We naturally begin with our peerless sun

The Story of the Heavens by Robert S. Ball is part of the HackerNoon Books Series. You can jump to any chapter in this book here. THE SUN.

CHAPTER II. THE SUN.

The vast Size of the Sun—Hotter than Melting Platinum—Is the Sun the Source of Heat for the Earth?—The Sun is 92,900,000 miles distant—How to realise the magnitude of this distance—Day and Night—Luminous and Non-Luminous Bodies—Contrast between the Sun and the Stars—The Sun a Star—Granulated Appearance of the Sun—The Spots on the Sun—Changes in the Form of a Spot—The Faculæ—The Rotation of the Sun on its Axis—View of a Typical Sun-Spot—Periodicity of the Sun-Spots—Connection between the Sun-Spots and Terrestrial Magnetism—Principles of Spectrum Analysis—Substances present in the Sun—Spectrum of a Spot—The Prominences surrounding the Sun—Total Eclipse of the Sun—Size and Movement of the Prominences—Their connection with the Spots—Spectroscopic Measurement of Motion on the Sun—The Corona surrounding the Sun—Constitution of the Sun.

In commencing our examination of the orbs which surround us, we naturally begin with our peerless sun. His splendid brilliance gives him the pre-eminence over all other celestial bodies.

The dimensions of our luminary are commensurate with his importance. Astronomers have succeeded in the difficult task of ascertaining the exact figures, but they are so gigantic that the results are hard to realise. The diameter of the orb of day, or the length of the axis, passing through the centre from one side to the other, is 866,000 miles. Yet this bare statement of the dimensions of the great globe fails to convey an adequate idea of its vastness. If a railway were laid round the sun, and if we were to start in an express train moving sixty miles an hour, we should have to travel for five years without intermission night or day before we had accomplished the journey.

When the sun is compared with the earth the bulk of our luminary becomes still more striking. Suppose his globe[Pg 30] were cut up into one million parts, each of these parts would appreciably exceed the bulk of our earth. Fig. 10 exhibits a large circle and a very small one, marked S and E respectively. These circles show the comparative sizes of the two bodies. The mass of the sun does not, however, exceed that of the earth in the same proportion. Were the sun placed in one pan of a mighty weighing balance, and were 300,000 bodies as heavy as our earth placed in the other, the luminary would turn the scale.

Fig. 10.—Comparative Size of the Earth and the Sun.

The sun has a temperature far surpassing any that we artificially produce, either in our chemical laboratories or our metallurgical establishments. We can send a galvanic current through a piece of platinum wire. The wire first becomes red hot, then white hot; then it glows with a brilliance almost dazzling until it fuses and breaks. The temperature of the melting platinum wire could hardly be surpassed in the most elaborate furnaces, but it does not attain the temperature of the sun.

It must, however, be admitted that there is an apparent discrepancy between a fact of common experience and the[Pg 31] statement that the sun possesses the extremely high temperature that we have just tried to illustrate. "If the sun were hot," it has been said, "then the nearer we approach to him the hotter we should feel; yet this does not seem to be the case. On the top of a high mountain we are nearer to the sun, and yet everybody knows that it is much colder up there than in the valley beneath. If the mountain be as high as Mont Blanc, then we are certainly two or three miles nearer the glowing globe than we were at the sea-level; yet, instead of additional warmth, we find eternal snow." A simple illustration may help to lessen this difficulty. In a greenhouse on a sunshiny day the temperature is much hotter than it is outside. The glass will permit the hot sunbeams to enter, but it refuses to allow them out again with equal freedom, and consequently the temperature rises. The earth may, from this point of view, be likened to a greenhouse, only, instead of the panes of glass, our globe is enveloped by an enormous coating of air. On the earth's surface, we stand, as it were, inside the greenhouse, and we benefit by the interposition of the atmosphere; but when we climb very high mountains, we gradually pass through some of the protecting medium, and then we suffer from the cold. If the earth were deprived of its coat of air, it seems certain that eternal frost would reign over whole continents as well as on the tops of the mountains.

The actual distance of the sun from the earth is about 92,900,000 miles; but by merely reciting the figures we do not receive a vivid impression of the real magnitude. It would be necessary to count as quickly as possible for three days and three nights before one million was completed; yet this would have to be repeated nearly ninety-three times before we had counted all the miles between the earth and the sun.

Every clear night we see a vast host of stars scattered over the sky. Some are bright, some are faint, some are grouped into remarkable forms. With regard to this multitude of brilliant points we have now to ask an important question. Are they bodies which shine by their own light like the sun,[Pg 32] or do they only shine with borrowed light like the moon? The answer is easily stated. Most of those bodies shine by their own light, and they are properly called stars.

Suppose that the sun and the multitude of stars, properly so called, are each and all self-luminous brilliant bodies, what is the great distinction between the sun and the stars? There is, of course, a vast and obvious difference between the unrivalled splendour of the sun and the feeble twinkle of the stars. Yet this distinction does not necessarily indicate that our luminary has an intrinsic splendour superior to that of the stars. The fact is that we are nestled up comparatively close to the sun for the benefit of his warmth and light, while we are separated from even the nearest of the stars by a mighty abyss. If the sun were gradually to retreat from the earth, his light would decrease, so that when he had penetrated the depths of space to a distance comparable with that by which we are separated from the stars, his glory would have utterly departed. No longer would the sun seem to be the majestic orb with which we are familiar. No longer would he be a source of genial heat, or a luminary to dispel the darkness of night. Our great sun would have shrunk to the insignificance of a star, not so bright as many of those which we see every night.

Momentous indeed is the conclusion to which we are now led. That myriad host of stars which studs our sky every night has been elevated into vast importance. Each one of those stars is itself a mighty sun, actually rivalling, and in many cases surpassing, the splendour of our own luminary. We thus open up a majestic conception of the vast dimensions of space, and of the dignity and splendour of the myriad globes by which that space is tenanted.

There is another aspect of the picture not without its utility. We must from henceforth remember that our sun is only a star, and not a particularly important star. If the sun and the earth, and all which it contains, were to vanish, the effect in the universe would merely be that a tiny star had ceased its twinkling. Viewed simply as a star, the sun must retire to a position of insignificance in the mighty fabric[Pg 33] of the universe. But it is not as a star that we have to deal with the sun. To us his comparative proximity gives him an importance incalculably transcending that of all the other stars. We imagined ourselves to be withdrawn from the sun to obtain his true perspective in the universe; let us now draw near, and give him that attention which his supreme importance to us merits.

Fig. 11.—The Sun, photographed on September 22, 1870.

To the unaided eye the sun appears to be a flat circle. If, however, it be examined with the telescope, taking care of course to interpose a piece of dark-coloured glass, or to employ some similar precaution to screen the eye from injury, it will then be perceived that the sun is not a flat surface, but a veritable glowing globe.

The first question which we must attempt to answer[Pg 34] enquires whether the glowing matter which forms the globe is a solid mass, or, if not solid, which is it, liquid or gaseous? At the first glance we might think that the sun cannot be fluid, and we might naturally imagine that it was a solid ball of some white-hot substance. But this view is not correct; for we can show that the sun is certainly not a solid body in so far at least as its superficial parts are concerned.

