Popular Science Monthly/Volume 5/September 1874/The Photosphere and Sun-Spots
|THE PHOTOSPHERE AND SUN-SPOTS.|
WHY is it that almost any one who was offered the opportunity of witnessing an eruption of Etna, or the effects of a tropical cyclone, would embrace such an occasion with eagerness, while phenomena so similar in kind, and on so far grander a scale, visible daily on the surface of the sun, excite a comparatively feeble interest in all but those devoted to their study?
It is doubtless, in part, because we have a more intimate and awe-inspired interest in disturbances which happen so near us, and which we do not extend to others with which we conceive we have less personal concern; but the difference of the kind, as well as the degree of our interest in solar phenomena, from that which we take in those which occur, as it were, at home, is due in part perhaps to a remoter cause, and forms a portion of the unconscious bias which the modern mind has inherited from ancient modes of thought.
Deeply impressed by the fact that the sun had warmed and lighted the world from unknown time, with a fire which never seemed to be fed, yet which never burned low like a terrestrial flame, ancient philosophy concluded that the sun was formed of something quite other than any gross earthly elements––of an element of pure fire, which shone and warmed forever without fuel, because it was its "nature" to; just as it is the "nature" of a fire on the hearth to burn only when fuel is supplied to it. The sun was, then, to the ancient world, a kind of supernatural phenomenon, interest in which partook more of the uninquiring awe due to an immediate miracle of Deity, than of the curiosity excited by a fact of the natural world; and whatever we may think of such a way of regarding the matter, the view of the ancient philosophy, that the sun was an immaculate orb of pure fire, self-sustained, continued to be accepted almost as a dogma of the faith down to times subsequent to the dawn of the modern philosophy.
When one of the first, possibly the first, of the observers of sunspots, Christopher Scheiner, a Jesuit, communicated his discovery to his provincial, the latter, Mr. Proctor relates, answered: "I have read Aristotle's writings from beginning to end, many times, and I can assure you I have nowhere found in them any thing similar to what you mention; go, therefore, my son, tranquillize yourself, be assured that what you take for spots in the sun are the faults of your glasses or your eyes."
Perhaps we are, however unconsciously, ourselves in some degree Aristotelians in such matters, and it is at least certain that the unrecognized influence of ancient modes of thought has delayed progress in solar physics, by preparing astronomers to admit theories which they could not have accepted with a clear recognition of the fact that physical laws are the same in the sun as here, however erroneously we learn them from our limited terrestrial experience. In the hypothesis of Wilson, for instance, that exhaustless flow of solar light and heat is made to come from a shallow stratum of brilliant cloud, surrounding a dark and presumably cold and solid globe. The elder Herschel adopts this hypothesis, with slight modification (it is not yet quite dislodged from the text-books), and even his eminent son appears to feel nothing like an imperious demand for a sustaining cause of the almost infinite flood of heat his own researches showed that these clouds must be giving.
It seems now extraordinary that men justly eminent as the Herschels could rest satisfied with an hypothesis which so evaded the consideration of the fundamental problem of the equality of the solar radiation, by tacitly assuming the suspension there of the most familiar laws of terrestrial experience.
The views now generally accepted contemplate the sun as hot throughout its mass, and in such a mass as containing indubitably an enormous though finite reservoir of heat. And, if, so far at least, there is an agreement even among those who differ as to the way in which this heat originated and is maintained, much is due to astronomers like Faye, who have insisted on this recognition of the need of accounting for the equable emission of heat with a success which may make us underrate our obligations to them, as this need, once enunciated, is so clear as to seem a truism, though it was far from being such till a very recent period.
We must, then, look at the sun with no more idea of witnessing any thing without the sphere of natural laws, than in looking at a fire across the street. It need not follow that we shall find the operation of those laws exactly the same that our limited experience has presented, and we shall still find abundant cause for admiration and wonder without introducing mysteries of our own creation.
When we telescopically examine that brilliant surface which we see daily with the naked eye (the photosphere), to study such of its phenomena as are here described, we do not need the spectroscope, but some means of protecting the eye from the blinding light and heat, and this should not involve the use of any colored shades. If we look into an unsilvered glass, as, for instance, into the panes of a shop-window from the street, we observe that it acts as a mirror, sending back a feeble reflection of ourselves, or other objects without; most of the rays from which have gone altogether through the glass, while a comparative few are returned to form the image. It may occur to us, then, instead of looking directly at the solar image formed by our telescope, to let it fall on a piece of plain glass, placed diagonally, through which about nineteen parts in twenty of the light will pass and be thrown away, the remaining twentieth being reflected and forming an exact though enfeebled image.
