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.
[Illustration:
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.