THE CONSTRUCTION OF THE HEAVENS.
"A Knowledge of the construction of the heavens," Herschel wrote in 1811, "has always been the ultimate object of my observations." All his other investigations—solar, planetary and stellar—were secondary to this great aim. Before the commencement of his career as an astronomer, the stars attracted very little attention. Star-catalogues had been formed, but, nevertheless, the stars were regarded chiefly as convenient reference-points for observations of the Moon and planets. A few double and variable stars and several star-clusters and nebulae had been discovered; theories of stellar distribution had been outlined by one or two obscure non-professional astronomers, such as Wright and Lambert. But there was little interest among astronomers in the study of the stars for their own sakes; and no great systematic effort had been made to discover the laws of stellar distribution and motion.
The field of sidereal astronomy, therefore, was virtually untrodden when, shortly after the beginning of his telescopic work, Herschel began his first review of the heavens. His second review, commenced in 1779, included stars down to the eighth magnitude. By-products of this review were the discovery of Uranus and the formation of his first catalogue of double stars. In December, 1781, he commenced his third review, which he completed in January, 1784, and which resulted in the publication of a second double-star catalogue. The problem which confronted Herschel was two-fold: (1) the scale, and (2) the structure of the stellar system. In 1781 he had written a paper on the parallaxes of the fixed stars, but his investigation only yielded a negative result. Accordingly he concentrated on the question of the structure of the universe and the arrangement of its component parts.
In 1783 the publication of Messier' s first catalogue directed Herschel's attention to the star-clusters and nebulae. As soon as it came into his hands, he says, he applied his 20-feet reflector to the nebulae, "and saw with the greatest pleasure that most of the nebulae which I had an opportunity of examining in proper situations yielded to the force of my light and power and were resolved into stars". Accordingly, Herschel decided to "sweep" the heavens with two main objects in view: (1) to search systematically for new nebulae, and (2) to gauge the extent of the sidereal system by counting the number of stars visible in different regions of the heavens. In 1784, in his preliminary paper on the construction of the heavens, he described his method of star-gauging, which, he said, "consists in repeatedly taking the number of stars in ten fields of view of my reflector very near each other, and by adding their sums and cutting off one decimal on the right, a mean of the contents of the heavens, in all the parts which are thus gauged, is obtained".
In his paper on "The Construction of the Heavens," dated 1st January, 1785, Herschel gave the results of his preliminary investigations and outlined his theory of the stellar system. "That the Milky Way," he said, "is a most extensive stratum of stars of various sizes admits no longer of the least doubt, and that our Sun is actually one of the heavenly bodies belonging to it is evident. I have now viewed and gauged this shining zone in almost every direction, and find it composed of stars whose number, by the account of these gauges, constantly increases and decreases in proportion to its apparent brightness to the naked eye." In the most crowded part of the Galaxy, Herschel occasionally counted as many as 588 stars in a field of view, and in quarter of an hour's time no fewer than 116,000 stars were thus enumerated, while other fields were almost destitute of stars. Herschel made two important assumptions—(1) that the stars were, roughly speaking, of the same size, and (2) that they were scattered throughout space with some approach to uniformity. As a result of his star-gauges, he was enabled on these two assumptions to estimate the possible extent and shape of the sidereal system. He sketched it as a cloven disc of irregular outline, extending much further in the direction of the Milky Way than in that of the galactic poles, the cleft representing the famous division in the Milky Way. The Milky Way was regarded as more or less an optical phenomenon, as a vastly extended portion of the stellar system.
Herschel's gauges led him to the view that the galactic system was strictly limited in extent. "It is true," he said, "that it would not be consistent confidently to affirm that we were on an island unless we had actually found ourselves everywhere bounded by the ocean, and therefore I will go no further than the gauges will authorise; but considering the little depth of the stratum in all those places which have been actually gauged, to which must be added all the intermediate parts that have been viewed and found to be much like the rest, there is but little room to expect a connection between our nebula and any of the neighbouring ones." The stellar system which he designated as "our nebula" was in his view an island universe—"a very extensive branching, compound congeries of many millions of stars". The majority of nebulae and clusters he believed to be independent stellar units. He divided these nebulæ, or milky ways—for at this time the two terms were interchangeable in his vocabulary—into four "forms," our "nebula" being regarded as of the third. In 1785 Herschel informed Miss Burney that he had discovered fifteen hundred universes,—"fifteen hundred whole sidereal systems, some of which might well outvie our Milky Way in grandeur".