When this has been done, if the reflected image be still too bright, we may reflect it again, this time only a twentieth of the first twentieth reaching the eye, and so on to any degree; but it is strikingly illustrative of the intensity of the solar splendor that, when, by three such reflections, the sunlight has been enfeebled 8,000 times, we yet find it intolerably bright. Instead of more mirrors, it is better to now arrange that the third mirror may rotate, so as to polarize the light. When this is done, the image of the sun appears distinct (if the optician have done his work well), colorless, and of any brightness desired.
The instrument just described in general terms is known as the polarizing eye-piece. All danger and discomfort in studying the sun disappear with its use, and we may look at its unclouded face as though the eye had been strengthened to bear its light; in fact, many hours of scrutiny of the solar disk with this instrument wearies the eye less than a few minutes' telescopic examination of the moon does without it.
What we shall see with it is far from being that sphere of dazzling light, everywhere equally brilliant, which we have been accustomed to consider the sun. The eye ranges over a vast surface, presenting at one view over five thousand times the entire area of our globe, to find everywhere diversity of shade. It is not only darker near the edges than at the centre, but the whole (apart from any consideration of the spots) presents an appearance somewhat like that very peculiar one which the ocean has when we obtain a bird's-eye view of it from some great height.
Any one, for instance, who has looked down upon the Mediterranean from the summit of Gibraltar or Capri, and can recall its curious unlikeness to its familiar aspect—its apparent stillness, the faint intricate bands of white and gray, which, thus seen, overlap it like a net-work of broad veins, and the strangely permanent patterns left by the foam, which are entirely lost to us as we approach its tossing surface—will have a not wholly inadequate idea of the first impression made by the sight of the photosphere. If we bring ourselves nearer, as it were, by an increase of magnifying power, we lose sight of the larger masses of light and shade, whose place is filled by a curious mottling of faint, inextricably confused, and intermingled moss-like patterns.
With the best optical aid, and in those rarer moments when our own atmosphere is comparatively tranquil, we discern that the whole of these cloud-like mottlings are composed of very minute definite oval forms, which have been compared to grains of rice.
Minute as they appear, their real size is very great; for, though in a large telescope they seem mere dots, the average area of each is certainly much over 100,000 square miles. Since we see them at all, it must be owing to some inequality of brightness which distinguishes them, and, in fact, they do not seem to be in absolute contact, but present rather the appearance of numberless little white clouds, arranged with a sort of order upon a background of darker sky, or, if we compare them to rice-grains, we may suppose the grains arranged in rude tesselated patterns upon a gray cloth.
The most extraordinary conjectures have been hazarded as to the real nature of these objects, which are of somewhat recent discovery, and which are so difficult of observation that few have distinctly seen them. Whatever these things may be, they are the principal source of the sun's light, and presumably of its heat, and this adds to the interest of their study.
The writer has given a considerable time to their observation, which can only be carried on successfully by patient waiting, and the employment of those scattered moments when the ever-perturbed atmosphere of the earth is relatively still. He has been led to conclude that these bodies are composed of still smaller forms, and that their total area is inconsiderable compared with that of the whole sun, for, though it is almost impossible to determine the aggregate space occupied by such minute things, the writer has been led to conclude that it can hardly exceed one-fifth of the solar surface, and may be much less. An inconsiderable part only of the solar light comes from the relatively dark background on which they appear, and, in reference to these still mysterious things, we may, then, partly adopt an expression which Huyghens used with regard to the faculæ, and say that there is indeed in the sun "something brighter than the sun itself." The expression will not appear a forced or exaggerated one if we reflect that (if we rely on the correctness of the observations just alluded to) the average brilliancy; of each of these bodies cannot be much less than five times that of full sunlight, "sunlight" itself being, in fact, caused by a dilution of their brilliance with that of the gray background which has been just compared to the cloth on which white grains are grouped.
