Chapter XXVII: 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.

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.