Some of the common terrestrial elements found in the sun are:

Aluminium.

Calcium.

Carbon.

Copper.

Hydrogen.

Iron.

Lead.

Nickel.

Pota.s.sium.

Silicon.

Silver.

Sodium.

Tin.

Zinc.

Oxygen (?)

Whatever differences of chemical structure may exist between the sun and the earth, it seems that we must regard these bodies as more like than unlike to each other in substance, and we are brought back to the second of our alternatives: there must be some influence opposing the force of gravity and making the substance of the sun light instead of heavy, and we need not seek far to find it in--

117. THE HEAT OF THE SUN.--That the sun is hot is too evident to require proof, and it is a familiar fact that heat expands most substances and makes them less dense. The sun's heat falling upon the earth expands it and diminishes its density in some small degree, and we have only to imagine this process of expansion continued until the earth's diameter becomes 58 per cent larger than it now is, to find the earth's density reduced to a level with that of the sun. Just how much the temperature of the earth must be raised to produce this amount of expansion we do not know, neither do we know accurately the temperature of the sun, but there can be no doubt that heat is the cause of the sun's low density and that the corresponding temperature is very high.

Before we inquire more closely into the sun's temperature, it will be well to draw a sharp distinction between the two terms heat and temperature, which are often used as if they meant the same thing. Heat is a form of energy which may be found in varying degree in every substance, whether warm or cold--a block of ice contains a considerable amount of heat--while temperature corresponds to our sensations of warm and cold, and measures the extent to which heat is concentrated in the body. It is the amount of heat per molecule of the body. A barrel of warm water contains more heat than the flame of a match, but its temperature is not so high. Bearing in mind this distinction, we seek to determine not the amount of heat contained in the sun but the sun's temperature, and this involves the same difficulty as does the question, What is the temperature of a locomotive? It is one thing in the fire box and another thing in the driving wheels, and still another at the headlight; and so with the sun, its temperature is certainly different in different parts--one thing at the center and another at the surface.

Even those parts which we see are covered by a veil of gases which produce by absorption the dark lines of the solar spectrum, and seriously interfere both with the emission of energy from the sun and with our attempts at measuring the temperature of those parts of the surface from which that energy streams.

In view of these and other difficulties we need not be surprised that the wildest discordance has been found in estimates of the solar temperature made by different investigators, who have a.s.signed to it values ranging from 1,400 C. to more than 5,000,000 C. Quite recently, however, improved methods and a better understanding of the problem have brought about a better agreement of results, and it now seems probable that the temperature of the visible surface of the sun lies somewhere between 5,000 and 10,000 C., say 15,000 of the Fahrenheit scale.

118. DETERMINING THE SUN'S TEMPERATURE.--One ingenious method which has been used for determining this temperature is based upon the principle stated above, that every object, whether warm or cold, contains heat and gives it off in the form of radiant energy. The radiation from a body whose temperature is lower than 500 C. is made up exclusively of energy whose wave length is greater than 7,600 tenth meters, and is therefore invisible to the eye, although a thermometer or even the human hand can often detect it as radiant heat. A brick wall in the summer sunshine gives off energy which can be felt as heat but can not be seen. When such a body is further heated it continues to send off the same kinds (wave lengths) of energy as before, but new and shorter waves are added to its radiation, and when it begins to emit energy of wave length 7,500 or 7,600 tenth meters, it also begins to shine with a dull-red light, which presently becomes brighter and less ruddy and changes to white as the temperature rises, and waves of still shorter length are thereby added to the radiation. We say, in common speech, the body becomes first red hot and then white hot, and we thus recognize in a general way that the kind or color of the radiation which a body gives off is an index to its temperature. The greater the proportion of energy of short wave lengths the higher is the temperature of the radiating body. In sunlight the maximum of brilliancy to the eye lies at or near the wave length, 5,600 tenth meters, but the greatest intensity of radiation of all kinds (light included) is estimated to fall somewhere between green and blue in the spectrum at or near the wave length 5,000 tenth meters, and if we can apply to this wave length Paschen's law--temperature reckoned in degrees centigrade from the absolute zero is always equal to the quotient obtained by dividing the number 27,000,000 by the wave length corresponding to maximum radiation--we shall find at once for the absolute temperature of the sun's surface 5,400 C.

