Chapter XXXVIII: Nondescript Nebulæ

The “dumb-bell” nebula in Vulpecula (M 27 = N.G.C. 6853) might almost be called a hybrid between the planetary and irregular species. Its affinities are indeed so numerous as to embarrass its classification. Imperfectly seen, it appears double, and such it was considered to be by the elder Herschel. What survives of it when the fainter parts are obliterated by dim air or bad definition, suggests the bell-shaped receptacles detached from the neck of an hour-glass. And, in fact, the succession of instances is unbroken, from bi-nuclear planetaries through the “dumb-bell” stage to unmistakable pairs of nebulæ thinly connected by hazy strips. This point has, however, been already referred to. The dumb-bell nebula, moreover, possesses a nuclear star, situated at the narrowest point of the neck; and it is difficult to conceive of the neck as other than a temporary arrangement. Many indications favour the idea that it will, sooner or later, break up and disappear, leaving the star regent over the masses finally disjoined. And it is curious to remember that just such an interjacent star occurs in several compound nebular systems.

A tendency to form a marginal ring is, on the other hand, visible in this nebula. A photograph taken by Mr. W. E. Wilson of Daramona, Ireland, which we are permitted by him to reproduce in Plate XXX., shows an incomplete bright border to be a leading feature in its construction. Whether in the future it is destined to become predominant, we can scarcely venture to surmise. The dumb-bell nebula is about 8′ in diameter. The bright framework can be seen in the photograph to be filled out into an elliptical shape by faint supplementary luminosity, which, however, is much stronger in the original negative. Dim protrusions at the ends of the major axis, traceable in Dr. Roberts’s picture,[1122] intimate opposite outflows, similar in cause and character to those suspected to proceed from the ring nebula in Lyra. The texture of the “dumb-bell” is lumpy and irregular. Presumably it is of an ellipsoidal form, and rotates on its transverse axis.

PLATE XXX.

Photograph of the Dumb-Bell Nebula. By W. E. Wilson, F.R.S.
Exposure, 60^m.
]

The spectrum is purely gaseous, and is dominated visually, with more than the usual emphasis, by the green nebulium line, while in the photographic region the ray at λ 3727 was found by Von Gothard to be of quite peculiar strength.[1123] Symptoms of helium-emission should be looked for.

Several objects have been described as miniatures of the Vulpecula dumb-bell, but the likeness may prove, on a more searching examination, to be only skin-deep. Among its supposed analogues are: (1) N.G.C. 1365, situated in Fornax, and described in Sir John Herschel’s _Cape Results_.[1124] It belongs, he says, to the class of “annular nebulæ with centres,” but the ring breaks off, leaving the oval incomplete at its extremities, while the shorter axis terminates with two bright masses, between which lies a “resolvable” nucleus. (2) N.G.C. 5189. In Musca. A “general similitude” to the dumb-bell was noticed by J. Herschel.[1125] (3) N.G.C. 3195. In Chameleon. A drawing is given in the _Cape Results_. (4) N.G.C. 1978. One of the nondescript inmates of the Magellanic Cloud. (5) N.G.C. 6905. A gaseous nebula in Cygnus, with a star in the centre, and four attendant upon it, like satellites. (6) N.G.C. 3226, 3227. A double nebula, near γ Leonis, consisting of two hazy masses subsensibly united by a “neck,” and enclosed in a dim, elliptical sheath. All these various structures may not, indeed, be traversing the same line of development. In some, the annular tendency will perhaps eventually prevail, in some the disruptive. The balance between opposing forces is unlikely to incline everywhere the same way, and upon its poise depends by supreme appointment the special form to be assumed in the course of ages by each individual system.

The “Crab” nebula in Taurus, discovered by Dr. Bevis in 1731, was again noted by Messier, 12th September 1758,[1126] as a “whitish light, elongated like the flame of a taper.” He took it at first for the comet he was in search of; and, to avert future confusion of the kind, adopted the memorable expedient of constructing a catalogue of nebulæ.[1127] The specimen at present in question measures 5′ along its greatest diameter, and about 3′ across. Lord Rosse gave it its distinctive name from the protruding filaments resembling the claws of a crustacean, brought to view by his great reflector.[1128] These have not yet been photographed. On a plate exposed during three hours by Dr. Roberts, 2nd February 1892, there came out an ovate body composed of “dense masses of clouds, with fainter areas between them,” a deep bay on one side being counterbalanced by a projecting limb on the other.[1129] No tentacular appendages were disclosed.

