Chapter XXXVII: Other Irregular Nebulæ

Irregular nebulæ are objects of large size, indeterminate outlines, and capricious shapes. They become fully apparent, as a rule, only by photographic means, their exterior sections and subordinate parts shining too dimly for distinct visual perception. The chemical retina, however, sees them with comparative ease; for their light consists mainly of isolated short-period vibrations. They are hence known to be of gaseous composition, and all are situated in or near the Milky Way. Many observers, especially those armed with large reflectors, can pick them out at sight by their greenish tinge; but they never appear blue like planetaries. There seems, indeed, to be a definite difference of hue between the two classes, approximate spectral identity notwithstanding. This probably indicates superior strength of the nebulium lines in irregular nebulæ; or the maximum of intensity in the dim continuous spectrum may be situated lower in them than in the planetary kind. Discrimination is not easy; for impressions of colour are often too subtle to be analysed.

Irregular nebulæ are of surprising variety. Each specimen has individual peculiarities, for the most part inimitable by any other. No copy of the Orion nebula is to be found in the heavens, and the Argo nebula, which comes next to it in importance, is equally _sui generis_. This magnificent edifice has no corner-stone corresponding to the trapezium; instead, a black opening of a lemniscate form, and as sharp to the eye as if cut with a punching instrument, yawns in its brightest part. The operation by which it came to be produced was, moreover, repeated in a fainter nebulous tract farther south, and was hence evidently controlled by some definite and special combination of circumstances. With reason, then, in view of the unique character of this feature, the formation has been surnamed the “Key-hole Nebula.”

At the eastern edge of the northern key-hole lies the extraordinary variable, η Carinæ, the vicissitudes of which cannot but be related to the tumultuous processes of change doubtless going on in the seething chaos around. The general surface of the nebula is emblazoned besides with a multitude of ordinary small stars, historically and telescopically undistinguished. Their scattering, however, is not at random; it has marks of _intention_. Sir John Herschel pointed out their disposal along the margins of dark rifts in the nebula; and many such allineations were traced by Mr. Ranyard[1098] in Dr. Russell’s photographs, taken at Sydney in 1894. The object as a whole seemed to him “a very fine specimen of a nebulous cluster with a central condensation, associated with dark structures and radiating streams of stars.” These “are in most cases accompanied by narrow black channels in the general nebulosity, which run parallel to, and alongside of the star-streams.” One is reminded of the dark lanes bordered with stars in the Hercules cluster; and the analogy, if genuine, is of no slight significance. Setting aside for the moment its implications of affinity between stellar globes and nebulæ, it would afford a certainty that the stars distributed over the surface of the Argo formation are really in and of it; and since they are obviously galactic, this would amount to a demonstration that the nebula too is galactic—that it belongs to the Milky Way, not geometrically by projection, but physically by collocation.

The first photograph of this fine object was obtained by Dr. Russell with a six-inch portrait-lens in June 1891; but it had an experimental rather than a delineative value. This could not be said of one taken nine months later by Sir David Gill. An exposure of twelve hours, spread over four nights, with the thirteen-inch photo-refractor of the International Survey, yielded the remarkable picture which forms our frontispiece. The advantages of the autographic process could not be more forcibly exemplified than by comparing it with Sir John Herschel’s drawing of the same object.[1099] Months of labour at the telescope were of less avail than half a day’s “following” with the camera. The artist fully recognised the inability of even his skilful hand to delineate the endless gradations of light and shade which his eye perceived. The elaborate pains taken by him tended, indeed, as in most similar cases, to exaggerate contrasts, and so vitiate the general effect. In the photograph, the disclosed nebulous fields are not only wider, but they are more harmoniously related and more intelligibly arranged. Nor has the distinctive trait of the nebula evaded chemical portraiture. The “key-hole” is conspicuous on the plate, although deformed by luminous inflows; Herschel’s “kidney-bean” opening to the south (sixth square from the bottom, fourth from the left side of the Plate) is scarcely encroached upon by chemical diffusion; and a third vacuity of similar design, though less perfect execution, occurs to the north-west of the key-hole (fourth square from the top, seventh from the left). There is, nevertheless, one striking discrepancy between the drawing and the photograph—a discrepancy which, on the face of it, implies the occurrence of genuine and extensive change. An isolated, trident-shaped structure prominent in the former is imperceptible in the latter, or survives, at the most, fragmentarily. Its disappearance was due to no accidental defect in the Cape negative; the Arequipa plates, exposed with the Bruce twenty-four-inch lens, show a corresponding effacement. It had, in fact, taken place even to the eye already in 1871, when Dr. Russell failed to perceive the “swan” form (as Sir David Gill called it) with the Sydney eleven-inch refractor. The evidence of light-extinction is almost conclusive; yet it should not be admitted without further question. Visual study of the nebula would perhaps be the most promising road towards the end in view. Such objects are now rarely _looked at_; observers adapt their apparatus and devote their energies to the exposure of plates. Yet in some cases—and this is surely one of them—the direct and indirect methods should, for completeness, be employed concurrently. Photographic and photometric brightness are commonly disparate in stars; they differ in nebulæ still more widely; it remains to be proved whether their differences may not be irregularly distributed or even variable with time. The spectrum of the Argo nebula is of the usual gaseous type. Further particulars about it are wanting.

