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.
[Illustration:
FIG. 48.—Drawing of Spindle Nebulæ (Spitaler).
]
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.