Chapter XLI: The Physics of the Milky Way

The Milky Way is an integral part of the great sidereal system. It marks the equatorial girdle of a sphere containing stars and nebulæ variously scattered and aggregated. The whole material creation is, to our apprehension, enclosed within this sphere. We know nothing of what may lie beyond. Thought may wander into the void, but observation cannot follow. And where its faithful escort halts, positive science comes to a standstill. Fully recognising the illimitable possibilities of Omnipotence, we have no choice but to confine our researches within the bounds of the visible world. That it _has_ bounds is evident from the consideration that it possesses shape and parts. Indefinitely extended, star-filled space could have neither. Hence it offers to the human mind an intelligible problem—a problem perhaps too intricate for definitive solution, yet coming well within range of attack. The siege operations may be protracted through many a campaign; but in conducting them we shall climb from peak to peak of the Alpine chain of truth, and gain continually wider views of the majestic scene that encompasses and enchants us.

The structural relations of the cosmos may evidently be looked at from many sides; our immediate concern is with but one of its aspects. We have only to consider the nature of the materials used for the building of the edifice and the plan of their apportionment to its different sections. These materials consist of gaseous and white nebulæ in all their varieties; of star clusters, globular and irregular, and of the sundry species of stars; and even a cursory inspection shows that they are not piled together at random. Each class, on the contrary, obeys its own law of distribution; and the distribution of sidereal, as of animal species, is the outcome of their history, a test of their longevity, an index to their nature. They are _where_ they are, because they are _what_ they are.

There seemed, twenty years ago, very little reason to anticipate that the photographic method could ever be used to advantage for investigating the physics of the Milky Way; yet it has, especially in Professor Barnard’s hands, proved most effective in that difficult branch of inquiry. The requisites were peculiar. The galactic drifts are made up of very small stars—usually of fourteenth to sixteenth magnitude, or even fainter,—and the sensitive plate perceives them, not, as the human eye does, collectively, merged into a nebulous surface, but one by one as light-points. Hence their rays need to be powerfully concentrated in order to make any chemical effect with a moderately long exposure. Moreover, the field must be large enough to show the colossal forms into which these stellar units are grouped. For each picture a canvas of at least 100 square degrees must be available. Hence telescopes of the ordinary type, however powerful, are, for this purpose, entirely useless. An ordinary village photographer’s apparatus would be better adapted to it. Professor Barnard obtained his remarkable series of delineations with the “Willard lens,” a doublet of six inches aperture and thirty-one focal length, constructed at New York in 1859.[1179] They extend over a large part of the Milky Way, but leave lacunæ, the filling up of which should be the diploma-performance of the new Bruce lens. It might indeed be supposed that a few specimen sky-scapes in galactic regions would suffice to afford practical acquaintance with the entire round; but this is very far from being the case. No feature of the Milky Way is more surprising than its inexhaustible variety. No “law of condensation,” such as prevails in globular clusters, is there traceable. Each section follows its own method of aggregation. In one, cloud-forms are met with of the cirrus type; in another, they recall breaking waves or tossing spray;[1180] again, groups of irregular bright spots alternate with extensive ramifications and rifts; here the starry fabric is coarse-grained, there of microscopic fineness; while for heterogeneous scattering in many quarters, there is substituted in others an apparently designed arrangement of the stars into rings, chains, and ellipses.

Barnard’s photograph in Ophiuchus, reproduced in Plate XXXI., has characteristics demanding earnest attention. The bright star below the middle of the plate encircled by a halation ring is θ Ophiuchi, an object spectrally akin to Bellatrix, radiantly white and marked by helium absorption. A “long, dull vacancy,” visible to the naked eye, stretching east and west below (south of) Theta, comes out as “an irregular rift in the sheeting of stars,” of highly complex relations. It circuits the east as well as the north and south sides of the bright mass in which the star is placed, breaks up to the north into scattered dark openings, and straggles on to the western edge of the plate, whence—as other photographs of the series show—it communicates with some of “the great vacant lanes” in the “wonderful nebulous region about ρ Ophiuchi.”[1181] The most singular feature, however, of the galactic chasms visible in our figure is the presence in them of two gradations of obscurity. Darker details can be made out on the dark background, recalling the analogy (which did not escape Professor Barnard’s notice) of the “black holes” in the umbræ of active sun-spots. The conviction was thus, he added, almost enforced that the Milky Way rests here upon a far-reaching stratum of nebulous, or quasi-nebulous matter.

PLATE XXXI.

