Chapter XXX: Globular Clusters

There is no possibility of failing to recognise in a globular cluster a true agglomeration—a structure _teres atque rotundus_. The systemic unity of such objects is as evident as that of a “globe of dew,” though it is by no means certain that they are not, like that “frail and fading sphere,” in course of more or less speedy evaporation. A gradual process of ejection is at least suggested by their streaming edges and filamentous appendages, formed of branching rows of stars, apparently on the move outward. One hundred and ten globular clusters were registered by Sir John Herschel in 1864 in his “General Catalogue” of nebulæ, and not many have since been identified. They are astonishing constructions. Their silvery radiance is a delight to the eye; the imagination is allured by their visionary beauty; reason is startled by the recondite nature of the problems they intimate. What, we cannot but ask ourselves, is the true nature of these mysterious “balls of stars”?[847] Are the luminous particles composing them suns in the proper sense? What are their mutual relations? How did they originate? In what are they to eventuate? Can mechanical stability be claimed for them, or must they be supposed to form temporary societies undermined by forces tending towards dissolution? On all these points definite information is still lacking; but there is no reason to despair of its future provision, since the inclusion of globular clusters within the scope of organised research is of quite recent date, and knowledge respecting them is accordingly in a nascent stage.

It may, however, safely be affirmed that their components are sun-like bodies—that they are spherical masses at an enormously high temperature, radiating into space by means of suitably adapted photospheric apparatus. Some are intensely actinic. They are much brighter chemically than visually. They hence presumably emit light mainly of the shorter wave-lengths, and are abnormally hot bodies. But the secret of their nature cannot be divined in our present ignorance of their spectroscopic peculiarities, and those of the individual star-points in clusters will long remain inaccessible. Their combined light, nevertheless, where it is concentrated into a “blaze” at the core, is capable of being effectually analysed with powerful instruments, and the determination of its quality is a _sine quâ non_ for progress in this branch. Until this has been effected, there is no possibility of assigning to globular clusters their proper place in the celestial hierarchy.

The great southern agglomerations, ω Centauri and 47 Toucani, seem almost untouched by the wear and tear of time. They show few signs of dilapidation. No dusky rifts, no glades or clearings are perceptible in them; the subtraction of material (if it be going on) has made little progress; they are as yet well compacted to the centre. Nor is a flow of stars outward hinted at unless obscurely. They are _cleaner_ at the edges than most objects of their class; tentacular appendages are wanting; their components are not visibly in marching order. This may mean that they have but newly arrived at their present state of being, and if so, their spectra should be of an early type. To the component stars, accordingly, a very low mean density may probably be attributed; their attractive power will prove small relatively to their light; the corresponding interstitial movements must be slow, and will be difficult of detection.

FIG. 47.—Photograph of ω Centauri (Bailey).
]

Just within the northern border of the Milky Way ω Centauri is visible to the naked eye as a hazy star of the fourth magnitude.[848] Telescopically it presents a grand aspect. Nothing more strikingly effective can be imagined than the transformation, by optical means, of a blurred light-spot into a glittering and multitudinous assemblage of separate suns. Nearly 6400 can be distinguished on sensitive plates, besides a crowd of others so small as to merge together into a grey mottling. No true nebulosity seems to be present. The photograph from which the counts were made by Professor and Mrs. Bailey is reproduced in Fig. 47. It was taken at Arequipa 19th May 1893 with an exposure of two hours in the thirteen-inch Boyden refractor. The area marked out for investigation covered 900 square minutes. It was “fairly well filled” with stars, and their statistical study defined the fact of their true central crowding.[849] They condense not merely in appearance, through the augmenting depth of spherical space in which they are distributed, but also as a consequence of actual compression inward. The number of stars per square minute was found, in fact, to increase in arithmetical progression with approach to the centre. Away from it the diminution ceased, a constant figure being reached along a line taken to be the boundary of the cluster; and this constant, since it must represent the areal population of the general sky, supplied a means of correcting the apparent results for that of the cluster. Deduction was accordingly made of 1616 unconnected stars, mere visual intruders from a limitless background; and there remained 5050 true components, collected within a circular space somewhat larger than that occupied by the full moon. They average about 12·5 magnitude.

