IRREGULAR STAR CLUSTERS.
Stars associated together into communities are probably subject to
mutual influences of special kinds. With the relative movements produced
in them by the stress or pull of gravity we are not here concerned; they
form an extraordinarily interesting subject for future inquiry, but as
yet no hint of their method is derivable from the scanty materials at
hand. The physics of clusters, however, falls within our scope, and is a
topic more immediately accessible. It is open to discussion chiefly in
two ways—by studying the spectral peculiarities, and the luminous
variability of the component objects.
It is not always easy to distinguish between a casual “sprinkle” and a
true cluster. The Pleiades are the only family of stars which shows a
wandering tendency; the rest are _adscripti glebæ_; they have no common
drift by which they could be set apart from casual inmates of the same
sky-region. There remains the argument from probabilities of
distribution, with its indefinite variations of conclusiveness according
to the circumstances of each particular case. Yet it usually suffices
for conviction. None, at least, of the five hundred registered clusters
present any real ambiguity of character, although many other groups
doubtless subsist unrecognised because poor in numbers and loosely
scattered.
Two varieties of stellar collections can be readily discriminated. One
is characterised by a spherical form; the constituent bright points
press inward towards a centre; they aggregate into “globular clusters.”
Those termed “irregular” appear to be constructed on a different plan.
Very slight traces of interior condensation are perceptible in them;
they are made up of star streams, branches, and spirals, more or less
closely intertwined and commingled. A glittering assemblage in Gemini
(Messier 35) has an obviously radiated structure. Lord Rosse was struck
with wonder at the arrangement into loops and arches of the stars in M
37, a similar object in Auriga. Still more definite and amazing are the
“patterns, consisting of lines, wreaths, and curves of stars,” in Dr.
Roberts’s photograph of a cluster in Cassiopeia (N.G.C. 7789), taken
with an exposure of ninety minutes, 26th November 1892. The effect of a
marshalled array is irresistible.
[Illustration:
PLATE XIX
Photograph of Messier 11. Taken by Dr. Roberts with an exposure of
90^m.
]
Interior vacancies seem correlative to a streaming conformation. They
perhaps represent spots denuded of their bright inhabitants by the
action of some unknown expulsive force. Irregular clusters, at any rate,
are often remarkably perforated or furrowed. Some are rendered, by the
development of “dark lanes,” essentially bifid or trifid. The well-known
star-throng in Antinous (M 11) is broken up by partial clearings in a
manner suggestive of eventual disruption. The breaches in the masonry
(so to speak) are finely shown in a photograph taken by Dr. Roberts,
10th August 1896, reproduced, by his kind permission, in Plate XIX. M.
Fenet, from an earlier Crowborough plate, mapped 395 components of this
cluster, which includes altogether about 1200, and remarked that most
were entitled, by the close attendance of satellite-stars, to be
regarded as forming multiple systems.[800] Their collection into seven
or eight separate allotments was evident to him, and could not, he
thought, fail to become further accentuated with time. All,
nevertheless, yield apparent allegiance to a ninth-magnitude star, which
fully sextuples the brightness of any of its followers. A spectroscopic
examination of this object would be desirable. The general quality of
its light might be readily ascertained, and the detection of
motion-displacements need not be despaired of. A catalogue of two
hundred members of this brilliant assemblage, referred to their leader,
was drawn up in 1870 by F. R. Helmert, and compared with measures
executed by Lamont in 1836–39.[801] The agreement was complete within
the limits of probable error; no discrepancies betrayed shiftings of
relative position, although sure provision was made against their
unnoticed occurrence in the future.
Helmert considered several of the stars observed by him to be slightly
variable; but their changes made no show on the Arequipa plates of the
cluster. Since, however, they were taken at an interval of a few days,
only variables of short periods or rapid vicissitudes could have been
disclosed, and none such, it is safe to say, are present. Yet there is
some probability that a temporary member was added to the group upwards
of sixty years ago. On the 12th of August 1839, Lamont entered No. 9 on
his list as a “new star”; he had not perceived it before, and on the
ensuing 9th of September it was gone. Helmert did not expressly look for
it, but could only have missed seeing it through its extreme
faintness.[802] Indeed, it was most likely, by that time, as hopelessly
extinct as Nova Andromedæ now is. The stars of M 11, although devoid of
nebulous attachments, are shown in one of Professor Barnard’s
small-scale photographs to form a knot at the margin of one of the great
cloudy formations in the Milky Way, their actual nuclear relation to
which he regards as “hardly questionable.”[803] This may be; the
supposition is plausible; yet it is a long way from being demonstrable.
Meanwhile the practical inquiries to be made in connection with the
cluster are these two: What is the spectrum of its leader star? and, Do
any of its components vary in light?
The lovely double cluster in Perseus (N.G.C. 869, 884) resembles it in
being non-nebulous, perhaps also in possessing galactic affinities. The
twin groups are known respectively as _h_ and χ Persei. Their connection
is remote; dynamically they can scarcely be mutually dependent, but they
are of closely analogous construction, and their components are
similarly linked into festoons and spirals.[804] None of them can be
perceived to drift, absolutely or relatively. This was put to the test
in 1884, when 172 stars in “χ Persei” were photographically determined
by O. Lohse for confrontation with the results of Vogel’s micrometrical
measures of them fourteen years earlier.[805] Their seeming immobility
will probably be maintained for many decades yet to come. Vogel’s
special catalogue of the thirty brightest among them (all above the
tenth magnitude) might, nevertheless, usefully be revised, from the
photometric point of view, for the purpose of detecting possible
alterations of brilliancy. The task would be the more hopeful that some
of the objects in question have a note of colour. A “ruby” star was
allotted a central position in χ Persei by Sir John Herschel;[806] the
Parsonstown reflectors displayed rosy, yellow, and bluish tints in many
of its sparkling associates; and Mr. Espin, much more recently, located
in the outskirts of the cluster eight reddish stars with fluted
spectra.[807] These may be expected to prove more or less variable,
though not in the prompt and definite fashion prevalent in globular
assemblages.