A general view of the sun as shown by a telescope of moderate dimensions may be seen in Fig. 11, which is taken from a photograph obtained by Mr. Rutherford at New York on the 22nd of September, 1870. It is at once seen that the surface of the luminary is by no means of uniform texture or brightness. It may rather be described as granulated or mottled. This appearance is due to the luminous clouds which float suspended in a somewhat less luminous layer of gas. It is needless to say that these solar clouds are very different from the clouds which we know so well in our own atmosphere. Terrestrial clouds are, of course, formed from minute drops of water, while the clouds at the surface of the sun are composed of drops of one or more chemical elements at an exceedingly high temperature.

The granulated appearance of the solar surface is beautifully shown in the remarkable photographs on a large scale which M. Janssen, of Meudon, has succeeded in obtaining during the last twenty years. We are enabled to reproduce one of them in Fig. 12. It will be observed that the interstices between the luminous dots are of a greyish tint, the general effect (as remarked by Professor Young) being much like that of rough drawing paper seen from a little distance. We often notice places over the surface of such a plate where the definition seems to be unsatisfactory. These are not, however, the blemishes that might at first be supposed. They arise neither from casual imperfections of the photographic plate nor from accidents during the development; they plainly owe their origin to some veritable cause in the sun itself, nor shall we find it hard to explain what that cause must be. As we shall have occasion to mention further on, the velocities with which the glowing gases on the sun are animated must be exceedingly great. Even in the hundredth part of a second (which is about the duration of the exposure of this plate) the movements of the solar clouds are sufficiently great to produce the observed indistinctness.

Fig. 12.—Photograph of the Solar Surface.
(By Janssen.)

Irregularly dispersed over the solar surface small dark objects called sun-spots are generally visible. These spots vary greatly both as to size and as to number. Sun-spots were first noticed in the beginning of the seventeenth century, shortly after the invention of the telescope. Their general appearance is shown in Fig. 13, in which the dark central nucleus appears in sharp contrast with the lighter margin or penumbra. Fig. 16 shows a small spot developing out of one of the pores or interstices between the granules.

Fig. 13.—An Ordinary Sun-spot.

The earliest observers of these spots had remarked that they seem to have a common motion across the sun. In Fig. 14 we give a copy of a remarkable drawing by Father Scheiner, showing the motion of two spots observed by him in March, 1627. The figure indicates the successive positions assumed by the spots on the several days from the 2nd to the 16th March. Those marks which are merely given in outline represent the assumed positions on the 11th and the 13th, on which days it happened that the weather was cloudy, so that no observations could be made. It is invariably found that these objects move in the same direction—namely, from the eastern to the western limb[3] of the sun. They complete the journey across the face of the sun in twelve or thirteen days, after which they remain invisible for about the same length of time until they reappear at the eastern limb. These early observers were quick to discern the true import of their discovery. They deduced from these simple observations the remarkable fact that the sun, like the earth, performs a rotation on its axis, and in the same direction. But there is the important difference between these rotations that whereas the earth takes only twenty-four hours to turn once round, the solar globe takes about twenty-six days to complete one of its much more deliberate rotations.

PLATE III.
SPOTS AND FACULÆ ON THE SUN.
(FROM A PHOTOGRAPH BY MR. WARREN DE LA RUE, 20TH SEPT., 1861.)

[Pg 37]

If we examine sun-spots under favourable atmospheric conditions and with a telescope of fairly large aperture, we perceive a great amount of interesting detail which is full of information with regard to the structure of the sun. The penumbra of a spot is often found to be made up of filaments directed towards the middle of the spot, and generally brighter at their inner ends, where they adjoin the nucleus. In a regularly formed spot the outline of the penumbra is of the same general form as that of the nucleus, but astronomers are frequently deeply interested by witnessing vast spots of very irregular figure. In such cases the bright surface-covering of the sun (the photosphere, as it is called) often encroaches on the nucleus and forms a peninsula stretching out into, or even bridging across, the gloomy interior. This is well shown in Professor Langley's fine drawing (Plate II.) of a very irregular spot which he observed on December 23–24, 1873.

The details of a spot vary continually; changes may often be noticed even from day to day, sometimes from hour to hour. A similar remark may be made with respect to the bright streaks or patches which are frequently to be observed especially in the neighbourhood of spots. These bright marks are known by the name of faculæ (little torches). They are most distinctly seen near the margin of the sun, where the light from its surface is not so bright as it is nearer to the centre of the disc. The reduction of light at the margin is due to the greater thickness of absorbing atmosphere round the sun, through which the light emitted from the regions near the margin has to pass in starting on its way towards us.

None of the markings on the solar disc constitute permanent features on the sun. Some of these objects may no doubt last for weeks. It has, indeed, occasionally happened that the same spot has marked the solar globe for many months; but after an existence of greater or less duration those on one part of the sun may disappear, while as frequently fresh marks of the same kind become visible in other places. The inference from these various facts is[Pg 38] irresistible. They tell us that the visible surface of the sun is not a solid mass, is not even a liquid mass, but that the globe, so far as we can see it, consists of matter in the gaseous, or vaporous, condition.

Fig. 14.—Scheiner's Observations on Sun-spots.

It often happens that a large spot divides into two or more separate portions, and these have been sometimes seen to fly apart with a velocity in some cases not less than a thousand miles an hour. "At times, though very rarely" (I quote here Professor Young,[4] to whom I am frequently indebted), "a different phenomenon of the most surprising and startling character appears in connection with these objects: patches of intense brightness suddenly break out, remaining visible for a few minutes, moving, while they[Pg 39] last, with velocities as great as one hundred miles a second."

Fig. 15.—Zones on the Sun's Surface in which Spots appear.

"One of these events has become classical. It occurred on the forenoon (Greenwich time) of September 1st, 1859, and was independently witnessed by two well-known and reliable observers—Mr. Carrington and Mr. Hodgson—whose accounts of the matter may be found in the Monthly Notices of the Royal Astronomical Society for November, 1859. Mr. Carrington at the time was making his usual daily observations upon the position, configuration, and size of the spots by means of an image of the solar disc upon a screen—being then engaged upon that eight years' series of observations which lie at the foundation of so much of our present solar science. Mr. Hodgson, at a distance of many miles, was at the same time sketching details of sun-spot structure by means of a solar eye-piece and shade-glass. They simultaneously saw two luminous objects, shaped something like two new moons, each about eight thousand miles in length and two thousand wide, at a distance of some twelve thousand miles from each other. These burst suddenly into sight at the edge of a great sun-spot with a dazzling brightness at least five or six times that of the neighbouring portions of the photosphere, and moved eastward over the spot in parallel lines, growing smaller and fainter, until in about five minutes they disappeared, after traversing a course of nearly thirty-six thousand miles."

The sun-spots do not occur at all parts of the sun's surface indifferently. They are mainly found in two zones (Fig. 15) on each side of the solar equator between the latitudes of 10° and 30°. On the equator the spots are rare except, curiously enough, near the time when there are few spots elsewhere. In high latitudes they are never seen. Closely connected with these peculiar principles of their distribution[Pg 40] is the remarkable fact that spots in different latitudes do not indicate the same values for the period of rotation of the sun. By watching a spot near the sun's equator Carrington found that it completed a revolution in twenty-five days and two hours. At a latitude of 20° the period is about twenty-five days and eighteen hours, at 30° it is no less than twenty-six days and twelve hours, while the comparatively few spots observed in the latitude of 45° require twenty-seven and a half days to complete their circuit.

