CHAPTER XIII

THE SUN--_continued_

The various parts of the Sun will now be treated of in detail.

I. PHOTOSPHERE.

The photosphere, or "light-sphere," from the Greek [phos] (_phos_), which means _light_, is, as we have already said, the innermost portion of the sun which can be seen. Examined through a good telescope it shows a finely mottled structure, as of brilliant granules, somewhat like rice grains, with small dark spaces lying in between them. It has been supposed that we have here the process of some system of circulation by which the sun keeps sending forth its radiations. In the bright granules we perhaps see masses of intensely heated matter, rising from the interior of the sun. The dark interspaces may represent matter which has become cooled and darkened through having parted with its heat and light, and is falling back again into the solar furnace.

The _sun spots_, so familiar to every one nowadays, are dark patches which are often seen to break out in the photosphere (see Plate V., p. 134). They last during various periods of time; sometimes only for a few days, sometimes so long as a month or more. A spot is usually composed of a dark central portion called the _umbra_, and a less dark fringe around this called the _penumbra_ (see Plate VI., p. 136). The umbra ordinarily has the appearance of a deep hole in the photosphere; but, that it is a hole at all, has by no means been definitely proved.

Sun spots are, as a rule, some thousands of miles across. The umbra of a good-sized spot could indeed engulf at once many bodies the size of our earth.

Sun spots do not usually appear singly, but in groups. The total area of a group of this kind may be of immense extent; even so great as to cover the one-hundredth part of the whole surface of the sun. Very large spots, when such are present, may be seen without any telescope; either through a piece of smoked glass, or merely with the naked eye when the air is misty, or the sun low on the horizon.

The umbra of a spot is not actually dark. It only appears so in contrast with the brilliant photosphere around.

Spots form, grow to a large size in comparatively short periods of time, and then quickly disappear. They seem to shrink away as a consequence of the photosphere closing in upon them.

That the sun is rotating upon an axis, is shown by the continual change of position of all spots in one constant direction across his disc. The time in which a spot is carried completely round depends, however, upon the position which it occupies upon the sun's surface. A spot situated near the equator of the sun goes round once in about twenty-five days. The further a spot is situated from this equator, the longer it takes. About twenty-seven days is the time taken by a spot situated midway between the equator and the solar poles. Spots occur to the north of the sun's equator, as well as to the south; though, since regular observations have been made--that is to say, during the past fifty years or so--they appear to have broken out a little more frequently in the southern parts.

From these considerations it will be seen that the sun does not rotate as the earth does, but that different portions appear to move at different speeds. Whether in the neighbourhood of the solar poles the time of rotation exceeds twenty-seven days we are unable to ascertain, for spots are not seen in those regions. No explanation has yet been given of this peculiar rotation; and the most we can say on the subject is that the sun is not by any means a solid body.

_Faculae_ (Latin, little torches) are brilliant patches which appear here and there upon the sun's surface, and are in some way associated with spots. Their displacement, too, across the solar face confirms the evidence which the spots give us of the sun's rotation.

Our proofs of this rotation are still further strengthened by the Doppler spectroscopic method of observation alluded to in Chapter XI. As was then stated, one edge of the sun is thus found to be continually approaching us, and the other side continually receding from us. The varying rates of rotation, which the spots and faculae give us, are duly confirmed by this method.

The first attempt to bring some regularity into the question of sunspots was the discovery by Schwabe, in 1852, that they were subject to a regular variation. As a matter of fact they wax and wane in their number, and the total area which they cover, in the course of a period, or cycle, of on an average about 11-1/4 years; being at one part of this period large and abundant, and at another few and small. This period of 11-1/4 years is known as the sun spot cycle. No explanation has yet been given of the curious round of change, but the period in question seems to govern most of the phenomena connected with the sun.

II. REVERSING LAYER.

This is a layer of relatively cool gases lying immediately upon the photosphere. We never see it directly; and the only proof we have of its presence is that remarkable reversal of the spectrum already described, when during an instant or two in a total eclipse, the advancing edge of the moon, having just hidden the brilliant photosphere, is moving across the fine strip which the layer then presents edgewise towards us. The fleeting moments during which this reversed spectrum lasts, informs us that the layer is comparatively shallow; little more indeed than about 500 miles in depth.

The spectrum of the reversing layer, or "flash spectrum," as it is sometimes called on account of the instantaneous character with which the change takes place, was, as we have seen, first noticed by Young in 1870; and has been successfully photographed since then during several eclipses. The layer itself appears to be in a fairly quiescent state; a marked contrast to the seething photosphere beneath, and the agitated chromosphere above.

