Chapter XIV: The Sun as a Whole

Modern science contemplates in the sun a huge sphere 867,000 miles in diameter, bounded by a dazzling cloud-shell, and composed of materials 1·4 times heavier than water, yet heated so far above the “critical temperature” of any terrestrial substance, that they must be regarded as in the gaseous state. The imperious demands of radiative emission can be met only by rapid exchanges, implying the unceasing activity of a system of profound vertical currents; while rotational surface-drifts of a peculiar kind notify complexities of internal movement defying speculation or research. Incidental to them, doubtless, are the dark spots marring, at times, the brightness of the disc, and giving evidence, by their more or less copious occurrence, of that far-reaching periodicity which may be called the central fact in solar physics. Spots are garlanded with faculæ, which represent photospheric upheavals; and above them, to a height of some hundreds of miles, rise the metallic vapours producing, by their absorption, the Fraunhofer lines. The reversing strata seem to be continuous with the chromosphere, although the nearly perfect transparency of the latter establishes a noteworthy distinction between the two formations; but the corona is a thing by itself, sharply separated, physically and spectroscopically, from every other appendage of the sun. It has, of course, relations with them, just as our air has with the ocean it surmounts—relations, however, that do not even verge towards a confusion of identity.

The two fundamental problems connected with the nature of the sun are its rotation and its periodicity. They may be quite closely allied, and in regard to both, “counsels of despair” have begun to prevail. The spot-cycle, like “Carrington’s law,” is set down as a congenital peculiarity, and the mists of the past are invoked to cover the perplexities of the present. There seems little immediate prospect of their being removed. Early modes of investigating the subject have had no striking success, and fresh ones await development. So operations have come to a pause, yet by no means to a dead-lock. The difficulty of learning how the phenomena are occasioned should only stimulate diligence in unmasking and tabulating them. The ramifications of the period can be followed out, even if we cannot get at its roots, and there are signs that they will be found to take unlooked-for directions. Similarly as regards the sun’s rotation. Explanatory hypotheses avail little, but the sifting of facts avails much. The case has not yet been fully stated. Lacunæ need to be filled up, anomalies to be smoothed away, errors to be corrected. In the doing of all this, a clue to the labyrinth may present itself.

The nature of sun-spots must long be under discussion. Questions of extreme interest are involved, some of them being visibly _answerable_, since criteria are at hand to determine which way the truth lies. Discriminative, rather than numerous observations will serve the purpose. The relations of faculæ with prominences offer another promising topic of inquiry, as well as those of prominences with coronal streamers. But this last is an eclipse-problem, and so, unluckily, is at present the entire subject of the corona. No pains, however, will be spared in order to bring to bear upon it the full resources of daylight investigation. As to whether they will be rewarded or no, the balance of forecast swings pretty even. The unfolding of some condition, now hidden, may incline it either way.

Solar spectroscopy presents a variety of aspects. It can be studied from the chemical, thermal, electrical point of view; pressure, density, motion, possibly magnetic stress, are concerned in it. Nothing in this branch, however, is more instructive than the spectral diversities of the various solar formations, or even of different parts of the same formation. Thus each section, some few miles thick, of the reversing layer is probably distinguished by emissive modifications; and the Fraunhofer spectrum integrates the absorption of them all. The spot-spectrum is a more pronounced variety; the chromospheric spectrum is largely, the coronal spectrum, wholly peculiar. Now the task of interpreting these several scripts is not—could not be—easy; yet solar chemistry is, in the main, less unfamiliar than might have been expected. It includes only one element—coronium—that can be clearly distinguished as terrestrially unknown. For unidentified lines are not necessarily of exotic origin, as is proved by the rapid progress of their recognition in the Fraunhofer spectrum _pari passu_ with advances in the photographic registry of metallic spectra. Nor is it probable that spot-cavities harbour strange forms of matter. The vaporisation in them of rare metals sufficiently explains what long appeared enigmatical in their absorptive action.

