Chapter XXXII: Double Nebulæ

“Double nebulæ,” Dr. See wrote in 1893,[906] “have been greatly neglected since the time of Sir John Herschel, but it is to be hoped that astronomers will again give adequate attention to these remarkable objects, which should be at once systematically studied and photographed. If accurate drawings or photographs of these objects were now made, it is not to be doubted that important changes could be observed fifty years hence.”

His special interest in them originated from a research into the “evolution of stellar systems.”[907] Sir John Herschel’s drawings of coupled nebulæ illustrated most aptly his theory of the origin by “fission” of double stars, some appearing actually modelled upon Poincaré’s “apioid”—the figure assumed by an ellipsoid when becoming unstable under the stress of increased axial rotation, and about to break up into unequal masses. Without entering into details regarding the process, it may be explained that disparity in the products of disruption indicates want of homogeneity in the parent body, so that the sooner the components separate, the greater the chance of their approximate equality. But the result of photographically investigating the pattern-objects was completely to alter the point of view from which they had to be regarded. “The actual nebulæ,” Professor Keeler stated,[908] “have almost no resemblance to the figures. They are, in fact, spirals sometimes of very beautiful and complex structure, and in any one of the nebulæ the secondary nucleus of Herschel’s figure is either a part of the spiral approaching the main nucleus in brightness, or it cannot be identified with any real part of the object.” There had been premonitions to this piece of “destructive criticism.” Many spirals are readily seen to be essentially duplex. Such is their exemplar in Canes Venatici, the second nucleus of which, separately catalogued by Sir John Herschel,[909] was brought into connection with the first only when the intervening whorl of nebulous matter disclosed itself at Parsonstown in April 1845. In many other cases, the brighter knots which tend to form on curving branches are seen isolated, for lack of light to bring the linking filaments into view, with the outcome of visually decomposing one formation into several. Thus telescopic improvements, which avail to analyse stars, have frequently a synthetic effect upon nebulæ. Even such adjacent objects as are presumably in mutual systemic relation, often show signs of being bound together by organic ties as well. Hence it is difficult to draw a line between single and double nebulæ. A pronounced “dumb-bell” form graduates insensibly into a pair of clearly individualised globes, barely united by a faint ligament. And their condition seems less alien to our ideas when we remember that the sun is nebulously connected with the earth by means of the zodiacal light.

Unification with increase of optical power was exemplified by Burnham’s observation, at the Lick Observatory in 1891,[910] of a nebula as single, though bi-nuclear, which Herschel had registered under two distinct headings (N.G.C. 7174, 7176). The condensations, which possibly offer to our view a double star in the making, are just 26″ apart, and belong to a nebular group in the Southern Fish. A more dubious object is situated in Aquarius (N.G.C. 7287). Detected by Müller at the M‘Cormick Observatory, it was described as an “excessively faint, slightly nebulous double star.” Burnham found the object to be indeed double at an interval of about 20″, yet not stellar, one component, at any rate, and perhaps both, appearing as small dim nebulæ. An authentic example of a double nebula was noted by Barnard in 1888, with the twelve-inch refractor of the Lick Observatory, in the neighbourhood of the wide double star 23 Orionis (Σ 696). The components are 36″ apart, faint and uncondensed. A tenth-magnitude star forms with them an equilateral triangle. Their measurement by Burnham in 1891[911] supplies a datum of first-class importance for the future determination of change in the system which they beyond question constitute.

