magnitude. Now they should in the fainter outlying parts of the nebula be more sparsely distributed than near the centre, and as they thinned off they would inevitably appear in their proper guise as fifteenth-magnitude stars. But the texture of the glimmering haze remains the same in every stage of attenuation.

It may then be taken as certain that, if this object be of stellar constitution, it is made up of stars smaller and closer together than we have supposed; unless we are prepared to lengthen still further, and very materially, a light-journey already protracted to the verge of the incredible. It is not, however, easy to conceive that bodies much less than half a million miles in diameter can be truly sun-like. An outpouring of light and heat in the profuse measure exemplified by the sun, implies storage-accommodation on a colossal scale; and the spectrum of the Andromeda nebula, so far as it can be deciphered, seems to correspond to a high standard of temperature.

Undoubtedly the path “of least resistance” is to accept the stellar origin of nebular radiance. It is not entirely practicable, but every other is impassable. The solar corona presents no real analogy to white nebulæ, since it is kept incandescent by the potent agency of the sun, while their glow is self-sustaining. This it can only be—setting aside the vague possibility of electrical discharges—by the sacrifice of motion in some form. According to Sir Norman Lockyer’s well-known hypothesis, the collisions of swarming meteorites supply the evolved energy; but there is little or no evidence that the cause acts, or would be adequate if it did act. We, at least, have no experience of its operation. The only meteoric collisions we know of are with the earth, which spreads a wide net for the capture of flying cosmic particles.

As an alternative suggestion it may be worth considering whether the shining of nebulæ might proceed from a very slow loss of circulatory speed through the resistance of a gaseous medium. A pulverulent constitution, resembling that of Saturn’s rings, should then be attributed to them; they would consist of relatively small masses interfused with some highly subtle aerial remnant, the distinctive bright lines of which add complexity to the nebular spectrum. But the velocity of circulation in such structures should increase outward. Other things being equal, they should accordingly, if arrested motion were the source of their luminosity, gain brightness with increasing distance from the centre. The reverse is very markedly the case; but the attendant conditions are so intricate that the contradiction need not be fatal to the speculation. It cannot, however, be usefully discussed apart from a profound study of the dynamical condition of such a peculiar system as that just indicated; and this we must leave to more competent authorities.