III. THE CHROMOSPHERE.

The Chromosphere--so called from the Greek [chroma] (_chroma_), which signifies _colour_--is a layer of gases lying immediately upon the preceding one. Its thickness is, however, plainly much the greater of the two; for whereas the reversing layer is only revealed to us _indirectly_ by the spectroscope, a portion of the chromosphere may clearly be _seen_ in a total eclipse in the form of a strip of scarlet light. The time which the moon's edge takes to traverse it tells us that it must be about ten times as deep as the reversing layer, namely, from 5000 to 10,000 miles in depth. Its spectrum shows that it is composed chiefly of hydrogen, calcium and helium, in the state of vapour. Its red colour is mainly due to glowing hydrogen. The element helium, which it also contains, has received its appellation from [helios] (_helios_), the Greek name for the sun; because, at the time when it first attracted attention, there appeared to be no element corresponding to it upon our earth, and it was consequently imagined to be confined to the sun alone. Sir William Ramsay, however, discovered it to be also a terrestrial element in 1895, and since then it has come into much prominence as one of the products given off by radium.

Taking into consideration the excessive force of gravity on the sun, one would expect to find the chromosphere and reversing layer growing gradually thicker in the direction of the photosphere. This, however, is not the case. Both these layers are strangely enough of the same densities all through; which makes it suspected that, in these regions, the force of gravity may be counteracted by some other force or forces, exerting a powerful pressure outwards from the sun.

IV. THE PROMINENCES.

We have already seen, in dealing with total eclipses, that the exterior surface of the chromosphere is agitated like a stormy sea, and from it billows of flame are tossed up to gigantic heights. These flaming jets are known under the name of prominences, because they were first noticed in the form of brilliant points projecting from behind the rim of the moon when the sun was totally eclipsed. Prominences are of two kinds, _eruptive_ and _quiescent_. The eruptive prominences spurt up directly from the chromosphere with immense speeds, and change their shape with great rapidity. Quiescent prominences, on the other hand, have a form somewhat like trees, and alter their shape but slowly. In the eruptive prominences glowing masses of gas are shot up to altitudes sometimes as high as 300,000 miles,[10] with velocities even so great as from 500 to 600 miles a second. It has been noticed that the eruptive prominences are mostly found in those portions of the sun where spots usually appear, namely, in the regions near the solar equator. The quiescent prominences, on the other hand, are confined, as a rule, to the neighbourhood of the sun's poles.

Prominences were at first never visible except during total eclipses of the sun. But in the year 1868, as we have already seen, a method of employing the spectroscope was devised, by means of which they could be observed and studied at any time, without the necessity of waiting for an eclipse.

A still further development of the spectroscope, the _Spectroheliograph_, an instrument invented almost simultaneously by Professor Hale and the French astronomer, M. Deslandres, permits of photographs being taken of the sun, with the light emanating from _only one_ of its glowing gases at a time. For instance, we can thus obtain a record of what the glowing hydrogen alone is doing on the solar body at any particular moment. With this instrument it is also possible to obtain a series of photographs, showing what is taking place upon the sun at various levels. This is very useful in connection with the study of the spots; for we are, in consequence, enabled to gather more evidence on the subject of their actual form than is given us by their highly foreshortened appearances when observed directly in the telescope.

V. CORONA. (Latin, _a Crown_.)

This marvellous halo of pearly-white light, which displays itself to our view only during the total phase of an eclipse of the sun, is by no means a layer like those other envelopments of the sun of which we have just been treating. It appears, on the other hand, to be composed of filmy matter, radiating outwards in every direction, and fading away gradually into space. Its structure is noted to bear a strong resemblance to the tails of comets, or the streamers of the aurora borealis.

Our knowledge concerning the corona has, however, advanced very slowly. We have not, so far, been as fortunate with regard to it as with regard to the prominences; and, for all we can gather concerning it, we are still entirely dependent upon the changes and chances of total solar eclipses. All attempts, in fact, to apply the spectroscopic method, so as to observe the corona at leisure in full sunlight in the way in which the prominences can be observed, have up to the present met with failure.

The general form under which the corona appears to our eyes varies markedly at different eclipses. Sometimes its streamers are many, and radiate all round; at other times they are confined only to the middle portions of the sun, and are very elongated, with short feathery-looking wisps adorning the solar poles. It is noticed that this change of shape varies in close accordance with that 11-1/4 year period during which the sun spots wax and wane; the many-streamered regular type corresponding to the time of great sunspot activity, while the irregular type with the long streamers is present only when the spots are few (see Plate VII., p. 142). Streamers have often been noted to issue from those regions of the sun where active prominences are at the moment in existence; but it cannot be laid down that this is always the case.