As the sun, so far at least as its outer regions are concerned, is a mass of gas and not a solid body, there would be nothing incredible in the supposition that spots are occasionally endowed with movements of their own like ships on the ocean. It seems, however, from the facts before us that the different zones on the sun, corresponding to what we call the torrid and temperate zones on the earth, persist in rotating with velocities which gradually decrease from the equator towards the poles. It seems probable that the interior parts of the sun do not rotate as if the whole were a rigidly connected mass. The mass of the sun, or at all events its greater part, is quite unlike a rigid body, and the several portions are thus to some extent free for independent motion. Though we cannot actually see how the interior parts of the sun rotate, yet here the laws of dynamics enable us to infer that the interior layers of the sun rotate more rapidly than the outer layers, and thus some of the features of the spot movements can be accounted for. But at present it must be confessed that there are great difficulties in the way of accounting for the distribution of spots and the law of rotation of the sun.

In the year 1826 Schwabe, a German astronomer, commenced to keep a regular register of the number of spots visible on the sun. After watching them for seventeen years he was able to announce that the number of spots seemed to fluctuate from year to year, and that there was a period of about ten years in their changes. Subsequent observations have confirmed this discovery, and old books and manuscripts have been thoroughly searched for information of early date.[Pg 41] Thus a more or less complete record of the state of the sun as regards spots since the beginning of the seventeenth century has been put together. This has enabled astronomers to fix the period of the recurring maximum with greater accuracy.

The course of one of the sun-spot cycles may be described as follows: For two or three years the spots are both larger and more numerous than on the average; then they begin to diminish, until in about six or seven years from the maximum they decline to a minimum; the number of the spots then begins to increase, and in about four and a half years the maximum is once more attained. The length of the cycle is, on an average, about eleven years and five weeks, but both its length and the intensity of the maxima vary somewhat. For instance, a great maximum occurred in the summer of 1870, after which a very low minimum occurred in 1879, followed by a feeble maximum at the end of 1883; next came an average minimum about August, 1889, followed by the last observed maximum in January, 1894. It is not unlikely that a second period of about sixty or eighty years affects the regularity of the eleven-year period. Systematic observations carried on through a great many years to come will be required to settle this question, as the observations of sun-spots previous to 1826 are far too incomplete to decide the issues which arise.

A curious connection seems to exist between the periodicity of the spots and their distribution over the surface of the sun. When a minimum is about to pass away the spots generally begin to show themselves in latitudes about 30° north and south of the sun's equator; they then gradually break out somewhat nearer to the equator, so that at the time of maximum frequency most of them appear at latitudes not greater than 16°. This distance from the sun's equator goes on decreasing till the time of minimum. Indeed, the spots linger on very close to the equator for a couple of years more, until the outbreak signalising the commencement of another period has commenced in higher latitudes.

We have still to note an extraordinary feature which[Pg 42] points to an intimate connection between the phenomena of sun-spots and the purely terrestrial phenomena of magnetism. It is of course well known that the needle of a compass does not point exactly to the north, but diverges from the meridian by an angle which is different in different places and is not even constant at the same place. For instance, at Greenwich the needle at present points in a direction 17° West of North, but this amount is subject to very slow and gradual changes, as well as to very small daily oscillations. It was found about fifty years ago by Lamont (a Bavarian astronomer, but a native of Scotland) that the extent of this daily oscillation increases and decreases regularly in a period which he gave as 10-1⁄3 years, but which was subsequently found to be 11-1⁄10 years, exactly the same as the period of the spots on the sun. From a diligent study of the records of magnetic observations it has been found that the time of sun-spot maximum always coincides almost exactly with that of maximum daily oscillation of the compass needle, while the minima agree similarly. This close relationship between the periodicity of sun-spots and the daily movements of the magnetic needle is not the sole proof we possess that there is a connection of some sort between solar phenomena and terrestrial magnetism. A time of maximum sun-spots is a time of great magnetic activity, and there have even been special cases in which a peculiar outbreak on the sun has been associated with remarkable magnetic phenomena on the earth. A very interesting instance of this kind is recorded by Professor Young, who, when observing at Sherman on the 3rd August, 1872, perceived a very violent disturbance of the sun's surface. He was told the same day by a member of his party, who was engaged in magnetic observations and who was quite in ignorance of what Professor Young had seen, that he had been obliged to desist from his magnetic work in consequence of the violent motion of his magnet. It was afterwards found from the photographic records at Greenwich and Stonyhurst that the magnetic "storm" observed in America had simultaneously been felt in England. A similar connection between sun-spots and the aurora borealis has[Pg 43] also been noticed, this fact being a natural consequence of the well-known connection between the aurora and magnetic disturbances. On the other hand, it must be confessed that many striking magnetic storms have occurred without any corresponding solar disturbance,[5] but even those who are inclined to be sceptical as to the connection between these two classes of phenomena in particular cases can hardly doubt the remarkable parallelism between the general rise and fall in the number of sun-spots and the extent of the daily movements of the compass needle.

Fig. 16.—The Texture of the Sun and a small Spot.

We have now described the principal solar phenomena with which the telescope has made us acquainted. But there are many questions connected with the nature of the sun which not even the most powerful telescope would enable us to solve, but which the spectroscope has given us the means of investigating.

What we receive from the sun is warmth and light. The intensely heated mass of the sun radiates forth its beams in[Pg 44] all directions with boundless prodigality. Each beam we feel to be warm, and we see to be brilliantly white, but a more subtle analysis than mere feeling or mere vision is required. Each sunbeam bears marks of its origin. These marks are not visible until a special process has been applied, but then the sunbeam can be made to tell its story, and it will disclose to us much of the nature of the constitution of the great luminary.

We regard the sun's light as colourless, just as we speak of water as tasteless, but both of those expressions relate rather to our own feelings than to anything really characteristic of water or of sunlight. We regard the sunlight as colourless because it forms, as it were, the background on which all other colours are depicted. The fact is, that white is so far from being colourless that it contains every known hue blended together in certain proportions. The sun's light is really extremely composite; Nature herself tells us this if we will but give her the slightest attention. Whence come the beautiful hues with which we are all familiar? Look at the lovely tints of a garden; the red of the rose is not in the rose itself. All the rose does is to grasp the sunbeams which fall upon it, extract from these beams the red which they contain, and radiate that red light to our eyes. Were there not red rays conveyed with the other rays in the sunbeam, there could be no red rose to be seen by sunlight.

The principle here involved has many other applications; a lady will often say that a dress which looks very well in the daylight does not answer in the evening. The reason is that the dress is intended to show certain colours which exist in the sunlight; but these colours are not contained to the same degree in gaslight, and consequently the dress has a different hue. The fault is not in the dress, the fault lies in the gas; and when the electric light is used it sends forth beams more nearly resembling those from the sun, and the colours of the dress appear with all their intended beauty.

The most glorious natural indication of the nature of the sunlight is seen in the rainbow. Here the sunbeams are refracted and reflected from tiny globes of water in the clouds; these convey to us the sunlight, and in doing so decompose the white beams into the seven primary hues—red, orange, yellow, green, blue, indigo, and violet.

PLATE A.
THE SUN.
Royal Observatory, Greenwich, July 8, 1892.

Fig. 17.—The Prism.