The Trifid nebula in Sagittarius (M 20 = N.G.C. 6514) affords another instance of ostensible change. Discovered by Messier in 1764, it appeared to Sir William Herschel in 1784 in the guise of “three nebulæ faintly joined into a triangle. In the middle,” he added, “is a double star.” And again, after two years, “About the double star is a black opening,” the combined effect of which recalled the Orion trapezium. He reiterated in 1811 that the position of the star was “in the middle” of the obscure space between the nebulæ.[1100] Sir John Herschel similarly assigned its place in 1827[1101] as “exactly in the central vacuity of a large irregular nebula, which appears to have been broken up into three portions by three rifts or cracks extending from its centre to its circumference, and whose directions meet at the double star.” Nothing could be more explicit; and his verbal description is authenticated by a rough sketch of high evidential value, though laying no claim to precision. Six years later, at Slough in 1833, he observed the double (really a sextuple) star to occupy “the centre of the trifid nebula.” Yet at the Cape in 1835, he drew it as adhering to the south-eastern lobe, and—stranger still—without comment on the alteration. And virtually under the same aspect the object was seen by the American observers, Mason and Smith, in 1839, as well as by Lassell at Malta in 1862. The complete immersion of the star-group in nebulosity, and its eccentric situation at the apex of a shining conical mass, are now patent to the merest tiro in telescopic scrutiny. Autographic impressions tell the same tale. One obtained by Dr. Roberts in ninety minutes, 13th July 1899, is shown in Plate XXVIII. Fig. 1. The open fan of nebulosity in the south-eastern quadrant has the multiple star at its apex, but indistinguishably, owing to the burnt-up condition of the plate in this bright region. The abruptness with which the luminous masses abut upon the dark rifts that divide them is most remarkable. On a Crossley plate of 6th July 1899,[1102] the small central block of nebulosity came out semi-detached, while in the Crowborough picture it appears as a simple prolongation of the great northern lobe. Here, too, by a further effect of light-concentration, the dependent nebula to the north is completely annexed by the adjacent triple structure. Its nuclear star would make an interesting subject for spectroscopic study.

Now Herschel’s Cape drawing of 1835 is in substantial agreement with the photographs of 1899. He saw about the same extent of nebulosity disclosed in them, distributed very much in the same way, and similarly related to the principal stars scattered through it. During sixty-four years, at any rate, fixity has prevailed. Mutual displacements are not sensibly in progress. The alleged variation must, we are driven to infer, have taken place suddenly between 1833 and 1835. This is certainly hard of credence; but it is still more difficult to admit that both Sir William and Sir John Herschel erred so egregiously as to locate the multiple star in the middle of a black space, if it really sparkled, as it does now, upon a background of lucent silver.

Professor Swift mentions having observed about 1888 “a luminous filament of the most delicate spider-like fineness stretched across the north-west cleft” of the Trifid nebula.[1103] It reminded him of a cable of the New York and Brooklyn suspension bridge, with the difference that it did not sag in the middle, but went straight from shore to shore. The installation of electric street lights at Rochester precluded him from keeping watch over this delicate and perhaps novel feature. It would be interesting to learn whether it continues visibly to span that strange abyss.