No hypothesis has yet been formulated which will account for the structure of the corona, or for its variation in shape. The great difficulty with regard to theorising upon this subject, is the fact that we see so much of the corona under conditions of marked foreshortening. Assuming, what indeed seems natural, that the rays of which it is composed issue in every direction from the solar body, in a manner which may be roughly imitated by sticking pins all over a ball; it is plainly impossible to form any definite idea concerning streamers, which actually may owe most of the shape they present to us, to the mixing up of multitudes of rays at all kinds of angles to the line of sight. In a word, we have to try and form an opinion concerning an arrangement which, broadly speaking, is _spherical_, but which, on account of its distance, must needs appear to us as absolutely _flat_.

The most known about the composition of the corona is that it is made up of particles of matter, mingled with a glowing gas. It is an element in the composition of this gas which, as has been stated, is not found to tally with any known terrestrial element, and has, therefore, received the name of coronium for want of a better designation.

One definite conclusion appears to be reached with regard to the corona, _i.e._ that the matter of which it is composed, must be exceedingly rarefied; as it is not found, for instance, to retard appreciably the speed of comets, on occasions when these bodies pass very close to the sun. A calculation has indeed been made which would tend to show that the particles composing the coronal matter, are separated from each other by a distance of perhaps between two and three yards! The density of the corona is found not to increase inwards towards the sun. This is what has already been noted with regard to the layers lying beneath it. Powerful forces, acting in opposition to gravity, must hold sway here also.

The 11-1/4 year period, during which the sun spots vary in number and size, appears to govern the activities of the sun much in the same way that our year does the changing seasonal conditions of our earth. Not only, as we have seen, does the corona vary its shape in accordance with the said period, but the activity of the prominences, and of the faculae, follow suit. Further, this constant round of ebb and flow is not confined to the sun itself, but, strangely enough, affects the earth also. The displays of the aurora borealis, which we experience here, coincide closely with it, as does also the varying state of the earth's magnetism. The connection may be still better appreciated when a great spot, or group of spots, has made its appearance upon the sun. It has, for example, often been noted that when the solar rotation carries a spot, or group of spots, across the middle of the visible surface of the sun, our magnetic and electrical arrangements are disturbed for the time being. The magnetic needles in our observatories are, for instance, seen to oscillate violently, telegraphic communication is for a while upset, and magnificent displays of the aurora borealis illumine our night skies. Mr. E.W. Maunder, of Greenwich Observatory, who has made a very careful investigation of this subject, suspects that, when elongated coronal streamers are whirled round in our direction by the solar rotation, powerful magnetic impulses may be projected upon us at the moments when such streamers are pointing towards the earth.

Some interesting investigations with regard to sunspots have recently been published by Mrs. E.W. Maunder. In an able paper, communicated to the Royal Astronomical Society on May 10, 1907, she reviews the Greenwich Observatory statistics dealing with the number and extent of the spots which have appeared during the period from 1889 to 1901--a whole sunspot cycle. From a detailed study of the dates in question, she finds that the number of those spots which are formed on the side of the sun turned away from us, and die out upon the side turned towards us, is much greater than the number of those which are formed on the side turned towards us and die out upon the side turned away. It used, for instance, to be considered that the influence of a planet might _produce_ sunspots; but these investigations make it look rather as if some influence on the part of the earth tends, on the contrary, to _extinguish_ them. Mrs. Maunder, so far, prefers to call the influence thus traced an _apparent_ influence only, for, as she very fairly points out, it seems difficult to attribute a real influence in this matter to the earth, which is so small a thing in comparison not only with the sun, but even with many individual spots.

The above investigation was to a certain degree anticipated by Mr. Henry Corder in 1895; but Mrs. Maunder's researches cover a much longer period, and the conclusions deduced are of a wider and more defined nature.

With regard to its chemical composition, the spectroscope shows us that thirty-nine of the elements which are found upon our earth are also to be found in the sun. Of these the best known are hydrogen, oxygen, helium, carbon, calcium, aluminium, iron, copper, zinc, silver, tin, and lead. Some elements of the metallic order have, however, not been found there, as, for instance, gold and mercury; while a few of the other class of element, such as nitrogen, chlorine, and sulphur, are also absent. It must not, indeed, be concluded that the elements apparently missing do not exist at all in the solar body. Gold and mercury have, in consequence of their great atomic weight, perhaps sunk away into the centre. Again, the fact that we cannot find traces of certain other elements, is no real proof of their entire absence. Some of them may, for instance, be resolved into even simpler forms, under the unusual conditions which exist in the sun; and so we are unable to trace them with the spectroscope, the experience of which rests on laboratory experiments conducted, at best, in conditions which obtain upon the earth.

[10] On November 15, 1907, Dr. A. Rambaut, Radcliffe Observer at Oxford University, noted a prominence which rose to a height of 324,600 miles.