Photograph of the Milky Way in Ophiuchus (Barnard).
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The reality, the frequency, and the determinateness of the black openings in the Milky Way, dimly seen with the naked eye, constitute one of the most important facts regarding the nature of that formation attested by the camera. It is true that their great exemplar in the southern heavens—the “Coalsack” in Crux—seemed, in a photograph taken by Dr. Russell of Sydney, 13th August 1890, to be embroidered with small stars over three-fourths of its area;[1182] but this is an unessential trait of the phenomenon. The fundamental circumstance connected with it is the extensive and complete perforation of the dense galactic stratum lying behind the starry network; and of this stratum there is no trace in the Sydney picture. Only the foreground of the scene is delineated in it; the backward stretching ranges remained inaccessible with the means employed. The true Coalsack, then, has not yet been photographed, and we are ignorant of the precise form which it will take upon the sensitive plate. The exposures made at Lick, however, effectively reached the piles of luminous dust which form the ultimate reality of the Milky Way, and brought into view with insistent clearness the pits and furrows of almost absolute darkness by which they are frequently interrupted. One is perpetually reminded, in looking at these autographic records, of Herschel’s exclamation of amazement when his telescope plunged suddenly into an unfathomable abyss in Scorpio: _Da ist wahrhaftig ein Loch im Himmel!_

Now “holes” of the kind have a very wide cosmical significance. The great rift, by which the entire galactic structure is divided throughout one-third of its circumference, is only a magnification of innumerable cracks and fissures yawning amid its component star-masses. And the dark lanes in globular clusters are plainly of kindred origin. Moreover, the riddled aspect of the Milky Way in certain of its sections is continually reproduced both in nebulæ and clusters—in nebulæ, whether gaseous or stellar, as well as in diverse species of clusters. The fact is a general one, that in all the forests of the universe there are glades and clearings. How they come to be thus diversified we cannot pretend to say; but we can see that the peculiarity is structural—that it is an outcome of the fundamental laws governing the distribution of cosmic matter. Hence the futility of trying to explain it as of incidental origin, as a consequence, for instance, of the stoppage of light by the interposition of obscure bodies, or aggregations of bodies, invisibly thronging space. That dark stars exist, singly and in systems, and dark nebulæ no less, we have been almost inevitably led to conclude; but the galactic clefts cannot be reckoned among their manifestations. They are, on the contrary, what they appear to be, intervals of starless space between neighbouring star-clouds, and suggest processes of disintegration[1183] advancing with inconceivable slowness towards unimagined issues. These wonderful collections are then in a state of flux; they are passing from one condition to another; Supreme Power is at work in dispersing or refashioning them, sending abroad their aggregated suns like flying sparks from the anvil.

But are those sparks indeed suns on the scale of our own? The answer to be given depends upon our estimate of their average distance. They are undoubtedly very remote. It may be taken as certain that they lie beyond the sphere occupied by the Durchmusterung stars—that is, by the ordinarily distributed stars of our system down to 9·5 magnitude. How much farther beyond, we are, however, unable to say. The thronging orbs of the Galaxy are by no means of uniform brightness. A recent inquirer[1184] concludes that they range from the sixth to the sixteenth magnitude; and the statement, which probably falls short of the truth, implies that stars enormously disparate in apparent lustre are at sensibly the same distance from ourselves; hence, that the crowd of small or medium-sized bodies collected in the Milky Way are dominated by veritable giants, more sparingly distributed. Professor Barnard endeavours to bring this state of things into conformity with received standards by levelling down the dimensions of galactic constituents.[1185] To the ruck of them he assigns scarcely more than planetary rank, so as to escape the necessity of admitting fabulous magnitudes for their leaders. But in truth we have no means of fixing a scale for such valuations. We know that there are suns hundreds, even thousands, of times larger than the sun. Why should there not be others larger still in a similar proportion? _Non est naturæ mensura hominis electio_, Kepler wrote to Herwart in 1599. Our estimates are, in many cases, minimum values; we can place no upper limit to the vastness of the orbs of space. It is only safe to affirm that the greatest and the least of them are associated and conjoined in the immense aggregations of “this gorgeous arch, with golden worlds inlaid, built with Divine ambition.”