Professor Barnard recognises two varieties of irregular clusters—the
purely stellar and the nebulously stellar.[808] They can be
distinguished with certainty only by chemical means. Long photographic
exposures are needed to test satisfactorily the condition of grouped
stars. The characteristic nebulosity of many clusters may in fact be
counted one of the most important discoveries made with the assistance
of the camera. But at present we are dealing with the non-nebulous kind,
such as the Hyades and Præsepe in Cancer.
Aldebaran, the great red “eye of the Bull,” is in, but not of the
Hyades. The disconnection will be rendered obvious, and the relative
drift more precisely definable, when the radial movements of the several
stars can be spectroscopically fixed. This, indeed, is already feasible,
were it not that the great telescopes of the world are otherwise
occupied.
Forty-five stars in Præsepe have been located with rigid accuracy.
Winnecke’s observations of them in 1858, Asaph Hall’s in 1870, above
all, Schur’s Catalogue for 1875,[809] laboriously constructed from a
triangulation with the Göttingen heliometer, ensure the detection of
their future movements. They are extremely minute. In the course of
thirty-two years they produced effects so small as to be barely
determinable. The spectra of ninety members of this stellar family were
photographed in or about the year 1896 at Harvard College.[810] Owing to
their faintness—they ranged from 6·5 to 9·5 magnitude—the details of
their classification remained uncertain; but the one main fact was
brought out that the collection is of mixed quality. It does not seem to
be expressly assorted in any way; no spectral pattern can be called
predominant. Twenty-eight of the ninety associates were recorded as
Sirian, sixty-one as solar or intermediate stars, and the third type was
represented by a solitary specimen; that is to say, the percentage of
first-type spectra in Præsepe is thirty-one, while it rises to
sixty-five in the Pleiades, and sinks to fifteen in Coma Berenices. In
this last asterism, however, the stars, although crowded, are not
_clustered_.[811] Yet their crowding obtains significance through
Professor Pickering’s notice of their almost exclusively solar
character.
A spectrographic survey, carried out at Harvard College, of four
southern clusters of the coarse-grained or irregular description, has
lifted another corner of the veil from this department of astrophysics.
One of these objects, still uncatalogued, is situated in the
neighbourhood of η Carinæ; a second (N.G.C. 3523) is not far off; the
third and fourth (N.G.C. 6405 and 6475 = M 6, M 7) are found in Scorpio.
The plates exposed showed in the aggregate 705 spectra capable of
characterisation, of which 576 were unmistakably of the first type.[812]
The average proportion, then, of hydrogen stars in these four clusters
is 82 per cent. Half a dozen helium stars were identified in the
anonymous group, none in the rest. On the whole, spectral uniformity may
be considered the ideal state towards which most clusters tend; it would
perhaps, if accidental components could be eliminated, prove to be more
nearly realised than it seems. In this connection it is of interest to
note that M 37 in Auriga consists wholly of yellow stars, presumably
belonging to the solar class. The sky, in Admiral Smyth’s phrase,
appears in that spot as if strewn with gold dust. The companion cluster
in Gemini is, on the other hand, resplendently white; and Dunlop
registered at Paramatta a bluish globular cluster (N.G.C. 6723) likely
to be packed with Sirian stars.
The jewel-cluster about κ Crucis is differently organised. Harmonies of
contrast rather than of consonance may here be observed; but the
collected brilliants display their various tints effectively only in the
fields of large telescopes. From measures of 130 of them, Mr. Russell of
Sydney derived in 1872 ostensible evidence of comparatively rapid
interstitial movements during the thirty-five years elapsed since the
date of Sir John Herschel’s corresponding work; but until a fresh set of
determinations gives assurance that they are pursued systematically,
little weight can be laid upon discrepancies otherwise explicable. His
suspicions of variability in twenty-five components have not so far been
verified. Several are bright enough to show distinctive spectra, the
nature of which it would be particularly interesting to ascertain. This
beautiful object lies near the northern border of the “Coal Sack.”
Irregular clusters obviously form systems of extreme intricacy. They
cannot be pieces of mechanism set in action by some uniformly operating
motive power. Their aspect is in most cases irreconcilable with the
hypothesis of a dynamical equilibrium. Few, if any, betray by movement
or conformation the influence of a preponderating centre of attraction.
They rather suggest inconceivably complex aggregations of partial
systems bound together loosely nor perhaps indissolubly. The
investigation of their mutual relations will tax the resources of the
old as well as of the new astronomy. Some of the problems to be
confronted have just begun to take shape, others loom on a remote
horizon. As a prelude to dealing with them, the separation might
gradually be effected of the really physical from the merely optical
components of clusters. The process will be greatly facilitated by the
ready help of the camera; and the slow evolution of telescopic or
tangential displacements can already in part be forestalled by the
spectroscopic disclosure of radial velocities.