Littrow described in 1835[912] a curious combination of three small nebulæ marking the angles of a triangle, the sides of which are formed by three nebulous bands, while a fine double star occupies the middle of the enclosure. The arrangement, met with near γ Pegasi, would make a promising subject for a photographic experiment. Close telescopic scrutiny, on the other hand, might advantageously be brought to bear upon a nebulous pair in Gemini (N.G.C. 2371, 2372). The distance from centre to centre of the components is only 32″, and they were seen at Parsonstown to be connected by “tails and filaments,” if not encircled by a filmy annulus.[913] An intermediate star, noted as “bright” 19th December 1848, was observed by Lassell in 1852,[914] and by d’Arrest in 1862, but has of late ceased to attract attention. Can it have lost light? D’Arrest seems to have had no difficulty in seeing it with an eleven-inch refractor, so the question might be readily answered. The preceding member of the pair was remarked by Dr. Dreyer in 1887 to be brighter and more condensed than its companion.[915] A similar nebular and stellar group was discovered by Dr. Common in 1880[916] in the constellation Crater. He regarded the nebulæ as planetaries; in the absence, however, of information concerning their spectra this cannot be held certain. The existence of the linking star—a feature of peculiar interest—has not, we believe, been verified, but need not be doubted.

Fine telescopic seeing avails to resolve, no less than to unify nebulæ. Some split up, like close double stars, under high powers. With a magnification of 250 Professor Swift perceived a nebula (N.G.C. 6679), earlier discovered by himself, to be a well-separated pair, and he obtained a similar result for one of Sir William Herschel’s. “It would,” he adds, “be a great satisfaction to be fully assured that they are binaries.”[917] We fear that the satisfaction is reserved for a future generation of astronomers. The Herschelian nebula in question was doubtless N.G.C. 3690 in Ursa Major, which had already in 1852 been divided with the Rosse reflector into two irregular masses at a distance of about 60″.[918] This is a coarse object compared with two delicate pairs discovered by Swift at Echo Mountain, California, in 1897. Each resembles a “double nebulous Uranus,” the conjoined discs being 5″ or 6″ apart.[919] They are numbered 6 and 27 on his eleventh catalogue; yet, although one seems the replica of the other, they are not near neighbours in the sky. Mixed pairs, stellar and nebulous, are less scarce than one might expect. Swift’s southern explorations yielded nearly a dozen specimens. Two are situated in Argo, near the small round nebula, N.G.C. 3267. Each proved resolvable, in the exquisite Californian air, into a star and nebula at a distance of 4″;[920] and the veteran observer’s concluding list of discoveries included five analogous couples, the widest having a span of 8″. They should at once be micrometrically measured; for until this is done they cannot be said to have started on their career in scientific history.

Double elliptical nebulæ are picked up now and again. They are not easily distinguishable from rifted nebulæ. Probably the true criterion is the duplicity of the nucleus. Rays stretched parallel to the main formation, but exhibiting no trace of independent condensation, can only be regarded as outlying portions of it; where there are two nuclei, there are, _in esse_ or _in posse_, two distinct bodies. As such two lens-shaped objects in Pegasus (N.G.C. 7814), photographed by Dr. Roberts,[921] should probably rank. The “dark lane” shown by the Rosse reflector was perceived in the negative to bisect the globe-like nucleus, and to widen out on either side of it; so that each oval is complete in itself; neither looks like a fragment of the other. A true pair seems also to be constituted by the lenticular nebulæ (N.G.C. 3786, 3788) delineated by Spitaler at Vienna in 1893;[922] while the status of many more cannot be fixed until they have been photographed with a variety of instruments and exposures. Among those of uncertain nature should be reckoned a cloven ray in Leo (N.G.C. 3628), 15′ in extent,[923] and probably annular;[924] a similar object in Centaur (N.G.C. 5128), viewed with amazement by Sir John Herschel; a fissured ellipse in Leo Minor (N.G.C. 2964), thought to be “almost double” at Parsonstown; and a spindle in Draco (N.G.C. 5866), described by Professor Keeler as “divided lengthwise by a narrow, perfectly dark straight rift, on each side of which, near the north-preceding end, and involved in the nebulosity, is a minute star of about the sixteenth magnitude.”[925] A bifid beam in Coma Berenices (N.G.C. 4565), 14′ long, and with a protruding nucleus, appears to be essentially single. Sir John Herschel noticed that the segments of rifted nebulæ are sharp on their confronted sides, diffuse outwardly.[926] They recall the gaping shell of a bivalve; and this peculiarity, if photographically persistent, might serve as a secondary mark of unity. The unity, as already pointed out, may be that of a ring viewed edgewise; or, in some cases, a formation primitively one may have been sundered by disintegration, as a rock-ledge is cut by a mountain torrent. This is, of course, said merely by way of illustration. We are unable to conceive how disintegrating forces in a nebula really act. Nor should the possibility be forgotten that the occurrence of black chasms may indicate, not the removal of matter, but an abolition of light. These apparently breached objects perhaps subsist, after all, in substantial entireness.