The bow set in the cloud is typical of that great department of modern science of which we shall now set forth the principles. The globes of water decompose the solar beams; and we follow the course suggested by the rainbow, and analyse the sunlight into its constituents. We are enabled to do this with scientific accuracy when we employ that remarkable key to Nature's secrets known as the spectroscope. The beams of white sunlight consist of innumerable beams of every hue in intimate association. Every shade of red, of yellow, of blue, and of green, can be found in a sunbeam. The magician's wand, with which we strike the sunbeam and sort the tangled skein into perfect order, is the simple instrument known as the glass prism. We have represented this instrument in its simplest form in the adjoining figure (Fig. 17). It is a piece of pure and homogeneous glass in the shape of a wedge. When a ray of light from the sun or from any source falls upon the prism, it passes through the transparent glass and emerges on the other side; a remarkable change is, however, impressed on the ray by the influence of the glass. It is bent by refraction from the path it originally pursued, and is compelled to follow a different path. If, however, the prism bent all rays of light equally, then it would be of no service in the analysis of light; but it fortunately happens that the prism acts with varying efficiency on the rays of different hues. A red ray is not refracted so much as a yellow ray; a yellow ray is not refracted so much as a blue one. It consequently happens that when the composite beam of sunlight, in which all the different rays are blended, passes through the prism, they emerge in the manner shown in the annexed figure (Fig. 18). Here then we have the source of the analysing[Pg 46] power of the prism; it bends the different hues unequally and consequently the beam of composite sunlight, after passing through the prism, no longer shows mere white light, but is expanded into a coloured band of light, with hues like the rainbow, passing from deep red at one end through every intermediate grade to the violet.

Fig. 18.—Dispersion of Light by the Prism.

We have in the prism the means of decomposing the light from the sun, or the light from any other source, into its component parts. The examination of the quality of the light when analysed enables us to learn something of the constitution of the body from which this light has emanated. Indeed, in some simple cases the mere colour of a light will be sufficient to indicate the source from which it has come. There is, for instance, a splendid red light sometimes seen in displays of fireworks, due to the metal strontium. The eye can identify the element by the mere colour of the flame. There is also a characteristic yellow light produced by the flame of common salt burned with spirits of wine. Sodium is the important constituent of salt, so here we recognise another substance merely by the colour it emits when burning. We may also mention a third substance, magnesium, which burns with a brilliant white light, eminently characteristic of the metal.

PLATE XIII.
SPECTRA OF THE SUN AND STARS.
I. SUN.
II. SIRIUS.
III. ALDEBARAN.
IV. BETELGEUZE.

[Pg 47]

The three metals, strontium, sodium, and magnesium, may thus be identified by the colours they produce when incandescent. In this simple observation lies the germ of the modern method of research known as spectrum analysis. We may now examine with the prism the colours of the sun and the colours of the stars, and from this examination we can learn something of the materials which enter into their composition. We are not restricted to the use of merely a single prism, but we may arrange that the light which it is desired to analyse shall pass through several prisms in succession in order to increase the dispersion or the spreading out of the different colours. To enter the spectroscope the light first passes through a narrow slit, and the rays are then rendered parallel by passing through a lens; these parallel rays next pass through one or more prisms, and are finally viewed through a small telescope, or they may be intercepted by a photographic plate on which a picture will then be made. If the beam of light passing through the slit has radiated from an incandescent solid or liquid body, or from a gas under high pressure, the coloured band or spectrum is found to contain all the colours indicated on Plate XIII., without any interruption between the colours. This is known as a continuous spectrum. But if we examine light from a gas under low pressure, as can be done by placing a small quantity of the gas in a glass tube and making it glow by an electric current, we find that it does not emit rays of all colours, but only rays of certain distinct colours which are different for different gases. The spectrum of a gas, therefore, consists of a number of detached luminous lines.

When we study the sunlight through the prism, it is found that the spectrum does not extend quite continuously from one end to the other, but is shaded over by a multitude of dark lines, only a few of which are shown in the adjoining plate. (Plate XIII.) These lines are a permanent feature in the solar spectrum. They are as characteristic of the sunlight as the prismatic colours themselves, and are full of interest and information with regard to the sun. These lines are the characters in which the history and the nature of the sun are written. Viewed through an instrument of adequate power, dark lines are to be found crossing the solar spectrum[Pg 48] in hundreds and in thousands. They are of every variety of strength and faintness; their distribution seems guided by no simple law. At some parts of the spectrum there are but few lines; in other regions they are crowded so closely together that it is difficult to separate them. They are in some places exquisitely fine and delicate, and they never fail to excite the admiration of every one who looks at this interesting spectacle in a good instrument.

There can be no better method of expounding the rather difficult subject of spectrum analysis than by actually following the steps of the original discovery which first gave a clear demonstration of the significance of the dark "Fraunhofer" lines. Let us concentrate our attention specially upon that line of the solar spectrum marked d. This, when seen in the spectroscope, is found to consist of two lines, very delicately separated by a minute interval, one of these lines being slightly thicker than the other. Suppose that while the attention is concentrated on these lines the flame of an ordinary spirit-lamp coloured by common salt be held in front of the instrument, so that the ray of direct solar light passes through the flame before entering the spectroscope. The observer sees at once the two lines known as d flash out with a greatly increased blackness and vividness, while there is no other perceptible effect on the spectrum. A few trials show that this intensification of the d lines is due to the vapour of sodium arising from the salt burning in the lamp through which the sunlight has passed.

It is quite impossible that this marvellous connection between sodium and the d lines of the spectrum can be merely casual. Even if there were only a single line concerned, it would be in the highest degree unlikely that the coincidence should arise by accident; but when we find the sodium affecting both of the two close lines which form d, our conviction that there must be some profound connection between these lines and sodium rises to absolute certainty. Suppose that the sunlight be cut off, and that all other light is excluded save that emanating from the glowing vapour of[Pg 49] sodium in the spirit flame. We shall then find, on looking through the spectroscope, that we no longer obtain all the colours of the rainbow; the light from the sodium is concentrated into two bright yellow lines, filling precisely the position which the dark d lines occupied in the solar spectrum, and the darkness of which the sodium flame seemed to intensify.

We must here endeavour to remove what may at first sight appear to be a paradox. How is it, that though the sodium flame produces two bright lines when viewed in the absence of other light, yet it actually appears to intensify the two dark lines in the sun's spectrum? The explanation of this leads us at once to the cardinal doctrine of spectrum analysis. The so-called dark lines in the solar spectrum are only dark by contrast with the brilliant illumination of the rest of the spectrum. A good deal of solar light really lies in the dark lines, though not enough to be seen when the eye is dazzled by the brilliancy around. When the flame of the spirit-lamp charged with sodium intervenes, it sends out a certain amount of light, which is entirely localised in these two lines. So far it would seem that the influence of the sodium flame ought to be manifested in diminishing the darkness of the lines and rendering them less conspicuous. As a matter of fact, they are far more conspicuous with the sodium flame than without it. This arises from the fact that the sodium flame possesses the remarkable property of cutting off the sunlight which was on its way to those particular lines; so that, though the sodium contributes some light to the lines, yet it intercepts a far greater quantity of the light that would otherwise have illuminated those lines, and hence they became darker with the sodium flame than without it.

We are thus conducted to a remarkable principle, which has led to the interpretation of the dark lines in the spectrum of the sun. We find that when the sodium vapour is heated, it gives out light of a very particular type, which, viewed through the prism, is concentrated in two lines. But the sodium vapour possesses also this property, that light from[Pg 50] the sun can pass through it without any perceptible absorption, except of those particular rays which are of the same characters as the two lines in question. In other words, we say that if the heated vapour of a substance gives a spectrum of bright lines, corresponding to lights of various kinds, this same vapour will act as an opaque screen to lights of those special kinds, while remaining transparent to light of every other description.