PLATE XXVIII.
1. Photograph of the Trifid Nebula. Taken by Dr. Roberts, 13th July
1899.
2. Photograph of Messier 77. Taken by Dr. Roberts, 26th November 1892.
]

The spectrum of this nebula was observed by Professor Keeler in 1890 as “continuous but short, being apparently confined to the blue and green.”[1104] Only a “brightening near the middle” could be detected. Nevertheless, on 3rd August 1894, Professor Campbell perceived at a glance the three usual nebula lines, the third (F) being relatively strong.[1105] An auroral glow almost effaced whatever continuous light was present; and the same accident, singularly enough, recurred at the date of a second observation 24th May 1895. Much might be learned about the nature of the Trifid nebula by a searching spectrographic inquiry. The condition, for instance, of the unknown line at λ 3727 would be important to ascertain. Its variations from one object to another, or possibly from one region to the next of the same object, are doubtless of a significance only to be fathomed by the patient collection and comparison of facts. The spectrum of the multiple star, which seems to have a nuclear relation to the south-eastern division of the nebula, was stated by Keeler to be devoid of marked features. Yet such may present themselves when its thorough examination becomes practicable. The components of the object were successively discovered in the course of nearly a century. Sir William Herschel saw it double in 1784; Herschel and South about 1826 found it to be triple; and no more than three members of the group were distinguished by Lassell with his four-foot reflector. The fourth and fifth stars—neither much brighter than the thirteenth magnitude—were added by Professor Langley, using the fifteen-inch Harvard College refractor, in 1866; finally, the Washington twenty-six-inch disclosed the sixth to Professor Holden 5th August 1875.

A fantastic structure, known as the “Omega” or “Horse-shoe” nebula (M 17 = N.G.C. 6618), is situated on the border of Scutum Sobieski, and, like the Trifid, invites inspection from southern latitudes. Sir John Herschel perceived it at Slough in the figure of a Greek Omega (Ω), with the left-hand baseline turned upward.[1106] He was surprised to see at the Cape a second arch springing from the same level as the first,[1107] besides other suspected convolutions. The subordinate appendage was again noticed by Swift in 1883.[1108] In a photograph taken by Dr. Roberts, 5th August 1893, the “horse-shoe” resemblance is almost obliterated.[1109] Much greater prominence is given to the spindle-shaped axis originally noticed by Messier in 1764. On the plate it is found to be encompassed by a dim envelope, uniting the various patches of nebulosity into a large oval, 18′ by 12′, to the north-western end of which an abortive “horse-shoe” is appended like an excrescence. A picture obtained under more favourable circumstances might bring these somewhat incongruous parts into an intelligible mutual dependence. Professor Holden collected evidence suggestive of variation in the Omega nebula,[1110] but none that could be regarded as conclusive. “There has certainly not been any bodily shifting,” Dr. Dreyer pronounced in 1887.[1111] He was not, however, equally clear that partial fluctuations in brightness might not have taken place. The question remains an open one.

The spectrum of the Omega nebula was recorded as gaseous by Sir William Huggins in 1864. Nothing further is known about it.

Perhaps the most important of the nebulæ for purposes of comparative study is “30 Doradûs.” Situated in the Greater Magellanic Cloud, the “looped” nebula may exercise, in Professor Pickering’s opinion, a dominating influence over that extraordinary mixed assemblage. Yet it has the filmy and unsubstantial appearance of silver filagree torn in shreds and hung in the black sky. Its spectrum offers a remarkable combination of linear elements with strongly continuous radiance. Here, if anywhere, a frontier-instance between “white” and “green” nebulæ is to be found. Burton in 1874 affirmed the predominance in it of the fundamental nebular line; but the Harvard observers are less explicit. Professor Pickering briefly announced in 1892[1112] the spectrum of this object to be “unlike that of other gaseous nebulæ”; adding in 1897[1113] that its “constitution appears to be partly stellar and partly gaseous.” Further, a sixth-magnitude star in Libra (A.G.C. 20,937) is said to reproduce the peculiarities of its mixed light, a discovery ranking among the most profoundly instructive of those made by Mrs. Fleming. Its full import may, however, develop only through prolonged investigation.