Paschen's law has been shown to hold true, at least approximately, for lower temperatures and longer wave lengths than are here involved, but as it is not yet certain that it is strictly true and holds for all temperatures, too great reliance must not be attached to the numerical result furnished by it.

[Ill.u.s.tration: FIG. 66.--The sun, August 11, 1894. Photographed at the Goodsell Observatory.]

[Ill.u.s.tration: FIG. 67.--The sun, August 14, 1894. Photographed at the Goodsell Observatory.]

119. THE SUN'S SURFACE.--A marked contrast exists between the faces of sun and moon in respect of the amount of detail to be seen upon them, the sun showing nothing whatever to correspond with the mountains, craters, and seas of the moon. The unaided eye in general finds in the sun only a blank bright circle as smooth and unmarked as the surface of still water, and even the telescope at first sight seems to show but little more. There may usually be found upon the sun's face a certain number of black patches called _sun spots_, such as are shown in Figs.

66 to 69, and occasionally these are large enough to be seen through a smoked gla.s.s without the aid of a telescope. When seen near the edge of the sun they are quite frequently accompanied, as in Fig. 69, by vague patches called _faculae_ (Latin, _facula_ = a little torch), which look a little brighter than the surrounding parts of the sun. So, too, a good photograph of the sun usually shows that the central parts of the disk are rather brighter than the edge, as indeed we should expect them to be, since the absorption lines in the sun's spectrum have already taught us that the visible surface of the sun is enveloped by invisible vapors which in some measure absorb the emitted light and render it feebler at the edge where it pa.s.ses through a greater thickness of this envelope than at the center. See Fig. 70, where it is shown that the energy coming from the edge of the sun to the earth has to traverse a much longer path inside the vapors than does that coming from the center.

[Ill.u.s.tration: FIG. 68.--The sun, August 18, 1894. Photographed at the Goodsell Observatory.]

Examine the sun spots in the four photographs, Figs. 66 to 69, and note that the two spots which appear at the extreme left of the first photograph, very much distorted and foreshortened by the curvature of the sun's surface, are seen in a different part of the second picture, and are not only more conspicuous but show better their true shape.

[Ill.u.s.tration: PLATE II. THE EQUATORIAL CONSTELLATIONS]

120. THE SUN'S ROTATION.--The changed position of these spots shows that the sun rotates about an axis at right angles to the direction of the spot's motion, and the position of this axis is shown in the figure by a faint line ruled obliquely across the face of the sun nearly north and south in each of the four photographs. This rotation in the s.p.a.ce of three days has carried the spots from the edge halfway to the center of the disk, and the student should note the progress of the spots in the two later photographs, that of August 21st showing them just ready to disappear around the farther edge of the sun.

[Ill.u.s.tration: FIG. 69.--The sun, August 21, 1894. Photographed at the Goodsell Observatory.]

Plot accurately in one of these figures the positions of the spots as shown in the other three, and observe whether the path of the spots across the sun's face is a straight line. Is there any reason why it should not be straight?

These four pictures may be made to ill.u.s.trate many things about the sun.

Thus the sun's axis is not parallel to that of the earth, for the letters _N S_ mark the direction of a north and south line across the face of the sun, and this line, of course, is parallel to the earth's axis, while it is evidently not parallel to the sun's axis. The group of spots took more than ten days to move across the sun's face, and as at least an equal time must be required to move around the opposite side of the sun, it is evident that the period of the sun's rotation is something more than 20 days. It is, in fact, rather more than 25 days, for this same group of spots reappeared again on the left-hand edge of the sun on September 5th.

[Ill.u.s.tration: FIG. 70.--Absorption at the sun's edge.]

