Chapter VI: The Photosphere and Its Dusky Veil

The sun is virtually bounded by a spherical envelope of intense lustre. What lies outside is negligible in mass and function. What is hidden within has its energies concentrated, so to speak, on the maintenance of the “photosphere” at the highest point of radiative efficiency. This implies enormous internal activity, the slackening of which would be the prelude to speedy extinction. The materials of a self-renewing stratum of concentrated emission are necessarily in a state of flux. Each constituent particle, as it delivers up its store of light and heat, becomes instantaneously effete, and is replaced by another. Charging and discharging processes pursue a ceaseless round, ceaseless, that is to say, until the growth of viscosity fatally impedes them. When that time comes, convection-currents cease to flow, superficial cooling advances rapidly, and the sun-like stage terminates. The epoch of inertness must in fact arrive when, for a circulatory, a fixed surface is substituted. During some long antecedent period, again, the same body was presumably too rare to be definitely limited, and might fitly be designated a nebula. So that a “sun” is definable as a gaseous mass clothed with a pellicle of dazzling luminosity, and organised for long continuance in the capacity of a distributor of light and heat.

The shining pellicle of our sun is, to all appearance, of a cloud-like nature. It is a locus of condensation, where uprushing gases, chilled by expansion, momentarily change their state, and thus acquire the power of suddenly parting with their stored energy. The “mottlings” of the photosphere mark visibly, perhaps, this rapid course of interchange, brilliant floccules denoting regions of _arterial_ ascent, dusky tracts those of corresponding _venous_ descent. That it is accompanied by violent turmoil, the evidence of the camera shows conclusively. The reticulated areas are highly evanescent. Ridges and brilliant cumuli, some hundreds of miles in extent, form only to be swept away. “When we come to study the minute details of the granulations,” Professor Young writes, “we find movements at the rate of a thousand miles an hour to be the rule rather than the exception.”[97]

Since photospheric light is purely continuous, photospheric chemistry remains a _terra incognita_. Only conjectures are possible regarding the kind of matter present in the solar condensations. The idea that they may be formed of carbon, started by Dr. Johnstone Stoney in 1867,[98] is still very generally entertained. It is indeed hampered by difficulties at present insurmountable; but the same may be said of every other hypothesis on the subject. Carbon was recommended for the position assigned to it by its refractoriness to heat and by its great radiative power. Lampblack, we need hardly say, is, in this latter respect, the standard substance. An unfavourable peculiarity, on the other hand, is its inability to exist as a liquid under conditions at all likely to be realised in the sun. Carbon has no fusing-point in the ordinary sense. At a temperature of about 3500° C. it sublimes without melting. Preparatory to crystallising as diamond it perhaps liquefies through the incomparable stress of molecular forces, but the process is transitory and obscure. It has never been observed; it is only reasonably supposed to take place. Moreover, at or near the photosphere, pressure of the required intensity certainly does not exist. The cumuli forming it should then consist, not of carbon droplets, but of carbon dust, and the analogy with terrestrial clouds would disappear. A still more serious objection is that carbon volatilises at a temperature far below that of the photosphere. Nor are we acquainted with any kind of matter the condensation of which might be thought of as possible under the conditions there prevailing.

The question of temperature is fundamental in solar physics. Everything that regards the nature, structure, and innate activity of the solar globe depends upon the answer furnished to it. And of late the answers have become much more plausible than those discordant to the extent of some millions of degrees arrived at thirty years ago. The main cause of this wide uncertainty lay, not in the actual measurements, which can be made sufficiently precise, but in the failure to establish on secure grounds some definite relation between temperature and radiation. There is no doubt that thermal outflows increase far more rapidly than the accompanying thermometric rise—that the heat received at a distance corresponds, in an augmented proportion, to a gain of heat at the source; but the correspondence has, until lately, been expressed only by empirical rules, not implicitly or unconditionally to be trusted. Boltzmann,[99] however, supplied an _ex post facto_ theoretical basis for a law published by Stefan of Vienna in 1879, according to which radiation grows as the fourth power of temperature. Its agreement with facts, so far as they are available, is besides tolerably close. Yet the security is precarious that it continues to match them in regions of cosmic heat, unattainable by experiment. It was, however, employed by Messrs. Wilson and Gray, with some modification and with excellent results, in their authoritative determination of the sun’s temperature.[100]

They adopted a method of direct observation, involving the fewest possible uncertainties of principle. Sun-heat, allowed to fall upon a “radio-micrometer”—an instrument of extreme sensitiveness invented by Professor Boys—was measured by the “balancing” of its effects against those of a strip of platinum heated to a known pitch. This gave the means, by the aid of Stefan’s law, of translating them into terms of temperature. Allowance had then to be made for a double absorption, first in the sun’s, again in the earth’s atmosphere. That only a fraction of the heat emanating from the solar condensations reached the apparatus in the West Meath observatory was unmistakable; how large a fraction was less easy to decide. Langley finds that the intensity of radiation at the centre of the disc is reduced near the limb by one-half,[101] and the total loss is estimated by Wilson and Rambaut at one-third of the whole.[102] Hence the sun’s thermal power would be one and a half times greater than it is if the emitting surface were stripped of its absorbent covering, and the correction of temperature demanded by its action amounts to at least 1000° C.