Double nebular ellipses do not invariably lie parallel to one another. The Andromeda nebula, for instance, is, in a manner, coupled with N.G.C. 205, the longer axis of which, as has been said, makes an angle of 60° with that of its primary. A pair in Virgo (N.G.C. 4567, 4568) stand in yet more singular relations to each other. They coalesce at their following extremities, and diverge at an angle of about 45°.[927] They might be conceived of—were this mechanically possible—as revolving on a pivot. The combination is essentially reproduced by two spindle nebulæ near the hind foot of the Great Bear (N.G.C. 3786, 3788), which meet almost rectangularly. Dr. Spitaler’s drawing of them is copied in Fig. 48. A third pair, similarly composed, was noted by Swift, 23rd September 1897, at the Lowe Observatory.[928] A bright nebula in Sculptor (N.G.C. 55) was then too seen to have a dim companion. Both are elongated; they meet obliquely and overlap. Possibly indeed they form together a single curved nebula; yet the indications are more in favour of a genuine coupled arrangement.

FIG. 48.—Drawing of Spindle Nebulæ (Spitaler).
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Elliptical nebulæ are sometimes found less congruously associated with round, perhaps globular attendants. Thus an enormously long ray in Canes Venatici (N.G.C. 4631) is preceded by a tenth-magnitude star, and that again by a nebulous orb.[929] Of the ray, it was recorded at Parsonstown, 26th March 1848, that “masses of light appear through it in knots”;[930] and the drawing made there exhibits helical lines corresponding presumably to an extensive system of foreshortened flat spires. The star does not appear to have been seen; nevertheless it ought, unless greatly diminished during the score of years elapsed since Herschel’s observation of it, to have been conspicuous with the six-foot speculum. A spindle-nebula in Eridanus (N.G.C. 1532) has also a round companion;[931] and two are attached to an ellipse depicted by Spitaler in 1893 (N.G.C. 2781, 2785). Triple combinations of round nebulæ are fairly common. The varieties of multiple stars are recapitulated in them. Single primaries have closely double satellites, or single satellites wait upon compound primaries. Three nebulæ, which, from their central brightening, may roughly be described as spherical, were detected by Barnard in 1886 lying close together in the field of his six-inch refractor.[932] Yet they are faint objects even with the Lick thirty-six-inch. The intervals between them were determined by Burnham in 1891 to be respectively 94″ and 78″. A century hence there will be hope of eliciting evidence of incipient revolution by the renewal of these measures. Another of Barnard’s new nebulæ (N.G.C. 6302) was resolved by Swift into a triplet. It is plunged deep in the Milky Way in Scorpio, and has a gaseous spectrum.[933]

Compound nebulæ lead the way to groups and clusters of such objects. Swift counted twelve in a single field near Algol, and perceived at least twenty collected into a slightly larger space between κ and γ Herculis.[934] Barnard explored in 1890 a nest of eighteen small separate nebulæ in Ursa Major; and Max Wolf observed in March 1901 a real “Nebelhaufen” surrounding, though probably disconnected from the star 31 Comæ Berenices.[935] No less than 108 components, some elongated, some roundish and of various degrees of faintness, were found included in a circle 30′ in diameter; and a similar group, photographed in 96 minutes, has η Virginis for its centre.[936] White nebulæ, in fact, tend very markedly to gather into flocks; whether as a consequence of their mode of origin, or through the compulsion of their mutual attraction, remains an open question. And since the masses of these bodies are likely to be very small, and their real distances very great, circulatory movements only of the most leisurely kind can be ascribed to them. We are unable as yet to forecast, even remotely, the establishment on a settled footing of the dynamics of nebular systems.