121. SUN SPOTS.--Another significant fact comes out plainly from the photographs. The spots are not permanent features of the sun's face, since they changed their size and shape very appreciably in the few days covered by the pictures. Compare particularly the photographs of August 14th and August 18th, where the spots are least distorted by the curvature of the sun's surface. By September 16th this group of spots had disappeared absolutely from the sun's face, although when at its largest the group extended more than 80,000 miles in length, and several of the individual spots were large enough to contain the earth if it had been dropped upon them. From Fig. 67 determine in miles the length of the group on August 14th. Fig. 71 shows an enlarged view of these spots as they appeared on August 17th, and in this we find some details not so well shown in the preceding pictures. The larger spots consist of a black part called the _nucleus_ or _umbra_ (Latin, shadow), which is surrounded by an irregular border called the _penumbra_ (partial shadow), which is intermediate in brightness between the nucleus and the surrounding parts of the sun. It should not be inferred from the picture that the nucleus is really black or even dark. It shines, in fact, with a brilliancy greater than that of an electric lamp, but the background furnished by the sun's surface is so much brighter that by contrast with it the nucleus and penumbra appear relatively dark.

[Ill.u.s.tration: FIG. 71.--Sun spots, August 17, 1894. Goodsell Observatory.]

[Ill.u.s.tration: FIG. 72.--Sun spot of March 5, 1873.--From LANGLEY, The New Astronomy. By permission of the publishers.]

The bright shining surface of the sun, the background for the spots, is called the _photosphere_ (Greek, light sphere), and, as Fig. 71 shows, it a.s.sumes under a suitable magnifying power a mottled aspect quite different from the featureless expanse shown in the earlier pictures.

The photosphere is, in fact, a layer of little clouds with darker s.p.a.ces between them, and the fine detail of these clouds, their complicated structure, and the way in which, when projected against the background of a sun spot, they produce its penumbra, are all brought out in Fig. 72. Note that the little patch in one corner of this picture represents North and South America drawn to the same scale as the sun spots.

[Ill.u.s.tration: FIG. 73.--Spectroheliograph, showing distribution of faculae upon the sun.--HALE.]

[Ill.u.s.tration: FIG. 74.--Eclipse of July 20, 1878.--TROUVELOT.]

122. FACULae.--We have seen in Fig. 69 a few of the bright spots called faculae. At the telescope or in the ordinary photograph these can be seen only at the edge of the sun, because elsewhere the background furnished by the photosphere is so bright that they are lost in it. It is possible, however, by an ingenious application of the spectroscope to break up the sunlight into a spectrum in such a way as to diminish the brightness of this background, much more than the brightness of the faculae is diminished, and in this way to obtain a photograph of the sun's surface which shall show them wherever they occur, and such a photograph, showing faintly the spectral lines, is reproduced in Fig.

73. The faculae are the bright patches which stretch inconspicuously across the face of the sun, in two rather irregular belts with a comparatively empty lane between them. This lane lies along the sun's equator, and it is upon either side of it between lat.i.tudes 5 and 40 that faculae seem to be produced. It is significant of their connection with sun spots that the spots occur in these particular zones and are rarely found outside them.

[Ill.u.s.tration: FIG. 75.--Eclipse of April 16, 1893.--SCHAEBERLE.]

123. INVISIBLE PARTS OF THE SUN. THE CORONA.--Thus far we have been dealing with parts of the sun that may be seen and photographed under all ordinary conditions. But outside of and surrounding these parts is an envelope, or rather several envelopes, of much greater extent than the visible sun. These envelopes are for the most part invisible save at those times when the brighter central portions of the sun are hidden in a total eclipse.

[Ill.u.s.tration: FIG. 76.--Eclipse of January 21, 1898.--CAMPBELL.]

Fig. 74 is from a drawing, and Figs. 75 and 76 are from eclipse photographs showing this region, in which the most conspicuous object is the halo of soft light called the _corona_, that completely surrounds the sun but is seen to be of differing shapes and differing extent at the several eclipses here shown, although a large part of these apparent differences is due to technical difficulties in photographing, and reproducing an object with outlines so vague as those of the corona. The outline of the corona is so indefinite and its outer portions so faint that it is impossible to a.s.sign to it precise dimensions, but at its greatest extent it reaches out for several millions of miles and fills a s.p.a.ce more than twenty times as large as the visible part of the sun.