Such was the famous "disc-theory " and its corollary—the hypothesis of island universes. For many years this theory was expounded in text-books of astronomy and popular science manuals as if it had been the outcome of Herschel's matured views on the stellar universe. The late R. A. Proctor, who was one of the first close students of Herschel's papers, truly remarked that "It seems to have been supposed that his papers could be treated as we might treat such a work as Sir J. Herschel's 'Outlines of Astronomy'; that extracts might be made from any part of any paper without reference to the position which the paper chanced to occupy in the entire series". The consequence of this method of expounding Herschel's views was that for many years astronomers were hardly aware of his gradual change of opinion.
The disc-theory and its corollary were, as already noted, based on the assumption of an equal scattering of stars in the Milky Way and involved the belief that all nebulæ were stellar clusters which would ultimately be resolved into stars. The first of these views was never held very confidently by Herschel. In his paper of 1785 he admitted that "in all probability there may not be two or three of them in the heavens, whose mutual distance shall be equal to that of any other two given stars, but it should be considered that when we take all the stars collectively there will be a mean distance which may be assumed as the general one". Even in the paper of 1785 Herschel remarked that it would not be difficult to point to two or three hundred gathering clusters in our system. Indeed, his classification of so-called "nebulæ" was based on his view that condensation gave evidence of age. Accordingly, he foresaw, as a result of "clustering power," the breaking-up of the galactic system into many small independent nebulæ. More and more evidences of this "clustering power" came to his notice until in 1802 he said of the Galaxy: "This immense starry aggregation is by no means uniform. The stars of which it is composed are very unequally scattered and show evident marks of clustering together into many separate allotments." He was coming gradually to the view that the fundamental assumption underlying his disc-theory that of an average equality of scattering was untenable. In his paper of 1811 he said: "I must freely confess that by continuing my sweeps of the heavens, my opinion of the arrangement of the stars and their magnitudes and of some other particulars has undergone a gradual change; and, indeed, when the novelty of the subject is considered, we cannot be surprised that many things formerly taken for granted should, on examination, prove to be different from what they were generally but incautiously supposed to be. For instance, an equal scattering of stars may be admitted in certain calculations; but when we examine the Milky Way, or the closely-compressed clusters of stars, of which my catalogues have recorded so many instances, this supposed equality must be given up." With the abandonment of this general assumption, the disc-theory became untenable.
Herschel's daring attempt to formulate a cosmology proved abortive. In place of this he was led to evolve a cosmogony. He appears to have been unaware of Kant's nebular hypothesis; indeed, he seems to have had, at the beginning of his career, no conception of evolutionary development among the celestial bodies. The dim, misty-looking nebulæ were all believed to be external galaxies, which increased telescopic power could resolve into their component stars. He was led to question and then to reject this generalisation by his study of a nebulous star of the eighth magnitude in the constellation Taurus, surrounded by a faintly-luminous atmosphere of considerable extent. The results of his investigations and reflections were contained in his remarkable paper "On nebulous stars properly so-called," dated 1st January, 1791. In regard to the nebulous star in Taurus, he said: "Our judgment will be that the nebulosity about the star is not of a starry nature". If, he pointed out, the nebulosity consisted of very remote stars, which appear nebulous on account of great distance, "then what must be the enormous size of the central point which outshines all the rest in so superlative a degree as to admit of no comparison?" If, however, the star is of average size, the smaller points composing the nebulosity must be almost infinitesimal. "We therefore either have a central body which is not a star, or have a star which is involved in a shining fluid of a nature totally unknown to us. I can adopt no other sentiment than the latter." And with characteristic caution he added in the same paper: "If therefore this matter is self-luminous, it seems more fit to produce a star by its condensation than to depend on the star for its existence".
This was in 1791, five years before Laplace suggested his classical hypothesis at the close of the "Systeme du Monde". The germ of the nebular theory, therefore, was present in the mind of Herschel at this early stage. In the paper of 1791, Herschel proceeded to apply his new view to the various nebulous regions all over the heavens. He concluded that he had been too hasty in his former surmise that all nebulæ were distant clusters. If the "shining fluid" can exist without stars, "we may with great facility explain that very extensive telescopic nebulosity" in the constellation Orion. "What a field of novelty is here opened to our conceptions!"