There are inequalities in the brightness of these bodies, some of which fall below, while others as certainly exceed, the average. If we remember that each of them occupies an area larger than Great Britain, that this area is, throughout, brighter than the sun (in fact, not in metaphor), and that such enormous bodies, whatever they may be, exist in the sun in numbers which are almost incalculable, we reach up to some idea, but doubtless an inadequate one, of the incomprehensible vastness of the solar sphere, and of the interest of the problems it offers for our study.
In order to examine these bodies under other conditions, we must look at a sun-spot. Here, again, we find it difficult to conceive the vastness of the field of operations, for, including both branches, the "spot" represented in our engraving covers over 1,000,000,000 square miles. If we fail utterly to "realize" the extent this represents, we may, perhaps, derive aid from its comparison with some familiar terrestrial object. In the small circle, accordingly, the continents of North and South America have been drawn on the same scale as the spot, as they would appear; that is, if they were actually transported to the solar surface, placed beside the spot, and viewed, together with it, from the distance at which the earth is from the sun.
The engraving is from a drawing by the writer. This drawing, while representing the general outline of a particular spot, seen in March, 1873, embodies the result of many previous studies on similar ones, and it has been made much less with an attempt to gain pictorial effect than to truthfully present such features as will help to give some idea of the constitution of the solar surface.
We see that each branch of the spot consists of two main parts, an outer (the penumbra) and an inner (the umbra), and beyond this rude division little seems to have been observed till recent years. The knowledge of the real complexity of spot-structure and the fullness of detail needed to represent it are of such recent origin that Sir John Herschel, who, in the Cape-of-Good-Hope observations, has given a number of sun-spot drawings, points out, in one of them, the tendency to a radial structure, as something remarkable and nearly unnoticed; and the fact that so eminent an observer should have made the spots his careful study without detecting more of the structure since discovered, will illustrate the difficulties attendant on such an investigation. If we look at this, not merely as at a picture, but in the way in which we should examine a geological map, with the purpose, that is,
of studying the superficial details as indices of the real nature of the structure, we are naturally led to inquire whether the surface before us is a solid like one of our continents, or a fluid like our seas, or a vapor like our clouds. We know, at the outset, from abundant evidence, that the temperature of the sun, whatever it may be, is far above that at which the materials chiefly composing the earth's crust would become fluid. It is, then, at the outset, an unlikely supposition that the surface of the sun should be solid; but, independently of such considerations, the behavior of this, or any other spot, is decisive as against this alternative. It was formed and grew to its present size in a comparatively short time, and, according to past experience, it will shortly break up and disappear.
Besides its rotation with the sun, the spot has an absolute motion on it, advancing, as a whole, at a greater angular velocity round the solar axis than spots nearer its poles, besides having a slight oscillating movement, which carries it alternately nearer to and farther from the sun's equator; all this going on simultaneously with changes in its form and size. The spot then moves about on the sun as a ship on the ocean, or, to employ a less inaccurate simile, like a rent in the clouds of our sky, which, while turning with the earth, both shifts its place and alters its appearance from hour to hour, the spot not being something above the sun's surface, but a gap in and below it. We seem irresistibly led to the conclusion, then, that the surface of the sun is not a solid, and, considering this freedom of motion, we are led to question if it can even be a liquid, and whether we must not look upon it as wholly vapor-like.
But we may approach the spot and look within it for answers to these questions, though, as we do so, the reader should distinguish between the facts stated and the inferences drawn from them. As to the former, observers may be said to agree with little exception; as to the latter, astronomers are, in some cases, at variance, and what follows is chiefly confined to a statement of fact, since a review of opposing hypotheses would not be at present in place.
The approach to the spot is scarcely marked in the engraving by any variation of the surface; though there is, in reality, a very slight blurring of the luminous masses (rice-grains) which makes these look less distinct as we draw near the edge. Here, all at once, the appearance changes. We are approaching what is really the outer rim of an enormous shallow funnel (that shown is 20,000 miles across); shallow, that is, in the outer portion only which is saucer-shaped, while the spout of the funnel is indefinitely deep. The first or gentle slope is the penumbra. It does not shade off into the photosphere, but begins, as has just been said, abruptly; and this sudden transition is a thing to be noted. We also observe that the edge is extremely irregular—full of indentations and subdivisions, patches of the photosphere pushing out here and there over it, and at times apparently hanging suspended above the abyss.