Despite its huge bulk, it is of most unsubstantial character, an airy nothing through which comets have been known to force their way around the sun from one side to the other, literally for millions of miles, without having their course influenced or their velocity checked to any appreciable extent. This would hardly be possible if the density even at the bottom of the corona were greater than that of the best vacuum which we are able to produce in laboratory experiments. It seems odd that a vacuum should give off so bright a light as the coronal pictures show, and the exact character of that light and the nature of the corona are still subjects of dispute among astronomers, although it is generally agreed that, in part at least, its light is ordinary sunlight faintly reflected from the widely scattered molecules composing the substance of the corona. It is also probable that in part the light has its origin in the corona itself. A curious and at present unconfirmed result announced by one of the observers of the eclipse of May 28, 1900, is that _the corona is not hot_, its effective temperature being lower than that of the instrument used for the observation.

[Ill.u.s.tration: FIG. 77.--Solar prominence of March 25, 1895.--HALE.]

124. THE CHROMOSPHERE.--Between the corona and the photosphere there is a thin separating layer called the _chromosphere_ (Greek, color sphere), because when seen at an eclipse it shines with a brilliant red light quite unlike anything else upon the sun save the _prominences_ which are themselves only parts of the chromosphere temporarily thrown above its surface, as in a fountain a jet of water is thrown up from the basin and remains for a few moments suspended in mid-air. Not infrequently in such a fountain foreign matter is swept up by the rush of the water--dirt, twigs, small fish, etc.--and in like manner the prominences often carry along with them parts of the underlying layers of the sun, photosphere, faculae, etc., which reveal their presence in the prominence by adding their characteristic lines to the spectrum, like that of the chromosphere, which the prominence presents when they are absent. None of the eclipse photographs (Figs. 74 to 76) show the chromosphere, because the color effect is lacking in them, but a great curving prominence may be seen near the bottom of Fig. 75, and smaller ones at other parts of the sun's edge.

[Ill.u.s.tration: FIG. 78.--A solar prominence.--HALE.]

125. PROMINENCES.--Fig. 77 shows upon a larger scale one of these prominences rising to a height of 160,000 miles above the photosphere; and another photograph, taken 18 minutes later, but not reproduced here, showed the same prominence grown in this brief interval to a stature of 280,000 miles. These pictures were not taken during an eclipse, but in full sunlight, using the same spectroscopic apparatus which was employed in connection with the faculae to diminish the brightness of the background without much enfeebling the brilliancy of the prominence itself. The dark base from which the prominence seems to spring is not the sun's edge, but a part of the apparatus used to cut off the direct sunlight.

Fig. 78 contains a series of photographs of another prominence taken within an interval of 1 hour 47 minutes and showing changes in size and shape which are much more nearly typical of the ordinary prominence than was the very unusual change in the case of Fig. 77.

[Ill.u.s.tration: FIG. 79.--Contrasted forms of solar prominences.--ZOELLNER.]

The preceding pictures are from photographs, and with them the student may compare Fig. 79, which is constructed from drawings made at the spectroscope by the German astronomer Zoellner. The changes here shown are most marked in the prominence at the left, which is shaped like a broken tree trunk, and which appears to be vibrating from one side to the other like a reed shaken in the wind. Such a prominence is frequently called an _eruptive_ one, a name suggested by its appearance of having been blown out from the sun by something like an explosion, while the prominence at the right in this series of drawings, which appears much less agitated, is called by contrast with the other a _quiescent_ prominence. These quiescent prominences are, as a rule, much longer-lived than the eruptive ones. One more picture of prominences (Fig. 80) is introduced to show the continuous stretch of chromosphere out of which they spring.

[Ill.u.s.tration: FIG. 80.--Prominences and chromosphere.--HALE.]

Prominences are seen only at the edge of the sun, because it is there alone that the necessary background can be obtained, but they must occur at the center of the sun and elsewhere quite as well as at the edge, and it is probable that quiescent prominences are distributed over all parts of the sun's surface, but eruptive prominences show a strong tendency toward the regions of sun spots and faculae as if all three were intimately related phenomena.

126. THE SUN AS A MACHINE.--Thus far we have considered the anatomy of the sun, dissecting it into its several parts, and our next step should be a consideration of its physiology, the relation of the parts to each other, and their function in carrying on the work of the solar organism, but this step, unfortunately, must be a lame one. The science of astronomy to-day possesses no comprehensive and well-established theory of this kind, but looks to the future for the solution of this the greatest pending problem of solar physics. Progress has been made toward its solution, and among the steps of this progress that we shall have to consider, the first and most important is the conception of the sun as a kind of heat engine.