This principle is of such importance in the theory of spectrum analysis that we add a further example. Let us take the element iron, which in a very striking degree illustrates the law in question. In the solar spectrum some hundreds of the dark lines are known to correspond with the spectrum of iron. This correspondence is exhibited in a vivid manner when, by a suitable contrivance, the light of an electric spark from poles of iron is examined in the spectroscope side by side with the solar spectrum. The iron lines in the sun are identical in position with the lines in the spectrum of glowing iron vapour. But the spectrum of iron, as here described, consists of bright lines; while those with which it is compared in the sun are dark on a bright background. They can be completely understood if we suppose the vapour arising from intensely heated iron to be present in the atmosphere which surrounds the luminous strata on the sun. This vapour would absorb or stop precisely the same rays as it emits when incandescent, and hence we learn the important fact that iron, no less than sodium, must, in one form or another, be a constituent of the sun.

Such is, in brief outline, the celebrated discovery of modern times which has given an interpretation to the dark lines of the solar spectrum. The spectra of a large number of terrestrial substances have been examined in comparison with the solar spectrum, and thus it has been established that many of the elements known on the earth are present in the sun. We may mention calcium, iron, hydrogen, sodium, carbon, nickel, magnesium, cobalt, aluminium, chromium, strontium, manganese, copper, zinc, cadmium, silver, tin, lead, potassium. Some of the elements which are of the greatest importance[Pg 51] on the earth would appear to be missing from the sun. Sulphur, phosphorus, mercury, gold, nitrogen may be mentioned among the elements which have hitherto given no indication of their being solar constituents.

It is also possible that the lines of a substance in the sun's atmosphere may be so very bright that the light of the continuous spectrum, on which they are superposed, is not able to "reverse" them—i.e. turn them into dark lines. We know, for instance, that the bright lines of sodium vapour may be made so intensely bright that the spectrum of an incandescent lime-cylinder placed behind the sodium vapour does not reverse these lines. If, then, we make the sodium lines fainter, they may be reduced to exactly the intensity prevailing in that part of the spectrum of the lime-light, in which case the lines, of course, could not be distinguished. The question as to what elements are really missing from the sun must therefore, like many other questions concerning our great luminary, at present be considered an open one. We shall shortly see that an element previously unknown has actually been discovered by means of a line representing it in the solar spectrum.

Let us now return to the sun-spots and see what the spectroscope can teach us as to their nature. We attach a powerful spectroscope to the eye-end of a telescope in order to get as much light as possible concentrated on the slit; the latter has therefore to be placed exactly at the focus of the object-glass. The instrument is then pointed to a spot, so that its image falls on the slit, and the presence of the dark central part called the umbra reveals itself by a darkish stripe which traverses the ordinary sun-spectrum from end to end. It is bordered on both sides by the spectrum of the penumbra, which is much brighter than that of the umbra, but fainter than that of the adjoining regions of the sun.

From the fact that the spectrum is darkened we learn that there is considerable general absorption of light in the umbra. This absorption is not, however, such as would be caused by the presence of volumes of minute solid or liquid particles like those which constitute smoke or cloud. This is indicated[Pg 52] by the fact, first discovered by Young in 1883, that the spectrum is not uniformly darkened as it would be if the absorption were caused by floating particles. In the course of examination of many large and quiescent spots, he perceived that the middle green part of the spectrum was crossed by countless fine, dark lines, generally touching each other, but here and there separated by bright intervals. Each line is thicker in the middle (corresponding to the centre of the spot) and tapers to a fine thread at each end; indeed, most of these lines can be traced across the spectrum of the penumbra and out on to that of the solar surface. The absorption would therefore seem to be caused by gases at a much lower temperature than that of the gases present outside the spot.

In the red and yellow parts of the spot-spectrum, which have been specially studied for many years by Sir Norman Lockyer at the South Kensington Observatory, interesting details are found which confirm this conclusion. Many of the dark lines are not thicker and darker in the spot than they are in the ordinary sun-spectrum, while others are very much thickened in the spot-spectrum, such as the lines of iron, calcium, and sodium. The sodium lines are sometimes both widened and doubly reversed—that is, on the thick dark line a bright line is superposed. The same peculiarity is not seldom seen in the notable calcium lines h and k at the violet end of the spectrum. These facts indicate the presence of great masses of the vapours of sodium and calcium over the nucleus. The observations at South Kensington have also brought to light another interesting peculiarity of the spot-spectra. At the time of minimum frequency of spots the lines of iron and other terrestrial elements are prominent among the most widened lines; at the maxima these almost vanish, and the widening is found only amongst lines of unknown origin.

The spectroscope has given us the means of studying other interesting features on the sun, which are so faint that in the full blaze of sunlight they cannot be readily observed with a mere telescope. We can, however, see them easily enough when the brilliant body of the sun is obscured during the rare occurrence of a total eclipse. The conditions necessary[Pg 53] for the occurrence of an eclipse will be more fully considered in the next chapter. For the present it will be sufficient to observe that by the movement of the moon it may so happen that the moon completely hides the sun, and thus for certain parts of the earth produces what we call a total eclipse. The few minutes during which a total eclipse lasts are of much interest to the astronomer. Darkness reigns over the landscape, and in that darkness rare and beautiful sights are witnessed.

Fig. 19.—Prominences seen in Total Eclipse.

We have in Fig. 19 a diagram of a total eclipse, showing some of the remarkable objects known as prominences (a, b, c, d, e) which project from behind the dark body of the moon. That they do not belong to the moon, but are solar appendages of some sort, is easily demonstrated. They first appear on the eastern limb at the commencement of totality. Those first seen are gradually more or less covered by the advancing moon, while others peep out behind the western limb of the moon, until totality is over and the sunlight bursts out again, when they all instantly vanish.

The first total eclipse which occurred after the spectroscope had been placed in the hands of astronomers was in 1868. On the 18th August in that year a total eclipse was visible in India. Several observers, armed with spectroscopes, were on the look-out for the prominences, and were able to announce that their spectrum consisted of detached bright lines, thus demonstrating that these objects were masses of glowing gas. On the following day the illustrious astronomer, Janssen, one of the observers of the eclipse, succeeded in seeing the lines in full sunlight, as he now knew exactly where to look for them. Many months before the eclipse Sir Norman Lockyer had been preparing to search for the prominences, as he expected them to yield[Pg 54] a line spectrum which would be readily visible, if only the sun's ordinary light could be sufficiently winnowed away. He proposed to effect this by using a spectroscope of great dispersion, which would spread out the continuous spectrum considerably and make it fainter. The effect of the great dispersion on the isolated bright lines he expected to see would be only to widen the intervals between them without interfering with their brightness. The new spectroscope, which he ordered to be constructed for this purpose, was not completed until some weeks after the eclipse was over, though before the news of Janssen's achievement reached Europe from India. When that news did arrive Sir N. Lockyer had already found the spectrum of unseen prominences at the sun's limb. The honour of the practical application of a method of observing solar prominences without the help of an eclipse must therefore be shared between the two astronomers.

