His contemplations of the heavens showed him everywhere traces of progress—of progress rising towards perfection, then sinking into decay, though with a sure prospect of renovation. He was thus led to arrange the nebulæ in a presumed order of development. The signs of interior condensation traceable in nearly all, he attributed to the persistent action of central forces. Condensation, then, gave evidence of age. Aggregated stars drew closer and closer together with time. So that scattered or branching formations were to be regarded as at an early stage of systemic existence; globular clusters, as representing universes still in the prime of life; while objects of the planetary kind were set down as “very aged, and drawing on towards a period of change, or dissolution.”
Our own nebula he characterised as “a very extensive, branching congeries of many millions of stars,” bearing upon it “fewer marks of profound antiquity than the rest.” Yet, in certain regions, he found “reason to believe that the stars are now drawing towards various secondary centres, and will in time separate into different clusters.” As an example of the ravages of time upon the galactic structure, he adverted to a black opening, four degrees wide, in the Zodiacal Scorpion, bordered on the west by an exceedingly compact cluster (Messier’s No. 80), possibly formed, he thought, of stars drawn from the adjacent vacancy. The chasm was to him one of the most impressive of celestial phenomena. His sister preserved an indelible recollection of hearing him, in the course of his observations, after a long, awful silence, exclaim, “Hier ist wahrhaftig ein Loch im Himmel!” (Here, truly, is a hole in the sky); and he recurred to its examination night after night and year after year, without ever clearing up, to his complete satisfaction, the mystery of its origin. The cluster significantly located at its edge was lit up in 1860 by the outburst of a temporary star.
This was not the sole instance noted by Herschel of the conjunction of a chasm with a cluster; and chasms and clusters alike told the same story of dilapidation. He foresaw, accordingly, as inevitable, the eventual “breaking-up” of the Milky Way into many small, but independent nebulæ. “The state into which the incessant action of the clustering power has brought it at present,” he wrote in 1814, “is a kind of chronometer that may be used to measure the time of its past and future existence; and although we do not know the rate of going of this mysterious chronometer, it is, nevertheless, certain that since the breaking up of the Milky Way affords a proof that it cannot last for ever, it equally bears witness that its past duration cannot be admitted to be infinite.”
Thus the idea of estimating the relative “ages” of celestial objects—of arranging them according to their progress in development, originated with Herschel in 1789. “This method of viewing the heavens,” he added, “seems to throw them into a new kind of light. They are now seen to resemble a luxuriant garden which contains the greatest variety of productions in different flourishing beds; and one advantage we may at least reap from it is that we can, as it were, extend the range of our experience to an immense duration. For, is it not almost the same thing whether we live successively to witness the germination, blooming, foliage, fecundity, fading, withering, and corruption of a plant, or whether a vast number of specimens, selected from every stage through which the plant passes in the course of its existence, be brought at once to our view?”
But while he followed the line of continuity thus vividly traced, another crossing, and more or less interfering with it, opened out before him. The discovery of a star in Taurus, “surrounded with a faintly luminous atmosphere,” led him, in 1791, to revise his previous opinions regarding the nature of nebulæ. He was not at all ashamed of this fresh start. No fear of “committing himself” deterred him from imparting the thoughts that accompanied his multudinous observations. He felt committed to nothing but truth. He was advancing into an untrodden country. At every step he came upon unexpected points of view. The bugbear of inconsistency could not prevent him from taking advantage of each in turn to gain a wider prospect.
Until 1791 Herschel never doubted that gradations of distance fully accounted for gradations of nebular resolvability. He had been led on, he explained, by almost imperceptible degrees from evident clusters to spots without a trace of stellar formation, no break anywhere suggesting the possibility of a radical difference of constitution. “When I pursued these researches,” he went on, “I was in the situation of a natural philosopher who follows the various species of animals and insects from the height of their perfection down to the lowest ebb of life; when, arriving at the vegetable kingdom, he can scarcely point out to us the precise boundary where the animal ceases and the plant begins; and may even go so far as to suspect them not to be essentially different. But, recollecting himself, he compares, for instance, one of the human species to a tree, and all doubt upon the subject vanishes. In the same manner we pass by gentle steps from a coarse cluster to an object such as the nebula in Orion, where we are still inclined to remain in the once adopted idea of stars exceedingly remote and inconceivably crowded, as being the occasion of that remarkable appearance. It seems, therefore, to require a more dissimilar object to set us right again. A glance like that of the naturalist, who casts his eye from the perfect animal to the perfect vegetable, is wanting to remove the veil from the mind of the astronomer. The object I have mentioned above is the phenomenon that was wanting. View, for instance, the nineteenth cluster of my sixth class, and afterwards cast your eye on this cloudy star, and the result will be no less decisive than that of the naturalist. Our judgment, I venture to say, will be that the nebulosity about the star is not of a starry nature.”
In this manner he inferred the existence of real nebulous matter—of a “shining fluid” of unknown and unimaginable properties. Was it perhaps, he asked himself, a display of electrical illumination, like the aurora borealis, or did it rather resemble the “magnificent cone of the zodiacal light?” A boundless field of speculation was thrown open. “These nebulous stars,” he added, “may serve as a clue to unravel other mysterious phenomena.”
As their close allies, he now recognised planetary nebulæ, the “milkiness, or soft tint of their light,” agreeing much better with the supposition of a fluid, than of a stellar condition. And he rightly placed in the same category the Orion nebula, and certain “diffused nebulosities” which he had observed just to tarnish the sky over wide areas. These last might, he considered, be quite near the earth, and the object in Orion not more distant than perhaps an average second magnitude star.
The relations of the sidereal to the nebular “principle” exercised Herschel’s thoughts during many years. He had no sooner reasoned out the existence in interstellar space of a rarefied, self-luminous substance, than he began to interrogate himself as to its probable function. Nature was to him the expression of Supreme Reason. He could only conceive of her doings as directed towards an intelligible end. Hence his confidence that rational investigation must lead to truth.
Already in 1791 he hinted at the conclusion which he foresaw. The envelope of a “cloudy star” was, he declared, “more fit to produce a star by its condensation than to depend upon the star for its existence.” And the surmise was confirmed by his detection, in a planetary nebula, of a sharp nucleus, or “generating star,” possibly to be completed in time by the further accumulation of luminous matter.