The sides of the slope are filled with what seem at first sight like long white threads, radially disposed, so that a circular penumbra, looked at casually, has somewhat the appearance of the iris surrounding the pupil. A closer look shows that these threads are not ranged exactly radially (in some spots not at all so). They are often contorted and thrown over each other, and evince a tendency to curl into sickle-shaped curves as they approach their inner extremities; while, if we examine them at the penumbral circumference, we find them to be apparent prolongations of those minute white objects to which the light of the sun has just been referred.
These "threads" or "filaments" are difficult of observation, for their average thickness is probably not over 200 miles, a width quite invisible at the sun's distance, in any thing but a very good telescope. Some appearances make it probable, however, that they are composed of filaments still finer, just as the finest silk thread is made up of numerous fibres, and they have a certain disposition to unite in fascicles, which are often mistaken for them. The dimension of 200 miles, then, is somewhat an arbitrary one, marking perhaps rather the present limit of vision of our telescopes than any real limit of the actual size; but, however this may be, the extraordinary length of these filaments is not open to question; they are quite commonly met with three or four thousand miles long, and the writer has occasionally distinctly traced one of these attenuated forms uninterruptedly through a much greater distance. What they are is still unknown.
What are the forces which cause the spot to move as a whole upon the solar surface, and what are the nature and direction of those which modify its form, and so completely change in a few days, or even hours, the disposition of its parts over its so vast area? To the first question there is, as yet, no satisfactory answer, though our knowledge, such as it is, seems to point to a constant interchange of matter between the surface of the sun and its interior, far within which seem to be impressed on the ascending currents velocities of rotation which so modify those which obtain at the surface. As to the second, the spectroscope, if appealed to, offers but very partial help, and we here restrict ourselves to a description of methods which do not involve its use. How may we determine the directions of the currents which we cannot doubt exist within the spot?
It has happened to the writer to be lost in one of the shallow, labyrinthine lakes, in the interior of our Northern wilderness, on whose still waters the canoe was left to drift aimlessly with the wind, while the guide sought, at first vainly, the traces of some current which would indicate the direction of the outlet; till, looking below the surface, the common direction of the extremities of the water-grasses, rooted at the bottom, showed the existence and direction of a current otherwise unperceived, and gave the question its solution.
The long filaments of the penumbra may be used in a similar way, flexible as they are, and rooted, as it were, at one end, while the other sways in the currents of the solar atmosphere, yielding to it as freely as the grasses to the water, or a streamer to the air; and the analogy is noteworthy in this: that one end of the filament is commonly made fast, while the other is left free. This is so very generally the case that we see that, if these filaments be clouds, they differ from ours in other circumstances than their shape. Those we are studying are bent into curves, which show that the solar winds frequently move in circular sweeps, and are, to a considerable degree, comparable with our cyclones. Long, twisted ropes are sometimes formed by them, one being thrown over another; and, in cloud-like masses, they at times move over and conceal lower portions of the penumbra—the abrupt changes in the directions of motion showing us that these are superposed strata of what, for want of a better word, we must call solar clouds, which drift across each other's course occasionally, nearly at right angles, while the ever-moving whirlwinds leave an unmistakable record of their action on these pliant forms.
In one part of the spot, one of these has been bent into a complete loop, or closed curve, the extremity showing a fringe of ragged strands, like that of a broken rope. The immensely more extended scale of the action here being kept in view, and the fact that the whole spot is being changed in all its parts—even while we are looking at it—by alterations which, though apparently gradual, are really the indications of an immense energy, it will be seen that, considered merely as a spectacle of the play of natural forces, we have before us something almost incomparably greater than any which the terrestrial volcano, earthquake, or cyclone, can offer. The entire surface of the earth, were it spread out into a plain, would be, in fact, of inconsiderable size as compared with either branch of the spot we are examining.
The quickness of the transformations that the observer sometimes notes here is wonderful. Lockyer, Young, and other observers, have demonstrated the existence of chromospheric movements, in some instances, at the rate of over 100 miles a second; and the velocities in the photosphere are, occasionally, of a similar order of magnitude. As an instance, it may be mentioned that the loop in question, which inclosed an area of about 3,000,000 square miles (not far from that of the United States), broke up, and seemingly melted away, like a snow-wreath before a fire, in little over a quarter of an hour. How vain the attempt must be to adequately realize to ourselves the features of such a cataclysm seen close at hand!