The despoiling effect of our own air has next to be considered. It is very large, and so are the discrepancies in its valuation. Rosetti of Padua, who in 1879 determined the temperature of the sun to be 20,000°,[103] concluded for a zenithal heat-stoppage of 29 per cent; Langley estimated it at 41; Knut Ångström[104] in 1890, laying stress for the first time upon the thermal opacity of the carbonic acid ingredient of the atmosphere, obtained 64 per cent as the ratio of absorption. This seemingly authentic result, namely, that only 36 per cent of the heat rays striking the earth vertically are transmitted to its surface, was provisionally admitted by Wilson and Gray, and after having made careful allowance for various kinds of possible error, they arrived in 1894 at an effective solar temperature of 8700° C. Substituting Langley’s value for terrestrial atmospheric absorption, and working up fresh experimental data, Mr. Wilson in 1901 reduced this figure to 6590°,[105] which probably underestimates the truth. At some such inconceivable degree of heat the undimmed photospheric clouds glow.

This is not all. The value just given belongs to an ideal stratum in the sun. It stands for the “effective,” not the actual temperature—the temperature, that is to say, which should be attributed to a surface of standard radiative capacity sending out the measured quantity of heat. Now it is certain that the photosphere falls very far short, in emissive power, of its imaginary substitute. There is no such thing in nature as a “perfectly black body,” or its correlative, a perfect radiator, the efficiency even of lampblack being only six-tenths of what it is assumed to be for purposes of calculation. And the sun is unlikely to be as good a radiator as lampblack. It must then be hotter in proportion to its inferiority, but to what extent it falls short of the ideal standard remains undetermined. It must also be very unequally hot. The brilliant granules giving its flocculent appearance to the photosphere radiate much more intensely than the gray interspaces. Hence computed temperatures represent an average higher than prevails in some formations, lower than is assignable to others. It is noticeable that several corrections based upon recent improvements in experimental data tend to enhance our conception of the tremendous energy of solar heat.

Le Chatelier’s method[106] of employing the intensities of selected rays in various light-sources as a criterion of temperature gave 7600° C. for that of the sun (uncorrected for solar absorption). It is, however, of doubtful validity. A parallel line of research was opened by Langley’s establishment of the principle that temperature is connected by a definite relation with the wave-length of maximum energy in the spectrum of a radiating body. Divergent views, nevertheless, prevail as to the _form_ of the relation. Michelson[107] and Rubens[108] agree that the wave-length of most powerful emission varies in length inversely as the square root of the temperature, while Paschen and Wien[109] maintain that the simple inverse ratio tallies more closely with facts. The outcome in determinations of the sun’s heat differs of course vastly with the law chosen. From Michelson’s, H. Ebert deduced in 1894 a temperature of 40,000° C., but added the qualifying remark, “The parts of the sun to which this value applies belong to the more interior regions; they are at any rate deep under the reversing layer, and therefore probably below the photosphere.”[110] Now subphotospheric heat may be of almost any intensity; hence the result, although not very informing, is safe to be in some sense correct. Paschen, on the other hand, obtained the low value of 5130°. It might be added that the law upon which he relied is suspiciously simple, “in view of the known complexity in the radiation of a solid body, and the various rates of increment with temperature attaching to different rays.”[111] It seems to be one of those formulæ which cannot be trusted far out of sight. They are _not true enough_ to bear extension into regions beyond experience. Useful over a moderate compass, they prove treacherous adjuncts to investigation. Difficulties, indeed, all but insuperable hamper attempts to infer the solar temperature from comparisons of spectral energy-curves. Unexpected peculiarities are found to characterise the modes of emission of solid bodies. Even continuous spectra are to some extent distinctive. Thus the same quantity of energy is very differently distributed in the rays sent out respectively by polished and lampblacked platinum, by carbon filaments, copper, and iron oxides; while with an equal increase of energy, the distribution becomes diversely modified for each substance. For each, that is to say, the maximum ordinate of the energy-curve creeps upward at a different rate. In the absence, then, of precise knowledge as to the composition and condition of the photosphere, inquiries as to its temperature, based on this principle, are futile. We should first need to be acquainted, in Professor Very’s words, with “the selective radiating power of the solar photosphere.”[112] Generalisations are here eminently unsafe, since laws of radiation derived from the experiments with one kind of material are by no means certain to prove applicable to others. Besides, the “absolute solar spectrum” (as Langley calls it) cannot be directly observed, and the shape of its representative curve is most materially altered by the effects of absorption in the solar atmosphere.