The south polar cluster, 47 Toucani, is of equal loveliness with ω Centauri, although on a smaller scale. Its “computed diameter” is 22′[850]—that is to say, the _extra_ stars which it projects upon the sphere die out completely at a distance of 11′ from its centre. They number about 2300, and are distributed in accordance with the same law noted as prevalent in the companion cluster. They are, however, even more densely aggregated; the realm is less spacious than ω Centauri proportionately to the throng of its inhabitants.

The great cluster in Hercules (M 13) presents a less uniform texture than its prototypes in the southern sky. Yet its constituent orbs follow virtually the same gradient of central compression.[851] Their distribution is affected besides by influences of an unimaginable kind. Three “dark lanes,” making an equiangular junction at a point south-east of the centre, were detected with the Rosse reflector in 1850.[852] An identical form of marking tends to recur in other parts of the cluster. On the Lick photographs of 1890–91, Professor Holden was able to trace no less than thirteen repetitions of it.[853] This insistence, he remarked in a local paper, made it evident “that a definite law was acting to produce this form, and that this law might be truly taken as representative for this cluster. In some way there are dark lanes produced and maintained among the hundreds of bright stars in this globular mass, and there are _many_ such channels. How can we conceive of such a system? It is tolerably clear that either the dark lanes are absolutely empty of matter, or at least that they are empty of luminous matter.” Yet neither of these alternatives seems to be in accord with fact.

A plate exposed during ten minutes with the Crossley reflector by Professor Keeler showed all the brighter stars in M 13.[854] Two hours were, however, needed to bring the swarms of their faint associates into view. In all, more than 5400 stars, fairly within the precincts of the cluster, were counted on a negative taken 13th July 1899. A study of their distribution, made by Mr. Palmer, Professor Keeler’s assistant, elicited some noteworthy peculiarities.[855] The components separated, speaking broadly, into two distinct orders of brightness, those of intermediate magnitudes being comparatively scarce. Out of the total number of 5482 counted on the plate, 1016 were classed as bright, 4466 as faint, or below 13·5 magnitude. Now the mode of scattering of these two radial—they extend outward in curved rows; that of their minute companions is more nearly globular. Moreover, the characteristic dusky tracks are vacant only as regards the former class of objects. They are lightly strewn with the diffusive star powder found everywhere in the cluster. To its presence Mr. Palmer attributes the effects of nebulosity noticed in earlier photographs. It requires a very high resolving power to distinguish the stellar haze created by flocks and throngs of sixteenth-magnitude stars from genuine “fluid haze”; but in this case there seems little doubt that the feat was performed. Confirmatory evidence of the best kind was afforded by Professor Barnard’s direct observations with the forty-inch Yerkes refractor. They convinced him that globular clusters are non-nebulous formations.[856] Spectrographic impressions will probably before long add their testimony, at least in a negative sense.

With the thirteen-inch photographic refractor of the Potsdam Observatory, Dr. Scheiner obtained, 9th September 1891, the first plate of M 13 on which the stars were sufficiently defined for exact measurement.[857] He accordingly prepared a catalogue by which the places of 833 were fixed with the utmost accuracy; and these fundamental stars, henceforward kept under watch and ward, will perhaps one day disclose the plan of their movements, and thus enable future astronomers to attack, with some possibility of success, one of the most arduous problems in celestial dynamics. It confronts them, under a still more bewildering form, in a superb cluster in Scorpio (M 62 = N.G.C. 6266). Here a second focus of condensation is obvious; two star-globes are fused into one.[858] The contorted growth of a twin cherry may help us to realise, however imperfectly, the attendant indefinite complexity of the conflicting forces.

