NEBULOUS CLUSTERS—_Continued._
Many nebulous clusters besides the Pleiades are known, but none in which
the relations of stars and nebulæ are so highly specialised. In general,
the stellar collection seems as if independently organised, and plunged
as a whole into an ocean of cosmic fog, without any strong tendency on
the part of its members to form individual nebulous attachments or
connections. When, however, the structural details of such formations
come more fully to our acquaintance, they may be found to contain
evidence of a closer association between particular objects of the two
species than can at present be vouched for. Such investigations can be
conducted effectively only by photographic means and with carefully
adapted instruments. For the purpose of bringing out the full extent of
the nebulosity, small portrait-lenses have prerogatives, illustrated
practically by Professor Barnard,[831] theoretically by Professor
Wadsworth.[832] But pictures on a larger scale than those obtained with
them are needed for the disclosure of many topographical minutiæ huddled
together by the strong concentration due to their short focal length.
For securing these, reflectors of powerful light-grasp are
unsurpassable. Thus two lines of photographic inquiry should be made to
converge upon nebulous clusters; one directed towards determining the
limits of the involving nebulosity, the other towards ascertaining its
constructive peculiarities.
To a superficial view the object we are now about to describe seems more
like a cluster and nebula than a nebulous cluster. The nebula is
“Messier 8”; it is just visible to the naked eye, and from the oval
vacancies which interrupt its light, it has received the descriptive
designation of the “Lagoon Nebula.”[833] The cluster—separately
catalogued as N.G.C. 6530—immediately follows it on the same parallel,
but not in complete detachment. The two formations unmistakably overlap,
and, to discerning vision, inextricably intermingle. Professor Barnard’s
negatives, taken with the Willard lens in 1892, showed the compound
object to be “a singular mixture of stars and nebulosity. East and west
its diameter is about 45′, and north and south some 42′. The southern
side is sharply defined and serrated, with three distinct pointed
projections. From its north-following corner a wisp of nebulosity
extends nearly to a group of nebulous stars, and possibly with a longer
exposure would be found to connect with them.”[834] A picture on so
small a scale could teach little regarding internal structure; the
“lagoons,” in fact, appear in it nearly _silted up_ with diffused
nebulosity. Many of the brighter stars are involved in the pervasive
haze, and one in particular occupies too critical a position at the edge
of “a very black hole” for the supposition of a mere chance arrangement
to be permissible. It may be doubted, however, whether the 5·7 magnitude
star, 9 Sagittarii, has more than an optical connection with the
cluster, and the multitude of stellar points glittering in the
background will almost certainly be separated from it by the slow
discrimination of drifting movement. A preliminary step towards applying
the test was taken by M. Comas Solà in 1898.[835] His photographic
triangulation of the group fixes a starting-point for comparative
inquiries which can yield tangible results only in the distant future.
His plates showed the stars to be doubly implicated with nebulosity. One
of the seventh magnitude acts as the focus to a cloudy mass, distinct,
apparently, from a diffuse, elongated structure projected upon the
central parts of the cluster, which it not impossibly encloses in
annular folds.[836] The Lagoon Nebula gives a spectrum of bright lines.
It will be interesting to learn whether they are displaced by motion,
and whether, if so, corresponding spectral shifts in the clustered stars
ratify the presumption of organic relationship between the two orders of
formation.
The optical history of nebulous objects is often curious and
instructive. As an open cluster, N.G.C. 2239, in Monoceros, was first
observed by Sir John Herschel. An adjacent patch of nebulosity, seen by
Swift in 1865,[837] again attracted Barnard’s attention in 1883. It
proved to be a kind of knot on a great nebulous ring discerned by the
latter in its entirety with the Lick twelve-inch refractor in 1889.[838]
This encloses the star group like a ring-fence, and may be said to
osculate with a second great filmy ellipse, of which only a section is
perceptible. Then came the turn of the camera. Professor Barnard
obtained a photograph of the complex arrangement 9th January 1894, which
confirmed visual impressions while demonstrating their inadequacy. They
were not misleading, but extremely partial. The photographed nebula is,
in Professor Barnard’s words, “about one degree in diameter, and very
irregular in brightness and outline.”[839] It involves the cluster with
unequal condensations, which are especially heavy north of the bright
stars. The nebulous knots, and the section of a large ellipse, fully
depicted in the sketch of 1889, reappeared in the chemical picture. In
this, the only effect of annularity left visible is that a vacant
interior space seems reserved for the conspicuously grouped stars. Their
chief being of the eighth magnitude, the determination of its spectral
character should present no difficulty, and would be of particular value
as a test of nebulous affinity.