On the whole, the straightforward plan of attack on the problem of the sun’s heat seems the most promising. Messrs. Wilson and Gray’s practical operations left little room for improvement, and the uncertainties affecting their final result will gradually diminish with the progress of other kinds of research. As higher temperatures, for instance, are brought under command, the range allowed to perilous processes of “extrapolation” can be restricted. And improvements, sure ere long to be realised, in the value assignable to telluric atmospheric absorption, will effectually reduce the marginal errors attached to present estimates of the _primitive_ heat-power of the sun.

Fluctuations in the sun’s heat-power must be regarded as possible, and they might be either irregular or periodical. Indeed, the superposition of both kinds of change would perhaps be more likely than the occurrence of either separately. Their detection would, in any case, be extremely difficult, although it is not, in Messrs. Wilson and Gray’s opinion, to be regarded as hopeless. The required measures would be simply differential; and differential measures escape many of the snares that hamper the execution of absolute measures. But comparisons in this matter are rendered almost nugatory by inconstancy of weather. Variations in the “solar constant,”[113] even if real, would probably be masked by local and temporary changes in the diathermancy of the air. Professor Very holds that “under these circumstances refinements in actinometry are of small avail,”[114] and he suggests “that the problem will have to be solved entirely by meteorological methods.”[115] “If temperature and humidity observations could be collated from the logs of vessels crossing the torrid zone, estimates of oceanic evaporation from day to day, combined with rainfall measures, might lead,” he believes, “to the detection of the variation of solar radiation.” But the chance of their doing so appears, all things considered, to be incalculably small. The elements of disturbance are too numerous and too strong to permit the emergence of the slight residual effects looked for. Far preferable appears Piazzi Smyth’s plan of earthing thermometers deeply enough to be inaccessible to superficial vicissitudes of temperature. And it can scarcely be without significance that the readings of those buried on the Calton Hill showed oscillations coincident in period with the sun-spot cycle.

Absorption in the sun’s atmosphere may also prove to be variable. And here again differential observations should suffice to test the question. They were undertaken by Wilson and Rambaut in 1892, but relinquished after one series had been made. The method employed was to pass an image of the sun across the radio-micrometer, while the motion of a spot of limelight, reflected from the mirror of the instrument, recorded the changing amounts of heat received from the different parts of the disc. The intention was to obtain such “curves of absorption” frequently throughout an eleven-year cycle, and thus determine the question of concurrent fluctuations in depth of the absorbing envelope. “If we find,” the authors wrote, “that such changes are taking place, as would be shown by the alteration in the ratio of the heat from the limb and centre of the disc, we think it will be quite possible, by an investigation of the co-ordinates of these curves, to determine the change in the value of the solar constant.”[116]

This theoretical possibility, nevertheless, is still a long way from realisation. Divers indications lead almost irresistibly to the conclusion that the sun is hotter at certain times than at others; and Professor Young counts it as “one of the most important and difficult problems of solar physics now pending to determine the actual amount of these variations and ascertain the laws that govern them.” But they are, as we have partly seen, disguised by manifold complications.

The one clear upshot of inquiries into the temperature of the sun is to show that it stands high above the boiling points of the most refractory among the chemical elements. The fact is embarrassing, but cannot be evaded. Apart from its consideration, no theory as to the nature of the photosphere is of the slightest value. And it is no easy task to frame one bringing it into harmony with other circumstances equally well assured, and equally rigid in their consequences. Three alternative hypothesis may be said to exhaust the possibilities of the subject. They are as follows:—

1. The photosphere is a surface of condensation for unknown materials capable of maintaining the liquid or solid state at a transcendent degree of heat.