A compressed cluster in Serpens (M 5 = N.G.C. 5904), discovered by Kirch in 1702, presents telescopically the appearance of a softly radiant globe with divergent outliers. A photograph taken by Dr. Roberts 25th April 1892, indicated a nebulous interior; but no such effect came out on negatives exposed with Dr. Common’s five-foot reflector, nor was it perceptible visually to Professor Barnard. It was due, presumably, to an amalgam of faint stars forming a kind of matrix for those bright enough to be individualised. The discovery of their variability was begun by Mr. Packer in 1890.[859] He remarked the fluctuations of two components, and Dr. Common suspected many more to share the same character.[860] This premonition was followed up, though not until after five years, by Professor Bailey’s announcement[861] that many globular clusters—say one in five—are veritable nests of variables. Their abundance is such that as many as a hundred—in Professor Barnard’s words—“have been found in a space in the sky that would be covered by a pin’s-head held at the distance of distinct vision.”[862] Of 3000 components of ω Centauri examined within a radius of 22′, 128 were found to fluctuate to the extent of half a magnitude or more.[863] In one case a range of five magnitudes was observed; but that of most of the objects investigated was limited to one and a half, or two magnitudes. Very short periods are the rule; three are of less than seven hours; yet one is protracted to 475 days, and there will be a special interest in determining the nature of the light-change comprised in so long a cycle. A true “Mira variable” would seem an anomaly in a cluster made up of silvery white stars; since we are taught by experience to associate periods of many months with strong absorption and consequent redness of colour. But cluster-variables belong, for the most part, to a type apart, the character of which has been described in an earlier chapter. They have long minima, and brief maxima attained with extraordinary rapidity. The activity of their changes, when they set in, contrasts singularly with the completeness of their suspension during the intervals of rest. There is an entire absence of concert among the affected stars. Each is an independent, self-regulated phenomenon. No more curious spectacle is afforded by the heavens than that of a throng of seeming signal-lights waxing and waning every few hours under the sway, obviously, of some common law, yet with no trace of unanimity; some fading while their neighbours are on the rise, others stationary and semi-extinct, though only biding their time to enter upon a phase of renewed brilliancy; and none deviating by a hair’s-breadth from the course of change individually prescribed for it.

Eighty-five variable stars have up to the present been recognised in Messier 5, and they agree, for the most part, quite closely in a mode of fluctuation elucidated by Professor Bailey’s persevering inquiries.[864] The form of their typical light-curve is not different from that assignable to the components of ω Centauri; but they tend unmistakably to obey a common period of approximately twelve hours, and the oscillations of nearly all are between the fourteenth and the fifteenth magnitudes.[865] Yet they are not executed simultaneously; a congruity of epochs is not even distantly indicated. Professor Barnard took visual charge in 1898 of some half a dozen of these strange objects,[866] and his list included Packer’s original variables, Nos. 42 and 84 of the Harvard enumeration, stars exceptional in the great assemblage of which they form part, both as to the length of their periods and the manner of their change. This copies the pattern set by δ Cephei; it proceeds continuously, although not symmetrically, along a curve steep in its upward branch, sloping gradually downward, and interrupted by a “hump,” significant of an abortive second maximum.[867] Its time-measure is about twenty-six days. Professor Barnard was struck with a number of ink-black holes rending the brilliant surface of the star-globe in Serpens. They are closely adjacent to the dense central portion, and suggest tunnelling operations on the scale of those progressing in the great Hercules cluster.

A cluster in Canes Venatici (M 3 = N.G.C. 5272) is similarly perforated. Lord Rosse observed “several small dark holes” at its core, from which “rays run out on every side.”[868] A “bifurcated dark lane” was, moreover, perceived in the northern segment of the nuclear “blaze.” From which we can gather that the distribution of stars in M 3 is controlled by forces of the same nature as those ruling in M 13. Yet the two clusters are markedly differentiated as regards light-stability. That in Canes is already known to contain 132 variables; while in the Hercules group diligent inquiry has failed to certify the presence of more than two, its components actually shining much more steadily than the average of the stellar multitude outside its limits. Eighteen hundred stars were counted by M. Orbinsky from photographs of M 3 taken at Pulkowa in 1894,[869] and his measurement of their places will in the future supply a test of their relative mobility.