The number of alternative titles by which the star catalogued by
Flamsteed as “15 Monocerotis” is known expresses the curious variety of
its claims to distinction. It is variable, multiple, and nebulous. In
the first capacity it is designated “S Monocerotis.” Its fluctuations
from 4·9 to 5·4 magnitude, in a period of 3^d 10^h 38^m, were noted by
Winnecke in 1867. These elements are indeed still somewhat uncertain.
They need verification and revision. The variable is, moreover, the
leading member of a triple combination, enrolled by Struve as “Σ 950.”
With a green companion at 2·8″, and a bluish one at 16·6″, it makes an
exquisite telescopic object, but gives no sign of orbital motion. It
occupies a dominating position in a collection of fifty or sixty stars,
ranging from the eighth to the thirteenth magnitude, one of which is
subject to fits and starts of extensive variation.[840] By virtue then
of its chiefship of a cluster, 15 Monocerotis is registered among nebulæ
as N.G.C. 2264. Nor solely on this account. Both Sir John Herschel and
Lord Rosse suspected it to be nebulous, and Bruno Peter, who measured
forty-five components of the group at Leipzig in 1879–82,[841] found
their leader visibly wrapt in a hazy envelope. But the wide extent of
its nebulous connections was entirely a photographic revelation. A
picture of the region about 15 Monocerotis, taken by Professor Barnard
on 1st February 1894,[842] showed it to be involved in a great nebula
some three degrees in diameter. “It clusters densely,” he says, “about
the groups of stars, and then spreads out in a weak, diffuse light, with
rifts in it, and irregularly terminated along the edges of a vast
vacancy in the Milky Way. The condensation, which is very strong, is not
at 15 Monocerotis, but twelve minutes south-preceding that star, where
it becomes a compact mass with numerous wisps and holes in it.” The
absence of nebulous concentration about the individual stars struck him
forcibly, especially by contrast with the different state of things in
the Pleiades.[843] The nebulosity is free and general; no single member
of the stellar assemblage appropriates a special share, or carves from
it an appendage of its own. Yet it is difficult to doubt that the
apparent association is real and physical.
The Wolf-Rayet spectrum is, broadly speaking, reversed in that of 15
Monocerotis. It includes both the hydrogen series and the upper “blue”
band; metallic lines scarcely appear, but those of oxygen, nitrogen, and
silicon are unlikely to be absent. About the peculiarities of the less
refrangible section nothing is yet known. Here, perhaps, symptoms of
emission will be found; indeed, a bright C might be looked for with
success not only in 15 Monocerotis, but in some one or two of its
principal associates.
An eighth-magnitude star in Auriga was noticed by Auwers about 1860 to
be projected on a hazy disc (N.G.C. 2175). The object is placed
centrally in a group of smaller stars, enveloped in nebulosity strongly
manifested in Professor Barnard’s photographs of 1894.[844] The
combination is quite similar to that just described in Monoceros. The
spectrum of N.G.C. 2175 was examined by Professor Keeler with an
indecisive upshot.[845]
Sir John Herschel observed at the Cape “a very remarkable object” in the
Milky Way, where it crosses the tail of Scorpio. It showed to him under
the guise of “a decided, tolerably defined, semi-nebulous mass, with
abundance of very small stars forming altogether a telescopic Magellanic
cloud. It fills about a field, and has branches and sinuses.”[846] This
miniature Nubecula (N.G.C. 6437) invites photographic delineation. Long
exposures may disclose in it constructive particulars of extreme
interest.
Nebulous clusters are connected, by insensible gradations, with certain
tracts of nebulosity in the Milky Way, which, since they are situated
in, or in the line of sight with a stellar stratum, necessarily appear
more or less densely star-strewn. The stars strewing them do not,
however, collect into groups capable of individualisation; hence they
cannot be termed “nebulous clusters.” These suggest some kind of
organisation; they are more or less isolated and coherent entities, and
are distinguishable as such from layers and beds of stars, however
closely packed. Not that a clear line separates the two kinds of
formation; the multiplicity of the heavens is too great for this to be
possible; but it is well to maintain differences ideally, even though
they be blurred, here and there in the concrete, beyond our perplexed
powers of recognition. Nebulous clusters, on the other hand, shade off,
as their members become fewer, into nebulous groups such as the Orion
Trapezium, and reduce, “in the limit,” to simple nebulous stars. These
will be the subject of a future chapter.