Fig. 162.
This heat would be sufficient to melt a layer of ice nearly fifty feet thick all around the sun in a minute. To develop this heat would require the hourly consumption of a layer of anthracite coal, more than sixteen feet thick, over the entire surface of the sun; and the mechanical equivalent of this heat is about ten thousand horse-power on every square foot of the sun's surface.
146. The Brightness of the Sun's Surface.—The sun's surface is a hundred and ninety thousand times as bright as a candle-flame, a hundred and forty-six times as bright as the calcium-light, and about three times and a half as bright as the voltaic arc.
The sun's disk is much less bright near the margin than near the centre, a point on the limb of the sun being only about a fourth as bright as one near the centre of the disk. This diminution of brightness towards the margin of the disk is due to the increase in the absorption of the solar atmosphere as we pass from the centre towards the margin of the sun's disk; and this increased absorption is due to the fact, that the rays which reach us from near the margin have to traverse a much greater thickness of the solar atmosphere than those which reach us from the centre of the disk. This will be evident from Fig. 162, in which the arrows mark the paths of rays from different parts of the solar disk.
Fig. 163.
147. The Spectroscope as an Astronomical Instrument.—The spectroscope is now continually employed in the study of the physical condition and chemical constitution of the sun and of the other heavenly bodies. It has become almost as indispensable to the astronomer as the telescope.
148. The Dispersion Spectroscope.—The essential parts of the dispersion spectroscope are shown in Fig. 163. These are the collimator tube, the prism, and the telescope. The collimator tube has a narrow slit at one end, through which the light to be examined is admitted, and somewhere within the tube a lens for condensing the light. The light is dispersed on passing through the prism: it then passes through the objective of the telescope, and forms within the tube an image of the spectrum, which is examined by means of the eye-piece. The power of the spectroscope is increased by increasing the number of prisms, which are arranged so that the light shall pass through one after another in succession. Such an arrangement of prisms is shown in Fig. 164. One end of the collimator tube is seen at the left, and one end of the telescope at the right. Sometimes the prisms are made long, and the light is sent twice through the same train of prisms, once through the lower, and once through the upper, half of the prisms. This is accomplished by placing a rectangular prism against the last prism of the train, as shown in Fig. 165.
Fig. 164.
Fig. 165.
149. The Micrometer Scale.—Various devices are employed to obtain an image of a micrometer scale in the tube of the telescope beside that of the spectrum.
Fig. 166.
One of the simplest of these methods is shown in Fig. 166. A is the telescope, B the collimator, and C the micrometer tube. The opening at the outer end of C contains a piece of glass which has a micrometer scale marked upon it. The light from the candle shines through this glass, falls upon the surface of the prism P, and is thence reflected into the telescope, where it forms an enlarged image of the micrometer scale alongside the image of the spectrum.
Fig. 167.
150. The Comparison of Spectra.—In order to compare two spectra, it is desirable to be able to see them side by side in the telescope. The images of two spectra may be obtained side by side in the telescope tube by the use of a little rectangular prism, which covers one-half of the slit of the collimator tube, as shown in Fig. 167. The light from one source is admitted directly through the uncovered half of the slit, while the light from the other source is sent through the covered portion of the slit by reflection from the surface of the rectangular prism. This arrangement and its action will be readily understood from Fig. 167.
Fig. 168.
151. Direct-Vision Spectroscope.—A beam of light may be dispersed, without any ultimate deflection from its course, by combining prisms of crown and flint glass with equal refractive, but unequal dispersive powers. Such a combination of prisms is called a direct-vision combination. One of three prisms is shown in Fig. 168, and one of five prisms in Fig. 169.
Fig. 169.
Fig. 170.
A direct-vision spectroscope (Fig. 170) is one in which a direct-vision combination of prisms is employed. C is the collimator tube, P the train of prisms, F the telescope, and r the comparison prism.
Fig. 171.