Near the star ξ Persei on 3rd November 1885, Barnard discovered, at Nashville with a six-inch telescope, a “very faint, very large, diffused” nebula (N.G.C. 1499).[1114] Six years later it came prominently into notice through a photograph taken at Halensee, near Berlin, by Dr. Archenhold.[1115] The Willard lens was then repeatedly brought to bear upon it at Lick, and one of the resulting pictures, to which Barnard gave six hours’ exposure 21st September 1895, is reproduced in Plate XXIX. The nebulosity extends over at least two degrees, and includes many “angular condensations.” A round dark spot near the northern border strikes the eye at once. That it is “doubtless a hole in the nebula,” Professor Barnard avers. But in a gaseous mass a “hole” could neither be produced nor maintained. Light in the perforated region must be suppressed unless it be intercepted, and the latter alternative involves consequences that may fairly be called inadmissible.

A drawing of the ξ Persei nebula, published by Dr. Scheiner in 1893,[1116] embodies five photographic delineations obtained with a Voigtländer “euryscope” of four inches aperture, in times of exposure varying from one to six hours. He found it to be little inferior in size to the great Orion nebula, but totally different in plan of construction. It has strongly luminous borders, and these are connected by bright causeways crossing a comparatively obscure interior. There is no sign of a nucleus, nor any tendency towards the formation of one. Portrait-lenses, or some modification of them, seem to be the only kind of instrument with which impressions of this object can be secured; a plate exposed during six hours at the focus of the Potsdam thirteen-inch astrographic refractor showed no trace even of veiling from the prolonged impingement of its rays. They are equally ineffective, Dr. Archenhold states, upon orthochromatic plates—a fact reasonably held to imply that the nebula emits chiefly light-waves of short periods. In other words, it is a _green_ nebula, and all but certainly gaseous.

A “vast and magnificent nebula” near Antares, seen imperfectly and fragmentarily, was disclosed in its entirety by Professor Barnard’s photographic researches in 1895.[1117] Its primary gathering-ground is about ρ Ophiuchi, a quadruple star of fourth magnitude giving a helium spectrum; but σ, γ^2, and 22 Scorpii, besides other smaller stars, form subordinate foci. Antares itself is involved in the trailing skirts of this cosmic cloud, but may in reality lie far away from them. Furrowed by an intricate system of rifts, and pierced by obscure cavities, the Antares nebula is evidently in an agitated and unstable condition; and its marked tendency to cling to individual stars suggests that its development will take the direction of accentuating such local condensations. The example of the Pleiades was recalled to Professor Barnard; and it may be that the formation in Scorpio presents us with an analogous aggregation in an earlier stage of growth.

One in many respects similar was photographed in Cepheus by Professor Barnard 13th October 1893. He traced in it “numerous irregular vacancies and zigzag lanes,” and noticed it to “mingle indefinitely with masses of small stars and become part of them.”[1118] This nebula is two degrees in diameter, and rudely circular in shape. Still more far-spreading and complex is a wonderful nebulous maze, vaguely centred at a point near ξ Cygni, but extending outward to a distance of at least eight degrees. Dr. Max Wolf, who virtually discovered this vast formation by his photographs of 1891, considers it to embrace all the stars, bright and faint, that come within its scope;[1119] and we cannot doubt that a heterogeneous system, partly stellar, partly nebular, is here presented to view. But the particularities of its composition evade for the present profitable inquiry.

Barnard’s circular nebula in Monoceros (N.G.C. 2237), which to the eye seems to draw a line of circumvallation round the cluster within, but loses in photographs all trace of annularity, may provisionally be classed as “irregular.” A fine picture taken by Dr. Roberts 5th March 1899[1120] shows the nebulosity to extend over a space about 77′ by 67′, in the form of a cloudy aggregation “broken up into wisps, streamers, and curdling masses, densely dotted with stars,” and including “many dark areas with and without either stars or nebulosity. Some remarkable black tortuous rifts meander through the nebulosity on the north-preceding half of the nebula; their margins are sharp and well defined in the midst of dense nebulosity. They are as clearly cut as we see the cañons of great rivers, but their width may in reality be millions of miles.”

PLATE XXIX.

Photograph of a Nebula in Perseus (N.G.C. 1499). By E. E. Barnard.
Exposure, 6^h.
]

Irregular, too, is a beautiful winged formation distantly resembling the Orion nebula, photographed by Schaeberle in the vicinity of Nova Aurigæ.[1121] The physical investigation of all these objects will prove an arduous but interesting task. The measurement of their radial movements, especially, should help to define ideas regarding their true status in the heavens.