When a spectroscope is pointed to the margin of the sun so that the slit is radial, certain short luminous lines become visible which lie exactly in the prolongation of the corresponding dark lines in the solar spectrum. From due consideration of the circumstances it can be shown that the gases which form the prominences are also present as a comparatively shallow atmospheric layer all round the great luminary. This layer is about five or six thousand miles deep, and is situated immediately above the dense layer of luminous clouds which forms the visible surface of the sun and which we call the photosphere. The gaseous envelope from which the prominences spring has been called the chromosphere on account of the coloured lines displayed in its spectrum. Such lines are very numerous, but those pertaining to the single substance, hydrogen, predominate so greatly that we may say the chromosphere consists chiefly of this element. It is, however, to be noted that calcium and one other element are also invariably present, while iron, manganese and magnesium are often apparent. The remarkable element, of which we have not yet mentioned the name, has had an astonishing history.

[Pg 55]

During the eclipse of 1868 a fine yellow line was noticed among the lines of the prominence spectrum, and it was not unnaturally at first assumed that it must be the yellow sodium line. But when careful observations were afterwards made without hurry in full sunshine, and accurate measures were obtained, it was at once remarked that this line was not identical with either of the components of the double sodium line. The new line was, no doubt, quite close to the sodium lines, but slightly towards the green part of the spectrum. It was also noticed there was not generally any corresponding line to be seen among the dark lines in the ordinary solar spectrum, though a fine dark one has now and then been detected, especially near a sun-spot. Sir Norman Lockyer and Sir Edward Frankland showed that this was not produced by any known terrestrial element. It was, therefore, supposed to be caused by some hitherto unknown body to which the name of helium, or the sun element, was given. About a dozen less conspicuous lines were gradually identified in the spectrum of the prominences and the chromosphere, which appeared also to be caused by this same mysterious helium. These same remarkable lines have in more recent years also been detected in the spectra of various stars.

This gas so long known in the heavens was at last detected on earth. In April, 1895, Professor Ramsay, who with Lord Rayleigh had discovered the new element argon, detected the presence of the famous helium line in the spectrum of the gas liberated by heating the rare mineral known as cleveite, found in Norway. Thus this element, the existence of which had first been detected on the sun, ninety-three million miles away, has at last been proved to be a terrestrial element also.

When it was announced by Runge that the principal line in the spectrum of the terrestrial helium had a faint and very close companion line on the red-ward side, some doubt seemed at first to be cast on the identity of the new terrestrial gas discovered by Ramsay with the helium of the chromosphere. The helium line of the latter had never been noticed to be double. Subsequently, however, several observers provided[Pg 56] with very powerful instruments found that the famous line in the chromosphere really had a very faint companion line. Thus the identity between the celestial helium and the gas found on our globe was established in the most remarkable manner. Certain circumstances have seemed to indicate that the new gas might possibly be a mixture of two gases of different densities, but up to the present this has not been proved to be the case.

After it had been found possible to see the spectra of prominences without waiting for an eclipse, Sir W. Huggins, in an observation on the 13th of February, 1869, successfully applied a method for viewing the remarkable solar objects themselves instead of their mere spectra in full sunshine. It is only necessary to adjust the spectroscope so that one of the brightest lines—e.g. the red hydrogen line—is in the middle of the field of the viewing telescope, and then to open wide the slit of the spectroscope. A red image of the prominence will then be displayed instead of the mere line. In fact, when the slit is opened wide, the prisms produce a series of detached images of the prominence under observation, one for each kind of light which the object emits.

We have spoken of the spectroscope as depending upon the action of glass prisms. It remains to be added that in the highest class of spectroscopes the prisms are replaced by ruled gratings from which the light is reflected. The effect of the ruling is to produce by what is known as diffraction the required breaking up of the beam of light into its constituent parts.

PLATE IV.
SOLAR PROMINENCES.
(DRAWN BY TROUVELOT AT HARVARD COLLEGE, CAMBRIDGE, U.S., IN 1872.)

[Pg 57]

Majestic indeed are the proportions of some of those mighty prominences which leap from the luminous surface; yet they flicker, as do our terrestrial flames, when we allow them time comparable to their gigantic dimensions. Drawings of the same prominence made at intervals of a few hours, or even less, often show great changes. The magnitude of the displacements that have been noticed sometimes attains many thousands of miles, and the actual velocity with which such masses move frequently exceeds 100 miles a second. Still more violent are the convulsions when, from the surface of the chromosphere, as from a mighty furnace, vast incandescent masses of gas are projected upwards. Plate IV. gives a view of a number of prominences as seen by Trouvelot at Harvard College Observatory, Cambridge, U.S.A. Trouvelot has succeeded in exhibiting in the different pictures the wondrous variety of aspect which these objects assume. The dimensions of the prominences may be inferred from the scale appended to the plate. The largest of those here shown is fully 80,000 miles high; and trustworthy observers have recorded prominences of an altitude even much greater. The rapid changes which these objects sometimes undergo are well illustrated in the two sketches on the left of the lowest line, which were drawn on April 27th, 1872. These are both drawings of the same prominence taken at an interval no greater than twenty minutes. This mighty flame is so vast that its length is ten times as great as the diameter of the earth, yet in this brief period it has completely changed its aspect; the upper part of the flame has, indeed, broken away, and is now shown in that part of the drawing between the two figures on the line above. The same plate also shows various instances of the remarkable spike-like objects, taken, however, at different times and at various parts of the sun. These spikes attain altitudes not generally greater than 20,000 miles, though sometimes they soar aloft to stupendous distances.

We may refer to one special object of this kind, the remarkable history of which has been chronicled by Professor Young. On October 7th, 1880, a prominence was seen, at about 10.30 a.m., on the south-east limb of the sun. It was then about 40,000 miles high, and attracted no special attention. Half an hour later a marvellous transformation had taken place. During that brief interval the prominence became very brilliant and doubled its length. For another hour the mighty flame still soared upwards, until it attained the unprecedented elevation of 350,000 miles—a distance more than one-third the diameter of the great luminary itself. At this climax the energy of the mighty outbreak seems to have at last become exhausted: the flame broke up into fragments,[Pg 58] and by 12.30—an interval of only two hours from the time when it was first noticed—the phenomenon had completely faded away.

No doubt this particular eruption was exceptional in its vehemence, and in the vastness of the changes of which it was an indication. The velocity of upheaval must have been at least 200,000 miles an hour, or, to put it in another form, more than fifty miles a second. This mighty flame leaped from the sun with a velocity more than 100 times as great as that of the swiftest bullet ever fired from a rifle.

The prominences may be generally divided into two classes. We have first those which are comparatively quiescent, and in form somewhat resemble the clouds which float in our earth's atmosphere. The second class of prominences are best described as eruptive. They are, in fact, thrown up from the chromosphere like gigantic jets of incandescent material. These two classes of objects differ not only in appearance but also in the gases of which they are composed. The cloud-like prominences consist mainly of hydrogen, with helium and calcium, while many metals are present in the eruptive discharges. The latter are never seen in the neighbourhood of the sun's poles, but generally appear close to a sun-spot, thus confirming the conclusion that the spots are associated with violent disturbances on the surface of the sun. When a spot has reached the limb of the sun it is frequently found to be surrounded by prominences. It has even been possible in a few instances to detect powerful gaseous eruptions in the neighbourhood of a spot, the spectroscope rendering them visible against the background of the solar surface just as the prominences are observed at the limb against the background of the sky.

