Also on the strength of photometric enumerations, Dr. Gould, of Boston, came to the conclusion, in 1879, that there is an extra thronging of stars about our sun, which forms one of a special group consisting of some four or five hundred members. The publication, in 1890, of the “Draper Catalogue,” of 10,530 photographed stellar spectra, has thrown fresh light on this interesting subject. Mr. Monck, of Dublin, gave reasons for holding stars physically like the sun to be generally nearer to us than stars of the Sirian class; and Professor Kapteyn, of Gröningen, as the result of a singularly able investigation, concluded with much probability that the sun belongs to a strongly condensed group of mostly “solar” stars, nearly concentric with the galaxy. It might, in fact, be said that we live in a globular cluster, since our native star-collection should appear from a very great distance under that distinctive form.
This modern quasi-discovery was anticipated by Herschel. He was avowedly indebted, it is true, to Michell’s “admirable idea” of the stars being divided into separate groups; but Michell did not trouble himself about the means of its possible verification, and Herschel did. He always looked round to see if there were not some touchstone of fact within reach.
His discussion of the solar cluster, though brief and incidental, is not without present interest. He found the federative arrangement of the stars to be “every day more confirmed by observation.” The “flying synods of worlds” formed by them must gravitate one towards another as if concentrated at their several centres of gravity. Accordingly, “a star, or sun, such as ours, may have a proper motion within its own system of stars, while the whole may have another proper motion totally different in quantity and direction.” We may thus, he continued, “arrive in process of time, at a knowledge of all the real, complicated motions of the planet we inhabit; of the solar system to which it belongs; and even of the sidereal system of which the sun may possibly be a member.” He proceeded to explain how stars, making part of the solar cluster, might be discriminated from those exterior to it; the former showing the perspective influence only of the sun’s translation among themselves, while the latter would be affected besides by a “still remoter parallax”—a secular drift, compounded of the proper motion of the sun within its cluster, and of its cluster relatively to other clusters.
The possibility of applying Herschel’s test is now fully recognised. Each fresh determination of the solar apex is scrutinised for symptoms of the higher “systematical parallax;” although as yet with dubious or negative results. Associated stellar groups are, nevertheless, met with in various parts of the sky. Herschel not only anticipated their existence, but suggested “a concurrence of proper motions” as the fittest means for identifying them.
His anticipation has been realised by Mr. Proctor’s detection of “star-drift.” Several stars in the Plough thus form a squadron sailing the same course; and similar combinations, on an apparently smaller scale, have been pieced together in various constellations. But the principle of their connection has yet to be discovered. They are evidently not self-centred systems; hence their companionship, however prolonged, must finally terminate. The only pronounced cluster with a common proper motion is the Pleiades; and its drift seems to be merely of a perspective nature—a reflection of the sun’s advance.
Bessel said of Herschel that “he aimed at acquiring knowledge, not of the motions, but of the constitution of the heavenly bodies, and of the structure of the sidereal edifice.” This, however, is a defective appreciation. He made, indeed, no meridian observations, and computed no planetary or cometary perturbations; yet if there ever was an astronomer who instinctively “looked before and after,” it was he. Could he have attained to a complete knowledge of the architecture of the heavens, as they stood at a given moment, it would not have satisfied him. To interpret the past and future by the present was his constant aim; from his “retired situation” on the earth, he watched with awe the grand procession of the sum of things defile through endless ages. He could not observe what was without at the same time seeking to divine what had been, and to forecast what was to come.
His nebular theory is now accepted almost as a matter of course. The spectroscope has lent it powerful support by proving the de facto existence of the “lucid medium,” postulated by him as a logical necessity. This was done August 1st, 1864, when Dr. Huggins derived from a planetary nebula in Draco a spectrum characteristic of a gaseous body, because consisting of bright lines. Their wave-lengths, which turned out to be identical for all objects of the kind, with one or two possible exceptions, indicated a composition out of hydrogen mixed with certain unfamiliar aeriform substances. Herschel’s visual discrimination of gaseous nebulæ was highly felicitous. Modern science agrees with him in pronouncing the Orion nebula, as well as others of the irregular class, planetaries, diffused nebulosities, and the “atmospheres” of “cloudy stars,” to be masses of “shining fluid.” As for his “ambiguous objects,” they remain ambiguous still. “Clusters in disguise” through enormous distance, give apparently the same quality of light with irresolvable nebulæ. His inference that stars and nebulæ form mixed systems has, moreover, been amply confirmed. No one now denies their significant affinity, and very few their genetic relationship.
Herschel gave a list in 1811 of fifty-two dim, indefinite nebulosities, covering in the aggregate 152 square degrees. “But this,” he added, “gives us by no means the real limits” of the luminous appearance; “while the depth corresponding to its superficial extent may be far beyond the reach of our telescopes;” so “that the abundance of nebulous matter diffused through such an expansion of the heavens must exceed all imagination.”
“The prophetic spirit of these remarks,” Professor Barnard comments, “is being every day made more evident through the revelations of photography.” He is himself one of the very few who have telescopically verified any part of these suggestive observations.
“I am familiar,” he wrote in Knowledge, January, 1892, “with a number of regions in the heavens where vast diffusions of nebulous matter are situated. One of these, in a singularly blank region, lies some five or six degrees north-west of Antares, and covers many square degrees. Another lies north of the Pleiades, between the cluster and the Milky Way; a portion of this has recently been successfully photographed by Dr. Archenhold. There is a large nebulous spot in that region, easily visible to the naked eye, which I have seen for many years. When sweeping there with a low power, the whole region between the Pleiades and the Milky Way is perceived to be nebulous. These great areas of nebulosity make their presence known by a singular dulling of the ordinarily black sky, as if a thin veil of dust intervened.” They “are specially suitable for the photographic plate, and it is only by such means that they can be at all satisfactorily located.”