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.
[Illustration:
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.”