These researches lay, as Professor Holden expresses it, “directly on the line of Herschel’s main work.” The separation of the stars into light-ranks intimates at once something as to their distribution in space; but the intimations may prove deceptive unless the divisions be accurately established. Hence, stellar photometry is an indispensable adjunct to the study of sidereal construction. Herschel prosecuted the subject besides with a view to ascertaining the constancy of stellar lustre. He had been struck with singular discordances between magnitudes assigned at different dates. Not to mention stars obviously variable, there were others which seemed to be affected by a slow, secular waxing or waning. In some of the instances alleged by him, the alteration was no doubt fictitious—a record of antique errors; but there was a genuine residuum. Thus, the immemorially observed constituents of the Plough preserve no fixed order of relative brilliancy, now one, now another of the septett having, at sundry epochs, assumed the primacy; while a small star in the same group, Alcor, the “rider” of the second “horse,” has, in the course of a millennium, plainly thrown off some part of its former obscurity. The Arabs in the desert regarded it as a test of penetrating vision; and they were accustomed to oppose “Suhel” to “Suha” (Canopus to Alcor) as occupying respectively the highest and lowest posts in the celestial hierarchy. So that Vidit Alcor, at non lunam plenam, came to be a proverbial description of one keenly alive to trifles, but dull of apprehension for broad facts. Now, however, Alcor is an easy naked-eye object. One needs not be a “tailor of Breslau,” or a Siberian savage, to see it. The little star is unmistakably more luminous than of old.
An inversion of brilliancy between Castor and Pollux, and between the two leading stars in the Whale, is further generally admitted to have taken place during the eighteenth century. The prevalence of such vicissitudes was deeply impressive to Herschel, especially through their bearing upon the past and future history of our own planet. “If,” he said, “the similarity of stars with our sun be admitted, how necessary will it be to take notice of the fate of our neighbouring suns, in order to guess at that of our own. The star which we have dignified by the name of Sun may to-morrow begin to undergo a gradual decay of brightness, like Alpha Ceti, Alpha Draconis, Delta Ursæ Majoris, and many other diminishing stars. It may suddenly increase like the wonderful star in Cassiopeia, or gradually come on like Pollux, Beta Ceti, etc. And, lastly, it may turn into a periodical one of twenty-five days’ duration (the solar period of rotation), as Algol is one of three days, Delta Cephei of five days, etc.” He found it, accordingly, “perhaps the easiest way of accounting for past changes in climate to surmise that our sun has been formerly sometimes more, sometimes less, bright than it is at present.” Herschel attempted, in 1798, to analyse star-colours by means of a prism applied to the eye-glasses of his reflector. Nothing of moment could at that time come of such experiments; but they deserve to be remembered as a sort of premonition of future methods of research into the physical condition of the stars.
His attention to the sun might have been exclusive, so diligent was his scrutiny of its shining surface. Many of its peculiarities were first described by him, and none escaped him, except the “deeper deep,” or black nucleus of spots, detected by Dawes in 1852. The dusky “pores” and brilliant “nodules,” the corrugations, indentations, and ridges; the manifold aspects of spots, or “openings;” their “luminous shelving sides,” known as penumbræ; were all noted in detail, ranged in proper order, and studied in their mutual relations. Spots presented themselves to him as evident depressions in the luminous disc; faculæ, “so far from resembling torches,” appeared “like the shrivelled elevations upon a dried apple, extended in length, and most of them joined together, making waves, or waving lines.” Towards the north and south, he went on, “I see no faculæ; there is all over the sun a great unevenness, which has the appearance of a mixture of small points of an unequal light; but they are evidently a roughness of high and low parts.”
His theory of the solar constitution was a development of Wilson’s. It was clearly conceived, firmly held, and boldly put forward. The definite picturesqueness, moreover, of the language in which it was clothed, at once laid hold of the public imagination, and gave it a place in public favour from which it was dislodged only by the irresistible assaults of spectrum analysis.