The spectrum of the Crab nebula appears to be rather more strongly continuous than is usual in the gaseous class, to which it unquestionably belongs. For at Harvard College in 1868 the green nebulium ray was recorded, and was suspected to have _less_ refrangible companions.[1130]

Another object of dubious relationships is Messier 77 (N.G.C. 1068), Lord Rosse’s “blue spiral” in Cetus.[1131] The description intimates an anomaly, since true spirals are “white,” and give a continuous spectrum. Now the colour of this object corresponds, as might have been expected, to a gaseous constitution, whether of the normal kind or in certain ways peculiar, remains to be proved. Its form, too, is ambiguous. A drawing published by Lord Rosse portrays a round, faint disc more than 2′ in diameter, upon which are relieved the brighter coils of a definitely separated central mass. In Dr. Roberts’s picture (see Plate XXVIII. Fig. 2), on the other hand, there is no trace of a convoluted structure. It shows “a stellar nucleus with projecting ansæ of dense nebulosity” surrounded by a faint zone, and that again “by a broad nebulous ring studded with strong condensations” like inchoate stars. A different aspect was, however, assumed by the object in a photograph taken by MM. Baillaud and Bourget in 1899 with the thirty-three-inch reflector of the Toulouse Observatory.[1132] The nucleus here again resolved itself into the winding folds seen at Parsonstown, and a paler spiral formation served for its background. Its character continues in many respects enigmatical.

M 77 is copied, in Professor Holden’s opinion, quite accurately by a nebula in the same constellation (N.G.C. 1084). Under the gaze of the camera it may resume the spiral shape obscurely visible to the Parsonstown observers, notwithstanding their final description of it as “a fine oval, with ragged edge and a mottled look,” indented by “a dark bay north of the nucleus.”[1133] Judging by analogy, it should yield a gaseous spectrum; though the circumstance, considering its remoteness from the Milky Way, would be exceptional.

Messier 78 (N.G.C. 2068), in Orion, is a “singular wispy nebula,” 6″ or 7″ across, enclosing a triple star, surmised to vary in light.[1134] An arrangement of the more lucent nebulous tufts along a spiral curve, affirmed visually,[1135] is denied photographically. Dr. Roberts’s plate exhibits a central cumulus, sharply terminated on one side, vaguely diffused on the other, the dense nuclear part being surrounded by dim floccules with wide dark spaces between.[1136] A smaller adjacent nebula (N.G.C. 2071) has a stellar focus, to which appurtenances like “mare’s tails” are attached. The distance from centre to centre of the two nebulæ is about 15′ of arc, and they can be made out to be in faint nebulous connection; but neither, Dr. Roberts expressly states, gives indications of possessing a spiral form. Their spectra, which are probably discontinuous, have scarcely been examined.

That of “a fine, pale-white object”[1137] in Canes Venatici is known to be continuous.[1138] Discovered by Méchain in 1781, M 94 (N.G.C. 4736) is large and bright;[1139] its nucleus, which is granular in texture, and evidently globular in shape, being surrounded by a zone of extremely dim, and that again by a zone of relatively intense luminosity. The outer annulus appears, in a photograph taken by Dr. Roberts,[1140] to be broken up into nearly a dozen irregular star-like condensations. Two abrupt opposite projections from it explain, possibly, the quasi-spiral aspect of the nebula in the Rosse reflector.

A circular object, one minute of arc in diameter, and of a “lucid pale-blue colour,” was met with by the elder Herschel in the constellation Hercules, and ranged in the planetary class[1141] (N.G.C. 6299). D’Arrest described it as a “nebulous disc,” invested with a “nebulous sheath,” so that its passing for a comet in 1819 is easily understood. Nevertheless, Sir William Huggins found it to shine with perfectly continuous light,[1142] and it was thought at Parsonstown, on 9th May 1872, to be resolved into a globular cluster.[1143] It should be added, however, that an undoubted planetary, N.G.C. 2022, was seen with the same instrument under a similar illusory aspect. There is indeed much difficulty in admitting the nebula in Hercules to be a genuine cluster. Its cometary envelope and the azure cast of its rays are almost contradictory of a stellar composition. But until a photograph of its spectrum has been obtained nothing can be definitely asserted on the subject. The quantity of light available for analysis is about equal to that given by a ninth-magnitude star.

A nebula in Virgo (N.G.C. 4900) looked, when ill-seen with the Rosse reflector, something like the Owl planetary in Ursa Major.[1144] A bright patch in the centre seemed to have dark spots on either side, the whole being surrounded by a lucid annulus or coil. Yet on a plate exposed at Crowborough, during three hours, no spiral structure emerged to view, although the nebulous condensations visible in the interior were judged to be of the kind usually found to accompany spirality.[1145] The formation, indeed, looks completely amorphous. It is probably of a non-gaseous nature.

The mutual relationships of many of the nebulæ just described are doubtless very close. Their elucidation offers a tempting and profitable field of research. Specimens in some respects anomalous are often the most instructive to study. Abortive features may be found in them, or half-developed characteristics, isolated from their accustomed surroundings, and thereby laid bare to scrutiny. Advantageous standpoints for comparison and correlation would thus be gained; and where these are effectively practicable, science cannot miss the path of progress.