The sidereal tribes are very differently affected towards the central plane of the system. Some, as it were, deliberately withdraw from it; others are exclusive in their preference for it; many press towards it, while maintaining a cosmopolitan status. That white stars largely preponderate in the Milky Way, is a fact made evident by Kapteyn’s discussion of the Cape Durchmusterung photographs.[1186] It might, nevertheless, as Mr. Monck of Dublin first noted, be due to their superior areal brilliancy, which would cause them, with increase of distance, to come into view preferentially, and at last exclusively. The percentage of white stars must, indeed, apart from very unlikely specialties of distribution, augment among the lower magnitudes, since a star emitting light of the Sirian quality would appear equally bright when fully twice as far off as one of the same size clothed with a solar atmosphere. Moreover, Kapteyn’s researches did not extend below the tenth magnitude, and the genuine galactic particles are of much lower grades of brightness. So that the nature of their spectra has not, up to the present, been disclosed to us. Yet a surmise regarding it may be hazarded. Mr. McClean perceived the bright helium stars included in his survey to be disproportionately numerous in the Milky Way zone. This is probably one among several symptoms of their great remoteness. They are sufficiently removed, perhaps, from the centre of the sidereal sphere to come within the sweep of the current of tendency setting in towards its equatorial regions. How it seems probable that fainter specimens of the type are similarly, but more strongly condensed; and a further step on the tempting road of conjecture leads us to the inference that the dim aggregations girdling the heavens are mainly composed of stars of the Orion family—of stars reversing the duplicate series of hydrogen lines, besides those of oxygen, helium, nitrogen, and silicon.

We have seen in an earlier chapter that the Milky Way is the chosen resort of Wolf-Rayet stars. They doubtless belong to it intimately and entirely; and the same may be said of Novæ. This involves the strange consequence that, amid the radiant galactic hordes, there must circulate a multitude of large obscure bodies, fitted on occasion to blaze into sudden conflagration, on a scale startling to intelligent beholders in every quarter of the universe. Must we then conclude that dark stars are relatively more plentiful in the Milky Way than elsewhere? Not necessarily, perhaps, for its scarcely infringed monopoly in the production of temporary stars might be explained equally well by the virtual limitation to it of the conditions needed for luminous explosions, as by the abundance in it of their appropriate fuel.

Very few gaseous nebulæ have any considerable galactic latitude; they are characteristically Milky Way objects. There are, indeed, exceptions. Some noted planetaries—those in Draco and Ursa Major for example—are situated far outside the zone of concentration, and they present the appearance of being nearer to the earth than most members of the class. The “stellar” variety, on the other hand, which are presumably small through remoteness, are, to the best of our knowledge, limited to the Milky Way; while irregular nebulæ occur either in the main stream or in some of its affluents. Among these the Magellanic Clouds are, in a sense, to be counted, though they should rather be described as pools, left behind as the waters contracted into their present bed. They are composed, like the Milky Way, of mixed ingredients, stellar and nebulous; they seem to reproduce its condition; they bear the same primitive stamp. Their globular shape, however, suggests an autonomous constitution, each being probably a self-regulated body; while the galactic aggregations may be supposed exempt from the efficient control of a central authority. Dr Russell’s photographs disclosed spiral tendencies in the Nubeculæ,[1187] destined perhaps to become more and more pronounced with time.

Through the agency of the portrait-lens and the sensitive plate, the nebulous affinities of the Milky Way have been more fully recognised than was possible by visual means alone. Several of Professor Barnard’s pictures exhibit an intermixture of vaguely diffused lucid matter with layers of minute stars, especially in parts of Cygnus, Cepheus, Perseus, Monoceros, and Scorpio. We see, then, that the great cosmic zone is not only frequented by nebulous objects, but is nebulous in itself. Yet it seems to repel from it the multitude of white nebulæ which tend to collect about its poles. This, at least, is the law of their visual distribution; but the camera threatens to abrogate it. Dr. Max Wolf holds that the results of his preliminary photographic surveys prove nebulæ to be in reality scattered pretty evenly over the heavens. Only their average brightness, he thinks, varies, the so-called “nebular regions” in Cetus and Virgo having acquired their reputation as such not because nebulæ are more numerous, but because they are there more conspicuous than in the intervening celestial tracts. The subject, in fact, of nebular distribution, which had been supposed practically disposed of, has been, by photographic explorations, reopened for fresh discussion.

The crowding of globular clusters upon the Milky Way is unmistakable; and the inwardness of their relation to its condensations is manifested by their avoidance of the vacuous rifts, and their adherence to the stream-lines of luminosity. Hence arises the logical necessity for their radical separation from white nebulæ, to which they seem, in most respects, so near akin that one might be led to believe mere difference of distance to occasion the distinction between resolvable and irresolvable objects. Only the opposite galactic proclivities of the two classes decisively place them apart.

Within sight of that ultimate problem, the structure of the sidereal universe, we pause. Our thoughts meet, but they cannot grapple with it; nor does it come within the scope of our present purpose to make the attempt. We must be content to register the marks of growth and change legible in the Milky Way; to note the evidence of its comparatively recent origin and inchoate state; to avow our impotence to comprehend the Supreme design which it is directed to realise; and to bend in awe and admiration before the unfathomable depths of difficulty and mystery towards which the study of sidereal development, in its larger bearings, inevitably leads. _Die Schöpfung_, as Kant discerned, _ist niemals vollendet_. _Sie hat zwar einmal angefangen, aber sie wird niemals aufhören._