In order to photograph a prominence we have, of course, to substitute a photographic plate for the observer's eye. Owing, however, to the difficulty of preventing the feeble light from the prominence from being overpowered by extraneous light, the photography of these bodies was not very successful until Professor Hale, of Chicago, designed his spectro-heliograph. In this instrument there is (in addition to the[Pg 59] usual slit through which the light falls on the prisms, or grating,) a second slit immediately in front of the photographic plate through which the light of a given wave-length can be permitted to pass to the exclusion of all the rest. The light chosen for producing an image of the prominences is that radiated in the remarkable "k line," due to calcium. This lies at the extreme end of the violet. The light from that part of the spectrum, though it is invisible to the eye, is much more active photographically than the light from the red, yellow, or green parts of the spectrum. The front slit is adjusted so that the k line falls upon the second slit, and as the front slit is slowly swept by clockwork over the whole of a prominence, the second slit keeps pace with it by a mechanical contrivance.

If the image of the solar disc is hidden by a screen of exactly the proper size, the slits may be made to sweep over the whole sun, thus giving us at one exposure a picture of the chromospheric ring round the sun's limb with its prominences. The screen may now be withdrawn, and the slits may be made to sweep rapidly over the disc itself. They reveal the existence of glowing calcium vapours in many parts of the surface of the sun. Thus we get a striking picture of the sun as drawn by this particular light. In this manner Professor Hale confirmed the observation made long before by Professor Young, that the spectra of faculæ always show the two great calcium bands.

The velocity with which a prominence shoots upward from the sun's limb can, of course, be measured directly by observations of the ordinary kind with a micrometer. The spectroscope, however, enables us to estimate the speed with which disturbances at the surface of the sun travel in the direction towards the earth or from the earth. We can measure this speed by watching the peculiar behaviour of the spectral lines representing the rapidly moving masses. This opens up a remarkable line of investigation with important applications in many branches of astronomy.

It is, of course, now generally understood that the sensation of light is caused by waves or undulations which impinge[Pg 60] on the retina of the eye after having been transmitted through that medium which we call the ether. To the different colours correspond different wave-lengths—that is to say, different distances between two successive waves. A beam of white light is formed by the union of innumerable different waves whose lengths have almost every possible value lying between certain limits. The wave-length of red light is such that there are 33,000 waves in an inch, while that of violet light is but little more than half that of red light. The position of a line in the spectrum depends solely on the wave-length of the light to which it is due. Suppose that the source of light is approaching directly towards the observer; obviously the waves follow each other more closely than if the source were at rest, and the number of undulations which his eye receives in a second must be proportionately increased. Thus the distance between two successive ether waves will be very slightly diminished. A well-known phenomenon of a similar character is the change of pitch of the whistle of a locomotive engine as it rushes past. This is particularly noticeable if the observer happens to be in a train which is moving rapidly in the opposite direction. In the case of sound, of course, the vibrations or waves take place in the air and not in the ether. But the effect of motion to or from the observer is strictly analogous in the two cases. As, however, light travels 186,000 miles a second, the source of light will also have to travel with a very high velocity in order to produce even the smallest perceptible change in the position of a spectral line.

We have already seen that enormously high velocities are by no means uncommon in some of these mighty disturbances on the sun; accordingly, when we examine the spectrum of a sun-spot, we often see that some of the lines are shifted a little towards one end of the spectrum and sometimes towards the other, while in other cases the lines are seen to be distorted or twisted in the most fantastic manner, indicating very violent local commotions. If the spot happens to be near the centre of the sun's disc, the gases must be shooting upwards or downwards to produce[Pg 61] these changes in the lines. The velocities indicated in observations of this class sometimes amount to as much as two or even three hundred miles per second. We find it difficult to conceive the enormous internal pressures which are required to impel such mighty masses of gases aloft from the photosphere with speeds so terrific, or the conditions which bring about the downrush of such gigantic masses of vapour from above. In the spectra of the prominences on the sun's limb also we often see the bright lines bent or shifted to one side. In such cases what we witness is evidently caused by movements along the surface of the chromosphere, conveying materials towards us or away from us.

An interesting application of this beautiful method of measuring the speed of moving bodies has been made in various attempts to determine the period of rotation of the sun spectroscopically. As the sun turns round on its axis, a point on the eastern limb is moving towards the observer and a point on the western limb is moving away from him. In each case the velocity is a little over a mile per second. At the eastern limb the lines in the solar spectrum are very slightly shifted towards the violet end of the spectrum, while the lines in the spectrum of the western limb are equally shifted towards the red end. By an ingenious optical contrivance it is possible to place the spectra from the two limbs side by side, which doubles the apparent displacement, and thus makes it much more easy to measure. Even with this contrivance the visual quantities to be measured remain exceedingly minute. All the parts of the instrument have to be most accurately adjusted, and the observations are correspondingly delicate. They have been attempted by various observers. Among the most successful investigations of this kind we may mention that of the Swedish astronomer, Dunér, who, by pointing his instrument to a number of places on the limb, found values in good agreement with the peculiar law of rotation which has been deduced from the motion of sun-spots. This result is specially interesting, as it shows that the atmospheric layers, in which that absorption takes[Pg 62] place which produces the dark lines in the spectrum, shares in the motion of the photosphere at the same latitude.

Fig. 20.—View of the Corona (and a Comet) in a Total Eclipse.

PLATE V.
TOTAL SOLAR ECLIPSE, JULY 29TH, 1878.
THE CORONA FROM THE PHOTOGRAPHS.
(HARKNESS.)

We have yet to mention one other striking phenomenon which is among the chief attractions to observers of total eclipses, and which it has hitherto not been found possible to see in full daylight. This is the corona or aureole of light which is suddenly seen to surround the sun in an eclipse when the moon has completely covered the last remaining crescent of the sun. A general idea of the appearance of the corona is given in Fig. 20, and we further present in Plate V. the drawing of the corona made by Professor Harkness from a comparison of a large number of photographs obtained at different places in the United States during the total eclipse of July 29th, 1878. In Fig. 21 we are permitted by the kindness of Mr. and Mrs. Maunder to reproduce the remarkable photograph of the corona which they obtained in India during the eclipse of January 22nd, 1898.

Fig. 21.—View of Corona during the Eclipse of Jan. 22nd, 1898
(Reproduced by kind permission of Mr. and Mrs. Maunder and of the proprietors of "Knowledge.")

The part of the corona nearest the sun is very bright, though not so brilliant as the prominences, which (as Professor Young says) blaze through it like[Pg 64] carbuncles. This inner portion is generally of fairly regular outline, forming a white ring about a tenth part of the solar diameter in width. The outer parts of the corona are usually very irregular and very extensive. They are often interrupted by narrow "rifts," or narrow dark bands, which reach from the limb of the sun through the entire corona. On the other hand, there are also sometimes narrow bright streamers, inclined at various angles to the limb of the sun and not seldom curved. In the eclipses of 1867, 1878, and 1889, all of which occurred at periods of sun-spot minimum, the corona showed long and faint streamers nearly in the direction of the sun's equator, and short but distinct brushes of light near the poles. In the eclipses of 1870, 1882, and 1893, near sun-spot maxima, the corona was more regularly circular, and chiefly developed over the spot zones. We have here another proof (if one were necessary) of the intimate connection between the periodicity of the spots and the development of all other solar phenomena.

