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.
[Illustration:
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.”
[Illustration:
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.