Somewhat inferior to Omega Centauri in size, though not at all in beauty, is 47 Toucani. So obvious is it to the naked eye that, for several nights after his arrival in Peru, Humboldt took it for a comet. Central condensation in this cluster appeared to Herschel as if marked off into three distinct stages; and to his delighted perception the whole interior offered, by its roseate hue, an exquisite contrast to the silvery radiance of the outer portions. No other observer has, however, noticed this chromatic peculiarity. The structure of 47 Toucani is almost perfectly uniform. It is broken by none of the “dark lanes,” rifts, or tunnels which so curiously diversify many globular clusters. The usual hirsute aspect lent by the spreading abroad of tentacles, or radiating stellar streams, is likewise scarcely distinguishable either in 47 Toucani or Omega Centauri. Indeed, Mr. Bailey noticed that the photographic images of both were all but perfectly circular. In a future age this may be otherwise. Streams of stars will, perhaps, set outward from these grand assemblages, leaving vacancies behind. Thus, if it be permissible to judge of the relative antiquity of clusters by their advance towards disruption, 47 Toucani and Omega Centauri may be reckoned among the youngest of the globular kind existing in the heavens.
The mechanism of clusters has received little attention from any astronomer beside Herschel. And a solution of an ideal case of the problem it presented was the utmost he could achieve.
“A quiescent spherical form,” he wrote in 1833, “may subsist as the bounding outline of an immense number of equal stars, uniformly distributed through its extent. In such a state of things each star might describe an ellipse in any plane, and in any direction in that plane, about the common centre without the possibility of collision. If the form be not spherical, and the distribution of the stars not homogeneous, the dynamical relations become too complicated to be distinctly apprehended.”
But the more closely these aggregations are examined, the less likely does it seem that they in any sense represent “quiescent forms.” The arrangement of the stars composing them rather suggests their being outward bound into the ocean of surrounding space, although the orders that they carry are to us sealed.
Herschel subsequently altered his views regarding the composition of clusters, and threw out in 1847 “the possibility of masses of luminous matter—of whatever density or rarity, of whatever bulk or minuteness—forming a connected system, and being prevented from collapse or from mutual interference by the resistance of a transparent and non-luminous medium.” For a “dynamical” he, in short, substituted a “statical equilibrium,” the interposed medium lending unity to the mixed aggregate, and enabling it to rotate, as a whole, upon an axis. But the rotation is more than questionable. It seems to be precluded by the ragged contours and indeterminate boundaries of all starry collections. Photographic evidence, on the other hand, favours Sir John Herschel’s surmise as to the composite nature of clusters. Some at least evidently unite within themselves the “two sidereal principles.” The stellar points they mainly consist of are immersed in, or linked together by, shining nebulous stuff.
Herschel provided a southern sequel to his father’s star-gauging work by counting 70,000 stars in 2,300 fields. Their distribution was in complete accordance with the results of the earlier experiments. “Nothing can be more striking,” Sir John wrote, “than the gradual, but rapid increase of density on either side of the Milky Way as we approach its course.” The existence of an “ecliptic of the stars” (in Lambert’s almost prophetic phrase) was demonstrated. Or, as Herschel himself put it, the plane of the Galaxy “is to sidereal, what the ecliptic is to planetary astronomy, a plane of ultimate reference, the ground-plan of the sidereal system.” He estimated, from the basis of his gauge-reckonings, that his twenty-foot reflector was capable of showing, in both hemispheres, about five and a half million stars. The smallest of these would be of 14·5 magnitude, on the strict photometric scale. But, unless his valuation was greatly too small, there must be a conspicuous falling off in stellar density beyond the region of tenth or eleventh magnitude. If this be so, scarcely one-quarter of the expected stars will make their appearance on the plates of the International Survey.
The grand feature of southern celestial scenery is the splendour of the Milky Way. One of the galactic condensations in Sagittarius actually seems to start out from the sky in a definite globular form; and the darkness of the great rift beginning near the Cross is so intensified by contrast with the strongly luminous branches it separates, as to throw the blackness of the exterior heavens into the shade. This part of the Milky Way may even be seen in southern latitudes—as it was by the present writer—reflected from a glassy ocean-surface. The section passing from Centaur through the Ship to Orion is, in some respects, still more striking. Captain Jacob remarked at Madras that “the general blaze from this portion of the sky is such as to render a person immediately aware of its having risen above the horizon, though he should not be at the time looking at the heavens.” Herschel commented on the singular interruptions of the shining zone by obscure spaces in Scorpio, near Alpha Centauri, and elsewhere; and admired the enhancement afforded to its magnificence by “a marvellous fringe of stars” attached pretty regularly to its southern border. “It is impossible,” he wrote to Sir William Hamilton in June, 1836, “to resist the conviction that the Milky Way is not a stratum, but a ring.”
His telescopic analysis disclosed in it a variety and complexity of structure for which he was wholly unprepared. “Great cirrous masses and streaks” of galactic light presented themselves in Sagittarius; and, as the telescope moves, the appearance is that of clouds passing in a scud.” “The Milky Way,” he continued, “is like sand, not strewn evenly as with a sieve, but as if flung down by handfuls, and both hands at once, leaving dark intervals, and all consisting of stars of the lowest magnitudes,” down to nebulosity, in a most astonishing manner.” As he proceeded, the stars became “inconceivably numerous and minute. There must be millions on millions, and all most unequally massed together; yet they nowhere run to nuclei, or clusters much brighter in the middle.”
In some regions, the formation proved unfathomable; all traces of stellar texture disappeared. In others, it was plainly perceived to consist of portions differing exceedingly in distance, but brought by projection into nearly the same visual line. Near the Trifid Nebula, “we see foreshortened,” he said, “a vast and illimitable area scattered over with discontinuous masses and aggregates of stars, in the manner of the cumuli of a mackerel-sky, rather than of a stratum of regular thickness and homogeneous formation.”
These varied observations compelled him to reject decisively Olbers’s hypothesis of light-extinction in space. For, if the possible range of ethereal messages be restricted in one direction, it must be equally restricted in all. “We are not at liberty,” he reasoned, “to argue that in one part of the circumference of the galaxy our view is limited by this sort of cosmical veil which extinguishes the smaller magnitudes, cuts off the nebulous light of distant masses, and closes our view in impenetrable darkness; while, at another, we are compelled, by the clearest evidence telescopes can afford, to believe that star-strewn spaces lie open, exhausting their powers and stretching out beyond their utmost reach.” These objections seem fatal to what we may call the “agnostic” theory of the sidereal world—the theory that investigations into its construction are for ever barred by failure of the means of communication—that we can never see more than a necessarily meaningless part of a possibly infinite, and, in any case, absolutely inscrutable whole.