In the spectrum of the corona there is a mysterious line in the green, as to the origin of which nothing is at present certainly known. It is best seen during eclipses occurring near the time of sun-spot maximum. It is presented in the ordinary solar spectrum as a very thin, dark line, which generally remains undisturbed even when lines of hydrogen and other substances are twisted and distorted by the violent rush of disturbed elements. The line is always present among the bright lines of the chromosphere spectrum. In addition to it the corona shows a few other bright lines, belonging, no doubt, to the same unknown element ("coronium"), and also a faint continuous spectrum, in which even a few of the more prominent dark lines of the solar spectrum have been sometimes detected. This shows that in addition to glowing gas (represented by the bright lines) the corona also contains a great deal of matter like dust, or fog, the minute particles of which are capable of reflecting the sunlight and thereby producing a feeble continuous spectrum. This matter seems to form the principal constituent of the long coronal rays and streamers, as the latter are not visible in the[Pg 65] detached images of the corona which appear instead of the bright lines when the corona is viewed, or photographed, during an eclipse, in a spectroscope without a slit. If the long rays were composed of the gas or gases which constitute the inner corona, it is evident that they ought to appear in these detached images. As to the nature of the forces which are continually engaged in shooting out these enormously long streamers, we have at present but little information. It is, however, certain that the extensive atmospheric envelope round the sun, which shows itself as the inner corona, must be extremely attenuated. Comets have on several occasions been known to rush through this coronal atmosphere without evincing the slightest appreciable diminution in their speed from the resistance to which they were exposed.

We have accumulated by observation a great number of facts concerning the sun, but when we try to draw from these facts conclusions as to the physical constitution of that great body, it cannot be denied that the difficulties seem to be very great indeed. We find that the best authorities differ considerably in the opinions they entertain as to its nature. We shall here set forth the principal conclusions as to which there is little or no controversy.

We shall see in a following chapter that astronomers have been able to determine the relative densities of the bodies in the solar system; in other words, they have found the relation between the quantities of matter contained in an equally large volume of each. It has thus been ascertained that the average density of the sun is about a quarter that of the earth. If we compare the weight of the sun with that of an equally great globe of water, we find that the luminary would be barely one and a half times as heavy as the water. Of course, the actual mass of the sun is very enormous; it is no less than 330,000 times as great as that of the earth. The solar material itself is, however, relatively light, so that the sun is four times as big as it would have to be if, while its weight remained the same, its density equalled that of the earth. Bearing in mind this lightness of the sun, and also the exceedingly high temperature which we know to prevail[Pg 66] there, no other conclusion seems possible than that the body of the sun must be in a gaseous state. The conditions under which such gases exist in the sun are, no doubt, altogether different from those with which we are acquainted on the earth. At the surface of the sun the force of gravity is more than twenty-seven times as great as it is on the earth. A person who on the earth could just lift twenty-seven equal pieces of metal would, if he were transferred to the sun, only be able to lift one of the pieces at a time. The pressure of the gases below the surface must therefore be very great, and it might be supposed that they would become liquefied in consequence. It was, however, discovered by Andrews that so long as a gas is kept at a temperature higher than a certain point, known as the "critical temperature" (which is different for different gases), the gas will not be turned into a liquid however great be the pressure to which it is submitted. The temperature on the sun cannot be lower than the critical temperatures of the gases there existing; so it would seem that even the enormous pressure can hardly reduce the gases in the great luminary to the liquid form.

Of the interior of the sun we can, of course, expect to learn little or nothing. What we observe is the surface-layer, the so-called photosphere, in which the cold of space produces the condensation of the gases into those luminous clouds which we see in our drawings and photographs as "rice grains" or "willow leaves." It has been suggested by Dr. Johnstone Stoney (and afterwards by Professor Hastings, of Baltimore) that these luminous clouds are mainly composed of carbon with those of the related elements silicon and boron, the boiling points of which are much higher than those of other elements which might be considered likely to form the photospheric clouds. The low atomic weight of carbon must also have the effect of giving the molecules of this element a very high velocity, and thereby enabling them to work their way into the upper regions, where the temperature has so fallen that the vapour becomes chilled into cloud. A necessary consequence of the rapid cooling of these clouds, and the consequent[Pg 67] radiation of heat on a large scale, would be the formation of what we may perhaps describe as smoke, which settles by degrees through the intervals between the clouds (making these intervals appear darker) until it is again volatilised on reaching a level of greater heat below the clouds. This same smoke is probably the cause of the well-known fact that the solar limb is considerably fainter than the middle of the disc. This seems to arise from the greater absorption caused by the longer distance which a ray of light from a point near the limb has to travel through this layer of smoke before reaching the earth. It is shown that this absorption cannot be attributed to a gaseous atmosphere, since this would have the effect of producing more dark absorption lines in the spectrum. There would thus be a marked difference between the solar spectrum from a part near the middle of the disc and the spectrum from a part near the limb. This, however, we do not find to be the case.

With regard to the nature of sun-spots, the idea first suggested by Secchi and Lockyer, that they represent down rushes of cooler vapours into the photosphere (or to its surface), seems on the whole to accord best with the observed phenomena. We have already mentioned that the spots are generally accompanied by faculæ and eruptive prominences in their immediate neighbourhood, but whether these eruptions are caused by the downfall of the vapour which makes the photospheric matter "splash up" in the vicinity, or whether the eruptions come first, and by diminishing the upward pressure from below form a "sink," into which overlying cooler vapour descends, are problems as to which opinions are still much divided.

A remarkable appendage to the sun, which extends to a distance very much greater than that of the corona, produces the phenomenon of the zodiacal light. A pearly glow is sometimes seen in the spring to spread over a part of the sky in the vicinity of the point where the sun has disappeared after sunset. The same spectacle may also be witnessed before sunrise in the autumn, and it would seem as if the material producing the zodiacal light, whatever it may be, had a[Pg 68] lens-shaped form with the sun in the centre. The nature of this object is still a matter of uncertainty, but it is probably composed of a kind of dust, as the faint spectrum it affords is of a continuous type. A view of the zodiacal light is shown in Fig. 22.

In all directions the sun pours forth, with the most prodigal liberality, its torrents of light and of heat. The earth can only grasp the merest fraction, less than the 2,000,000,000th part of the whole. Our fellow planets and the moon also intercept a trifle; but how small is the portion of the mighty flood which they can utilise! The sip that a flying swallow takes from a river is as far from exhausting the water in the river as are the planets from using all the heat which streams from the sun.

The sun's gracious beams supply the magic power that enables the corn to grow and ripen. It is the heat of the sun which raises water from the ocean in the form of vapour, and then sends down that vapour as rain to refresh the earth and to fill the rivers which bear our ships down to the ocean. It is the heat of the sun beating on the large continents which gives rise to the breezes and winds that waft our vessels across the deep; and when on a winter's evening we draw around the fire and feel its invigorating rays, we are only enjoying sunbeams which shone on the earth countless ages ago. The heat in those ancient sunbeams developed the mighty vegetation of the coal period, and in the form of coal that heat has slumbered for millions of years, till we now call it again into activity. It is the power of the sun stored up in coal that urges on our steam-engines. It is the light of the sun stored up in coal that beams from every gaslight in our cities.

For the power to live and move, for the plenty with which we are surrounded, for the beauty with which nature is adorned, we are immediately indebted to one body in the countless hosts of space, and that body is the sun.

Fig. 22.—The Zodiacal Light in 1874.

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This book is part of the public domain. Robert S. Ball (2008). The Story of the Heavens. Urbana, Illinois: Project Gutenberg. Retrieved October 2022 https://www.gutenberg.org/cache/epub/27378/pg27378-images.html

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