In 1802 Herschel dealt with the subject again in his "Catalogue of 500 new nebulæ". But it was not till 1811, in another epoch-making paper on the construction of the heavens, that Herschel enunciated his nebular hypothesis. In this paper he gave a complete list of nebulæ which he had discovered and studied, "assorting them into as many classes as will be required to produce the most gradual affinity between the individuals contained in any one class with those contained in that which precedes and that which follows it". Those contained in one class and those in the next class in order, he declared, have not so much difference between them, in his own suggestive remark, "as there would be in an annual description of the human figure, were it given from the birth of a child till he comes to be a man in his prime". He traced the evolutionary sequence from extensive diffused nebulosities, through irregular nebulæ, "nebulæ a little brighter in the middle," "nebulæ a little brighter" and "much brighter in the middle," nebulæ showing the progress of condensation, planetary nebulæ and stellar nebulæ, to "nebulæ nearly approaching to the appearance of stars". He declared it highly probable that "every succeeding state of the nebulous matter is the result of the action of gravitation upon it while in a foregoing one, and by such steps the successive condensation of it has been brought up to the planetary condition. From this the transit to the stellar form, it has been shown, requires but a very small additional compression of the nebulous matter." In 1814 he drew attention to double nebulæ joined by nebulosity between them. "It seems," he said, "as if we had these double objects in three different successive conditions: first as nebulæ; next as stars with remaining nebulosity; and lastly as stars completely free from nebulous appearance". Herschel's nebular hypothesis has never received in text-books of astronomy the attention it deserves. It was the result of long years of patient study, and is one of the most perfect examples of inductive reasoning in the history of science. Herschel, as has been already remarked, had sought a cosmology and he had found a cosmogony. Nevertheless, he did not abandon his attempt to discover the structure of the sidereal system. In 1817 and 1818, when nearly eighty years of age, he communicated two remarkable papers to the Royal Society on the extent and condition of the Milky Way and on the relative distances of clusters of stars. In these papers Herschel explained his new method of star-gauging, which some writers have confused with his first. The two methods, however, were quite distinct. In the first, one telescope was used in different regions of the heavens; whereas in the second, various telescopes were turned on the same region. The new method assumed the distribution of the stars to approximate to a certain properly-modified equality of scattering, and also a certain equality of real brightness. In the paper of 1817 he applied this new principle to the Milky Way, and in the paper of 1818 to star-clusters, assuming that the relative distances of "globular and other clusters "can be determined by the telescopic powers necessary to reveal and resolve them, provided that the component stars are, generally speaking, comparable to Sirius in size. Proctor, writing in 1872, contended that "the principle is unsound and that Herschel himself would have abandoned it had he tested it earlier in his observing career". Most writers have agreed with his estimate: yet recent work on star-clusters would seem to indicate that Herschel's second method was not so unsound as has been generally believed. Herschel propounded no hypothesis to take the place of the disc-theory. Indeed, his later view was that the sidereal system was much more extended in the plane of the Galaxy than he had previously believed. "The utmost stretch of the space-penetrating power of the 20-foot telescope could not fathom the profundity of the Milky Way." In 1817, he gave expression to the view that "not only our sun, but all the stars we can see with the eye are deeply immersed in the Milky Way and form a component part of it". Nevertheless, in the paper of 1818, he held that some of the nebulae, not obviously composed of true nebulous matter, which he called "ambiguous objects" are "clusters of stars in disguise, on account of their being so deeply immersed in space that none of the gauging powers of our telescopes have hitherto been able to reach them". Obviously, he still clung to the view that some of these dim, misty objects were "island universes".
The paper of 1818 was the last which Herschel wrote on the construction of the heavens. He failed in the object of his search, but countless others have failed since his day, and at the present time astronomers are still groping after the solution of the great problem. Herschel did not labour in vain: his papers on the structure of the universe form the foundation of all subsequent research. In the eloquent words of the late Miss Clerke: "One cannot reflect without amazement that the special life-task set himself by this struggling musician—originally a penniless deserter from the Hanoverian Guard—was nothing less than to search out the construction of the heavens. He did not accomplish it, for that was impossible; but he never relinquished it, and, in grappling with it, laid deep and sure the foundations of sidereal science."
- By Dr. Harlow Shapley, at Mount Wilson Observatory, California.