The sun was regarded by Herschel as a cool dark body surrounded by an extensive atmosphere made up of various elastic fluids. Its upper stratum—Schröter named it the “photosphere”—was of cloud-like composition, and consisted of lucid matter precipitated from the elastic medium by which it was sustained. Its depth was estimated at two or three thousand miles, and the nature of its emissions suggested a comparison with the densest coruscations of the aurora borealis. Below lay a region of “planetary,” or protective clouds. Dense, opaque, and highly reflective, “they must add,” he said, “a most capital support to the splendour of the sun by throwing back so great a share of the brightness coming to them.” Their movements betrayed the action of vehement winds; and indeed the continual “luminous decompositions” producing the radiating shell, with the consequent regeneration of atmospheric gases beneath, “must unavoidably be attended with great agitations, such as with us might even be called hurricanes.” The formation and ascent of “empyreal gas” would cause, when moderate in quantity, pores, or small openings in the brilliant layers. But should it happen to be generated in uncommon quantities, “it will burst through the planetary regions of clouds, and thus will produce great openings; then, spreading itself above them, it will occasion large shallows, and, mixing afterwards gradually with other superior gases, it will promote the increase, and assist in the maintenance of the general luminous phenomena.”
The solid globe thus girt round with cloud and fire was depicted as a highly eligible place of residence. An equable climate, romantic scenery, luxuriant vegetation, smiling landscapes, were to be found there. It might, accordingly, be admitted without hesitation that “the sun was richly stored with inhabitants.” For the lucid shell visible from the exterior possessed, according to this theory, none of the all-consuming ardour now attributed to it. Its blaze was a superficial display; beneath, “the immense curtain of the planetary clouds was everywhere closely drawn” round a world perfectly accommodated to vital needs.
In order to reconcile this supposed state of things with the observed order of nature, it was suggested that traces of it subsist in the planets, “all of which, we have pretty good reason to believe, emit light in some degree.” The night-side illumination of Venus, the sinister glare of the eclipsed moon, the auroral glimmerings of the earth, were adduced as evidence to this effect. The contrast between the central body and its dependants was softened down to the utmost.
“The sun, viewed in this light,” Herschel wrote in 1794, “appears to be nothing else than a very eminent, large, and lucid planet, evidently the first, or, in strictness of speaking, the only primary one of our system; all others being truly secondary to it. Its similarity to the other globes of the solar system with regard to its solidity, its atmosphere, and its diversified surface; the rotation upon its axis, and the fall of heavy bodies, lead us on to suppose that it is also most probably inhabited, like the rest of the planets, by beings whose organs are adapted to the peculiar circumstances of that vast globe.”
To us, nearing the grey dawn of the twentieth century, the idea seems extravagant; it was, in the eighteenth, plausible and alluring. The philosophers of that age regarded the multiplicity of inhabited worlds as of axiomatic certainty. The widest possible diffusion of life followed, they held, as a corollary from the beneficence of the Creator; while their sense of economy rendered them intolerant of wasted globes. Herschel was then reluctant to attribute to the sun a purely altruistic existence. Only from the point of view of our small terrestrial egotism could so glorious a body figure as solely an attractive centre, and a focus of warmth and illumination to a group of planets. Besides, looking abroad through the universe, we see multitudes of stars which can exercise no ministerial functions. Those united to form compressed clusters, or simply joined in pairs, are unlikely, it was argued, to carry a train of satellites with them in their complex circlings. Unless, then, “we would make them mere useless brilliant points,” they must “exist for themselves,” and claim primary parts in the great cosmical life-drama.
Herschel’s sun is to us moderns a wholly fabulous body. Still, there is a fantastic magnificence about the conception so strongly realised by his powerful imagination. Moreover, its scientific value was by no means inconsiderable. It represented the first serious effort to co-ordinate solar phenomena; it implied the spontaneous action of some sort of machinery for the production of light and heat. Spots were associated with a circulatory process; the photosphere was portrayed under its true aspect. The persistence of its hollows and heights, its pores and rugosities, convinced Herschel that the lustrous substance composing it was “neither a liquid nor an elastic fluid,” which should at once subside into an unbroken level. “It exists, therefore,” he inferred, “in the manner of lucid clouds swimming in the transparent atmosphere of the sun.”