152. The Telespectroscope.—The spectroscope, when used for astronomical work, is usually combined with a telescope. The compound instrument is called a telespectroscope. The spectroscope is mounted at the end of the telescope in such a way that the image formed by the object-glass of the telescope falls upon the slit at the end of the collimator tube. A telespectroscope of small dispersive power is shown in Fig. 171; a being the object-glass of the telescope, cc the tube of the telescope, and e the comparison prism at the end of the collimator tube. A more powerful instrument is shown in Fig. 172. A is the telescope, C the collimator tube of the spectroscope, P the train of prisms, and E the telescope tube. Fig. 173 shows a still more powerful spectroscope attached to the great Newall refractor (18).
Fig. 172.
Fig. 173.
153. The Diffraction Spectroscope.—A diffraction spectroscope is one in which the spectrum is produced by reflection of the light from a finely ruled surface, or grating, as it is called, instead of by dispersion in passing through a prism. The essential parts of this instrument are shown in Fig 174. This spectroscope may be attached to the telescope in the same manner as the dispersion spectroscope. When the spectroscope is thus used, the eye-piece of the telescope is removed.
Fig. 174.
154. Continuous Spectra.—Light from an incandescent solid or liquid which has suffered no absorption in the medium which it has traversed gives a spectrum consisting of a continuous colored band, in which the colors, from the red to the violet, pass gradually and imperceptibly into one another. The spectrum is entirely free from either light or dark lines, and is called a continuous spectrum.
155. Bright-Lined Spectra.—Light from a luminous gas or vapor gives a spectrum composed of bright lines separated by dark spaces, and known as a bright-lined spectrum. It has been found that the lines in the spectrum of a substance in the state of a gas or vapor are the most characteristic thing about the substance, since no two vapors give exactly the same lines: hence, when we have once become acquainted with the bright-lined spectrum of any substance, we can ever after recognize that substance by the spectrum of its luminous vapor. Even when several substances are mixed, they may all be recognized by the bright-lined spectrum of the mixture, since the lines of all the substances will be present in the spectrum of the mixture. This method of identifying substances by their spectra is called spectrum analysis.
The bright-lined spectra of several substances are given in the frontispiece. The number of lines in the spectra of the elements varies greatly. The spectrum of sodium is one of the simplest, while that of iron is one of the most complex. The latter contains over six hundred lines. Though no two vapors give identical spectra, there are many cases in which one or more of the spectral lines of one element coincide in position with lines of other elements.
156. Methods of rendering Gases and Vapors Luminous.—In order to study the spectra of vapors and gases it is necessary to have some means of converting solids and liquids into vapor, and also of rendering the vapors and gases luminous. There are four methods of obtaining luminous vapors and gases in common use.
Fig. 175.
(1) By means of the Bunsen Flame.—This is a very hot but an almost non-luminous flame. If any readily volatilized substance, such as the compounds of sodium, calcium, strontium, etc., is introduced into this flame on a fine platinum wire, it is volatilized in the flame, and its vapor is rendered luminous, giving the flame its own peculiar color. The flame thus colored may be examined by the spectroscope. The arrangement of the flame is shown in Fig. 175.
Fig. 176.
(2) By means of the Voltaic Arc.—An electric lamp is shown in Fig. 176. When this lamp is to be used for obtaining luminous vapors, the lower carbon is made larger than the upper one, and hollowed out at the top into a little cup. The substance to be volatilized is placed in this cup, and the current is allowed to pass. The heat of the voltaic arc is much more intense than that of the Bunsen flame: hence substances that cannot be volatilized in the flame are readily volatilized in the arc, and the vapor formed is raised to a very high temperature.
(3) By means of the Spark from an Induction Coil.—The arrangement of the coil for obtaining luminous vapors is shown in Fig. 177.
Fig. 177.