2. It is a surface of condensation for known materials under unknown conditions.

3. It is no true surface of condensation, the substances composing it being, although viscous, still vaporous.

Now each of these explanations is largely an appeal to ignorance, and so far scarcely deserves to be ranked as an explanation at all. Yet one of them must be fundamentally true. The first may be dismissed as contradictory of a strong consensus of evidence. The third involves glaring incongruities, both with what can be seen and with what must be inferred. There remains only the second. We seem bound to adopt the view that the sun is veritably clothed in a kind of cocoon—a web of incandescent filaments. It is perhaps of mixed composition. The surface is irregular. It comprises “fleece-like floors” at apparently different levels. Possibly they represent the successive condensations of various substances—silicon, carbon, titanium, vanadium, platinum, to mention a few of those most resistant to heat. The diversity of their emissive powers might contribute to produce the _tonings_ brought out in photographs of the disc, and the arrangement would be analogous to the surmised replacement in our upper air of aqueous by carbonic acid cloud-fields. But the postulated “unknown conditions” needed to enforce condensation at the enormous temperature of the photosphere may long continue to baffle the scientific imagination.

A darkening of the sun’s disc towards the limb is obvious telescopically, and conspicuous photographically. Its amount, measured by Bouguer in 1729, formed the basis of Laplace’s calculation that the arrest of light indicated was no less than eleven-twelfths of the entire. The data were correct, but the result, owing to certain mistaken assumptions, was greatly in error. Modern authorities, nevertheless, are far from being unanimous on the subject. Pickering finds that the intrinsic lustre of the sun exceeds its apparent lustre four and two-third times; the disparity, according to Vogel, is about twofold. There are, however, distinctions to be made. The absorption in the solar, as in the terrestrial atmosphere, is markedly selective. The brunt of its attack falls upon the most refrangible rays. Father Secchi noticed in 1870, and Professor Langley again in 1875, that the light from the limb is, in consequence, tinged with chocolate brown, while that from the central parts of the disc seems bluish by comparison. This general indication was, in 1877, analysed by Dr. Vogel,[117] who, by detailed measurements with a spectral photometer constructed on the polarising principle, ascertained that 30 per cent of the red, but only 13 per cent of the violet marginal rays penetrate the solar atmosphere. Hence an alteration in tint corresponding in its mode of origin to the ruddy suffusion of the setting sun. Now Seeliger has pointed out that selective absorption implies a medium of high refractive power; but equivalent conditions might, according to E. von Oppolzer,[118] be supplied by “a rare atmosphere in which flying particles are suspended.” It is, beyond doubt, an exceedingly shallow one. This was inferred by Vogel from the rapid degradation of light towards the edge of the disc, and it is rendered patent to sense by the brilliancy of facular summits, which, rising above the absorptive strata, shine unveiled against the dusky limb. Obviously, then, the darkening effect is produced in the immediate neighbourhood of the photosphere. It cannot be due to _cool_ gases, and _hot_ gases stop light distinctively in isolated beams. An alternative hypothesis was suggested some time ago by Professor Hastings of New Haven. The sun’s so-called “atmosphere” is, in this view, nothing more than a smoke-laden stratum.[119] Minute solid particles of carbon or silicon, carried upward from the photospheric clouds, are the agents of obscuration. The assumption of a solar analogue to a London fog is certainly a daring expedient, yet none more satisfactory is at present available.

The sun’s “veil” is indeed particularly difficult to fit in with the rest of its economy. It manifestly exists, and the position seemingly prescribed for it is between the photosphere and the reversing layer, although Dr. Scheiner prefers to place it in chromospheric regions.[120] In some torrid locality, at any rate, it exercises a kind of action characteristic of cool substances. Its composition out of refrigerated materials is strongly indicated. The refrigeration, however, may be excessively transient as regards each individual particle, although permanent in their aggregate. The general effect is to diminish the sun’s heat by one-third or one-half, and its light by fully two-thirds, with an attendant change to pale primrose of its original glacier-green tint.

Among the many enigmas of solar physics there is none more curious or more evasive than that which confronts us in this intimate appurtenance of the photosphere. Even the lines of approach to it are very few. Yet some are practicable, and almost untried. Researches of a special kind into the spectra of sun-spots should help towards its elucidation; still more, perhaps, careful spectroscopic comparisons with the obscurer interstitial spaces of the brilliant granules strewing the solar surface. If these are relatively dark through a mere lowering of temperature, then little can be learned from them in this connection. But if they are dark through increased absorption, it will be important to determine whether the absorption corresponds to that produced by the problematical “veil.” Are they, in other words, sinks for solar _soot_? Do they mark the lines of subsidence of the same refuse materials which by their interposition dim and tarnish the shining face of the sun? A definite reply to the question would bring us preceptibly nearer to the goal of our inquiries into the nature of the sun’s “smoke” envelope. What cannot be doubted is the importance of its function as a regulator of the sun’s output of energy. This has for the first time been adequately discussed by Dr. J. Halm in a paper incorporated with the _Annals of the Royal Observatory, Edinburgh_.[121] His views on the subject demand careful consideration.