It is quite impracticable to convey an adequate idea of the complexity, strangeness, and beauty of these penumbral forms by an engraving; and the description is likely to fail equally, both on account of the unlikeness of the appearances to any thing with which we are familiar, and the difficulty of using any descriptive terms which, drawn from terrestrial analogies, will not here prove inaccurate. A plume-like form, in the upper portion of the spot, is necessarily but an imperfect memorandum of an appearance, in reality all but impossible to render with the pencil, even on a scale which depicted it a hundred times this size. It might be likened to a sheet of glass, covered with the most intricate and capricious patterns the frost ever traces on our window-panes in winter; but, together with this, there was something flame-like in the graceful terminal curves, and something strangely suggestive of fern-like vegetation about the whole. This double and apparently incompatible impression of something at once crystalline and plant-like is strikingly conveyed by many of the penumbral forms, and yet the description will doubtless appear incongruous to any but the few who have seen for themselves. The comparison of the frost-figures is the least inapt, perhaps, to be found, but it is really impossible to obtain an accurate one, when we have no single thing on earth which we can exactly liken it to. When we consider that this extraordinary shape occupied a greater area than the North and South American Continents united while that, over the whole, obtained a temperature far above that of the white flame which plays about the mouth of a furnace, and that its parts turned, as the observer looked, from one evanescent beautiful form to another, with a rapidity of change which indicated the existence of inconceivable force, we need feel less surprise that any metaphor, necessarily drawn from our limited terrestrial analogies, should so fail to convey an adequate idea of what the writer is certain he has seen, but confesses he cannot properly describe.
The umbra or dark inner shade, commences as abruptly as the penumbra, but the contrast between it and the penumbral edge is far greater than between that and the photosphere. We possess no very accurate photometric determinations of the relative light of these portions of the spot, and nothing seems practicable beyond a rough averaging where the umbræ are themselves of such various tints. If we adopt the somewhat crude determinations of the elder Herschel, we may assume that the penumbra is, as a whole, rather less than half as bright as the photosphere, and the umbra about one-seventieth of the brightness of the penumbra. More accurately, if we represent the average brightness of the photosphere by 1,000, that of the penumbra will be denoted by 469, and that of the umbra by only 7. The umbra appears, at first sight, to be black, but this is only from contrast with the superior brightness around it. It is certain, for instance, that sunlight is at least 200,000 times brighter than moonlight (probably more). The umbra, then, if it be but seven thousandths of the brightness of the surface, is still 1,400 times (at least) as bright as the moon, or far brighter than the calcium-light. The absolute depth of the inner edge of the penumbra, below the surface, is not very great, according to M. Faye, and probably not over from 2,000 to 4,000 miles. (Every thing is relative; and, on the sun, 2,000 miles is little for the depth of a cavity which may be from ten to twenty times this width.)
Somewhere about this lower level commences the umbra, which has been already compared to the spout of the funnel, of which the penumbra formed the upper shallow cone; and, through these umbral shades, the eye looks down to quite unknown depths.
The darkest parts are far from being black, however they may appear by contrast, the very "blackest" part being radiant with a light which would appear intolerable to the unshielded eye. Brown and reddish tints are occasionally seen here with the polarizing eye-piece. These, the spectroscope shows, are due to incandescent hydrogen; but a common tint, which is particularly that of the nuclei or deeper umbral shades, is a very pure, violet.
It is impossible to do more, in such an article as this, than to outline, in the briefest way, a few of the more prominent appearances of the spots and solar surface, without attempting any description of the laws which regulate their respective motions, and the emission of their light and heat, and without alluding to the numerous other topics of interest to the student. The reader will not, it may be hoped, on this account derive the impression that his attention has been invited to a description of superficial solar phenomena, merely as spectacles. In this point of view alone, certainly, we cannot contemplate them without lively wonder, but their deeper interest will lie in the light they shed on the nature of the sun itself, and the laws which govern that flow of light and heat through which alone we ourselves live and move. Experience seems to indicate that, according as these wonderful phenomena are studied with or without the spectroscope, they are assimilated more, in the observer's mind, with such terrestrial motions as those we call eruptions in the first instance, or cyclones in the second. It would be generalizing from a partial view, therefore, to present the reader with any single hypothesis at present, especially while those versed in the study find so much that, on any hypothesis, is still mysterious.