A starry sphere in Pegasus (M 15 = N.G.C. 7078) seemed nebulous in Dr. Roberts’s photographs;[870] but they were perhaps clouded by stellar dust, not by true cosmical fog. Of 900 members of this collection examined at Harvard College, 51 proved variable. The proportion in Ihle’s great cluster in Sagittarius (M 22 = N.G.C. 4424) is much smaller—16 to 1550; and only 10 among 600 stars tested for stability in M 2 gave responsive signs of fluctuation. This cluster, which is situated in Aquarius, might be the twin of that in Hercules plunged in a deeper depth of space.[871] The clusters ω Centauri and 47 Toucani, so much alike in other respects, deviate widely in the matter of variability. Periodical stars by the score occur, as we have seen, in the former stately assemblage; in the latter, only six have been registered, notwithstanding the most careful scrutiny. Whence the diversity? Professor Pickering[872] surmises that it depends upon the relation of a common plane of revolution to the line of sight. Each globular cluster would be, on this view, a system, the movements of which, whether axial or orbital, are conducted on the same level. And should this level happen to coincide with the visual ray, variability would result, either through the rotation of such components as possessed unequally luminous surfaces, or as a consequence of the eclipses of those provided with closely revolving satellites, Yet neither rationale of light-change can, without grave misgivings, be admitted. Suns with dusky hemispheres, or permanently spotted, may be treated as mathematical fictions. Nor is any evidence as yet forthcoming that genuine eclipse-stars ever find a habitat in clusters. Certainly none of the cluster-variables so far investigated can be accounted such.[873] That they are rapid binaries may be plausibly surmised, but they must be of the non-occulting sort. In eclipsing stars the maximum is essentially permanent; the minimum is accidental. In cluster-variables opposite conditions prevail. Habitually obscure, they brighten incidentally.

Professor Bailey’s discovery throws open a spacious field of research. Each variable cluster might well claim a sentinel appointed for the exclusive following of the complex, elusive, and rapid changes which ceaselessly develop within its compass. In December 1901, 509 components of star-globes were reckoned as periodical. Every one of these is perhaps a system apart; every one has its peculiarities, the inner meaning of which can only be drawn out by sustained attention. Powerful instruments are, moreover, required. The objects in question lie near the limit of practicable observation; work upon them taxes modern resources to the utmost. Nor can the photographic method alone be relied on. Long exposures are needed to show the stars at all, and they can naturally give no more than the “mean magnitude” during the intervals they cover. But when these intervals bear a large proportion to the entire period of change, such coarse-grained data cannot satisfactorily represent the manner of its progress. The resulting light-curve, as Professor Bailey says,[874] is always smoothed down; and it is smoothed to the limit of a straight line, in the ultimate case of the exposure equalling the period of a star’s variation. Hence the absolute necessity for supplementary visual determinations to fill out the peaks and corners rounded off by the camera. At the critical epoch, when the flash is being turned on, every minute counts. Estimates of brightness at the rate of ten or twelve an hour are not too numerous for the purpose of keeping guard over the swift alterations going forward. The hourly or two-hourly averages given on sensitive plates are wholly inadequate.

Two questions of fundamental interest present themselves in connection with the variability of clusters. Why, we must ask, are the stars in one globular assemblage luminously unstable, while in others, its strict analogues, they shine quite steadily? The contrast is not explained by any visible difference of constitution. Variability does not appear to come in at any particular stage of growth; it is not associated with a definite situation in the heavens; it does not characterise pierced and outworn globes preferentially to compact ones, or _vice versâ_; it can, in short, be correlated with no feature obvious to direct notice. It remains to be seen whether any spectroscopic peculiarity corresponds to it.

Again, the absence from, or extreme scarcity of Algol-stars in clusters occasions perplexity. We are led to believe that rapid variables are, in truth, binaries revolving in the light-period. But if so, the orbital planes of a certain proportion of them ought to pass through the earth, with the outcome of affording us the spectacle of so many occulting pairs. If these do not exist, we shall be forced to conclude that cluster-variables owe their punctuality to some other cause than the strict time-keeping of satellites.

One of the most signal services rendered by photography to astronomy has been in the facilities supplied by it for the measurement of star-clusters. The relative positions, especially of the components of compressed groups, can only with extreme difficulty be established by direct triangulations; while every negative taken of them fixes their configuration at a given epoch, and gives the means of determining its changes, should they occur. Thus the places of sixty-two stars in M 5, catalogued at Harvard College in 1897, were estimated by Professor Pickering to be of so high a degree of accuracy that annual displacements amounting to one-hundredth part of a second of arc can be detected by their comparison with results similarly obtained from plates taken a few years hence.[875] The foundations have then been laid for an extensive superstructure of knowledge, as regards both the physical and the dynamical condition of globular clusters; yet centuries may elapse before it becomes possible—in Kepler’s phrase—to “think over again,” with apprehensive minds, those wonderful “thoughts of God.”