With the substitution of known for unknown substances in the sun, a leading argument for dissociation vanished. In this view, the supramundane species giving lines experimentally unrecorded were chemical fragments of our elements broken up by enormous heat. But if no supramundane species exist, the elements presumably remain intact. Absolute stability it would indeed be extremely rash to ascribe to them; yet it is certain that their individuality survives fierce ordeals. Evidence of other kinds tends towards the same conclusion. Mr. Jewell’s critical examination of the Fraunhofer rays indicated for the shadings attached to certain iron and calcium lines an origin quite close to the photosphere.[317] This goes far towards demonstrating the integrity of extremely complex molecules at the highest temperature prevailing near the sun. Outbursts of heat from the interior there may be, reaching a still more exalted pitch; but they must be transitory, since cooling by expansion should promptly and potently affect them. Further, the thermal relations of the terrestrial elements are distinctive and uniform. If these bodies are not really simple, they are at least compounds of a different order from those known to be such, and artificially producible. Finally, the extensive detection of spectral series invalidates the “one line, one element” principle which underlies most of the arguments for dissociation. Unity of origin is emphatically claimed, not only by each series, but by each connected set of series. Researches into the essential nature of matter have, however, entered upon a new phase through the aid of what we may call electrical analysis, in which “ions” play the part of atoms in chemical analysis; and their outcome may perhaps aid in the solution of many intricate solar problems.

There is a strong temptation to transfer to the sun the conditions prevailing on the earth, and to model solar upon terrestrial meteorology. This was the line taken by M. Egon von Oppolzer in 1893. He admitted, indeed, that it traversed dangerous ground, in view of the wide divergences of opinion as to the nature of atmospheric processes belonging to immediate and everyday experience. Still he judged it safe to apply the kinetic theory of gases and the dynamical theory of heat to the determination of the state of equilibrium in the sun’s aerial appendages. The results were scarcely encouraging. At elevations of one second of arc, or 450 miles, above the photosphere, the prevalent temperature was found to be 14,000° C. lower than at its surface, which should hence be extravagantly hot. And although this inconvenience was abated by forced assumptions, it could not, on the principles adopted, be removed. They involved, for example, a rise of temperature in downward currents to the extent of at least 5000° for each descent of 450 miles. Spots were regarded as “places of extreme alternations of temperature,”[318] where abnormally hot layers cover anticyclonic regions of increased pressure and reduced thermal excitement. They are “produced indirectly through a sinking down of masses upon the photosphere, and directly through extraordinary radiation, brought about by transparency of the overlying region.” To counterbalance descending movements in the spot-zones, a continuous uprising of heated matter was supposed to progress at the poles, constituted so far the analogues of our equatorial belt of calms. Two great permanent cyclones were thus centred on the axis of the sun. They had important functions assigned to them. Upon their regulative power was made to depend the working of the entire machine, and solar periodicity itself was referred to the alternate relaxation and enhancement of their activity. They were, nevertheless, a purely arbitrary creation. Not so much as a tortoise in mid-air was provided for the earth-bearing elephant to stand upon. The author claimed, it is true, only the merit of simplification; yet the contrast of conditions between the earth and the sun largely vitiated his reasonings. Atmospheric circulation on the earth is maintained by external heat; agitations on the sun by internal heat; they depend absolutely upon processes of cooling. Hence the impossibility of assimilation. Trade winds and cyclones lack in the sun the driving power by which they are kept going on the earth. They are characteristic of a planetary body—of a globe _vitalised_ from without.

Eruptive hypotheses of the solar constitution have been proposed under various forms by a succession of writers—by Secchi, Faye, Lockyer, Schaeberle,[319] Young,[320] Sidgreaves.[321] Here, at any rate, we are in touch with reality. Volcanic forces are powerful in the sun, which might, in a sense, be described as organised upon a volcanic basis. The conditions of upheaval are everywhere at hand; only some casual relief of pressure, or access of heat is needed to provoke an actual explosion. Settling down on the photosphere in a cooled, though still gaseous state, the products of eruption would then give rise to spot-phenomena. Dark patches would mark the effects of their general and special absorption of light, while faculæ and flames attested the vehemence both of the original outbursts, and of those reactively started by their partial subsidence. Procedures of this kind on the sun appear inevitable; they at least count for something, if they do not explain everything.

A curiously subversive theory in solar physics was propounded in 1891 by Dr. August Schmidt.[322] Its interest, however, is largely academic. Yet it is no illusory speculation. It rests upon a sound foundation, and emphasises an undeniable truth. The mode of action brought into prominence by it necessarily plays a part in modifying the aspect of the heavenly bodies; the only question open is whether the part is conspicuous or insignificant.

The laws of atmospheric refraction were traced to their ultimate consequences by Kummer in 1860.[323] Some of them are remarkable.

The visual lifting of extra-terrestrial objects results, as is familiarly known, from the inflected character of the paths pursued by light-rays through our air. A beam reaching a spectator at sea-level from a horizontal direction is really bent upward with a curvature one-seventh that of the earth itself. But on a globe of seven times the earth’s radius—other things remaining the same—the refracted ray would possess identically the curvature of its surface, to which it would accordingly run parallel for ever, or until extinguished by absorption. It would never reach the eye of a spectator. “Circular refraction” would take effect upon it. A similar fate would befall a ray starting horizontally from the surface towards outer space, instead of attaining to which, it should follow an unending round within the air. On Jupiter, this critical stage must be considerably overpassed. From a Jovian atmosphere of the proportionate mass of the earth’s, light could only escape at angles of elevation exceeding 3° 22′.[324]

Applying his formulæ to the sun, Schmidt found that circular refraction would there be produced in a hydrogen atmosphere at a temperature of 10,000°C., and of one-ninth the standard density of air. The upshot was to “explain away” most of the solar appendages. The photosphere he showed to be an optical illusion, arising at the surface where circular refraction just comes into operation in an incandescent gaseous globe diffusing uniformly outward. The difficulty was thus overcome of accounting for the visibly sharp separation between the dense body of the sun and its tenuous surroundings; while a phantasmagoria of granules, spots, flames, and faculæ was easily evoked on the supposition of irregular refractions within the seeming disc.[325] To this dubious locality, the reversing layer, too, was transferred by Dr. Knopf of Jena, who hailed the speculation as the dawn of a new era in solar physics. Hence, opposite displacements of the Fraunhofer lines cannot be a rotational effect, and the singular accordance of what they should be on that supposition with the measures actually obtained, is the outcome of pure accident. But this is extravagant and unthinkable—a _reductio ad absurdum_ of the optically composed sun. Plainly, the whole theory is in the air. Its reasonings, to be sure, are mathematically valid; but they refer, as Dr. Seeliger has pointed out,[326] to an ideally transparent globe, from which the absorptive effects, prominent in the sun, are absent. The solar spectrum, besides, fails to be accounted for by them “on the accepted principles of physics.”[327] Nor can practised observers of the solar surface readily be persuaded that they have watched, not realities, but mirage-effects. In dismissing the hypothesis as untenable, the important inference was, however, retained by Professor Frost “that refraction within and on the sun itself may modify in some considerable degree” observed phenomena.

An ingenious corollary has been added to Schmidt’s theory by M. Julius of Amsterdam.[328] It was suggested by certain anomalies in the dispersive action of sodium-vapour upon light, noticed by Becquerel, and confirmed by himself. They led him far. He was conducted by them to optical explanations of the “flash” spectrum, of the broadening of lines in sun-spot spectra, and of line-displacements, usually interpreted on Doppler’s principle. They are, however, given under reserve, and can be tested by comparing the lines affected in the sun with those in the vicinity of which special refraction is exerted. This is a laboratory task, which M. Julius himself is well qualified to discharge. Should the provisionallyassumed correspondences be found to subsist, the arguments for the phantasmic character of solar appurtenances will have gained imposing strength. But such a result cannot reasonably be anticipated. Refraction on the sun is likely, owing to the extreme rarity of the medium in which it should take place, to be of evanescent effect. Only in spot-cavities, it may perceptibly affect appearances. The entire matter, nevertheless, deserves to be thoroughly sifted, and cannot in future be forgotten or ignored.

Here again a fresh prospect is opening into view, and many others invite exploration by difficult and devious routes. For solar science does not become less arduous as it advances. One of the surest marks of progress in any branch is, indeed, the development of new and unforeseen problems. And the sun, as we have learned by degrees to recognise, is a body organised in too complex a method for easy apprehension. We perceive that its energies are specially directed; the purpose of the machine is obvious, and it is admirably fulfilled. In part we can see how, but much remains mysterious. Right to the core of the mystery we may never penetrate; nature is virtually invincible by man; but in urging our way forward, we shall gain continually larger and more lucid views.