The terminals of the coil between which the spark is to pass are brought quite close together. When we wish to vaporize any metal, as iron, the terminals are made of iron. On the passage of the spark, a little of the iron at the ends of the terminals is evaporated; and the vapor is rendered luminous in the space traversed by the spark. A condenser is usually placed in the circuit. With the coil, the temperature may be varied at pleasure; and the vapor may be raised even to a higher temperature than with the electric lamp. To obtain a low temperature, the coil is used without the condenser. By using a larger and larger condenser, the temperature may be raised higher and higher.
By means of the induction coil we may also heat gases to incandescence. It is only necessary to allow the spark to pass through a space filled with the gas.
Fig. 178.
(4) By means of a Vacuum Tube.—The form of the vacuum tube commonly used for this purpose is shown in Fig. 178. The gas to be examined, and which is contained in the tube, has very slight density: but upon the passage of the discharge from an induction coil or a Holtz machine, through the tube, the gas in the capillary part of the tube becomes heated to a high temperature, and is then quite brilliant.
157. Reversed Spectra.—If the light from an incandescent cylinder of lime, or from the incandescent point of an electric lamp, is allowed to pass through luminous sodium vapor, and is then examined with a spectroscope, the spectrum will be found to be a bright spectrum crossed by a single dark line in the position of the yellow line of the sodium vapor. The spectrum of sodium vapor is reversed, its bright lines becoming dark and its dark spaces bright. With a spectroscope of any considerable power, the yellow line of sodium vapor is resolved into a double line. With a spectroscope of the same power, the dark sodium line of the reversed spectrum is seen to be a double line.
It is found to be generally true, that the spectrum of the light from an incandescent solid or liquid which has passed through a luminous vapor on its way to the spectroscope is made up of a bright ground crossed by dark lines; there being a dark line for every bright line that the vapor alone would give.
158. Explanation of Reversed Spectra.—It has been found that gases absorb and quench rays of the same degree of refrangibility as those which they themselves emit, and no others. When a solid is shining through a luminous vapor, this absorbs and quenches those rays from the solid which have the same degrees of refrangibility as those which it is itself emitting: hence the lines of the spectrum receive light from the vapor alone, while the spaces between the lines receive light from the solid. Now, solids and liquids, when heated to incandescence, give a very much brighter light than vapors and gases at the same temperature: hence the lines of a reversed spectrum, though receiving light from the vapor or gas, appear dark by contrast.
159. Effect of Increasing the Power of the Spectroscope upon the Brilliancy of a Spectrum.—An increase in the power of a spectroscope diminishes the brilliancy of a continuous spectrum, since it makes the colored band longer, and therefore spreads the light out over a greater extent of surface; but, in the case of a bright-lined spectrum, an increase of power in the spectroscope produces scarcely any alteration in the brilliancy of the lines, since it merely separates the lines farther without making the lines themselves any wider. In the case of a reversed spectrum, an increase of power in the spectroscope dilutes the light in the spaces between the lines without diluting that of the lines: hence lines which appear dark in a spectroscope of slight dispersive power may appear bright in an instrument of great dispersive power.
160. Change of the Spectrum with the Density of the Luminous Vapor.—It has been found, that, as the density of a luminous vapor is diminished, the lines in its spectrum become fewer and fewer, till they are finally reduced to one. On the other hand, an increase of density causes new lines to appear in the spectrum, and the old lines to become thicker.
161. Change of the Spectrum with the Temperature of the Luminous Vapor.—It has also been found that the appearance of a bright-lined spectrum changes considerably with the temperature of the luminous vapor. In some cases, an increase of temperature changes the relative intensities of the lines; in other cases, it causes new lines to appear, and old lines to disappear.
In the case of a compound vapor, an increase of temperature causes the colored bands (which are peculiar to the spectrum of the compound) to disappear, and to be replaced by the spectral lines of the elements of which the compound is made up. The heat appears to dissociate the compound; that is, to resolve it into its constituent elements. In this case, each elementary vapor would give its own spectral lines. As the compound is not completely dissociated at once, it is possible, of course, for one or more of the spectral lines of the elementary vapors to co-exist in the spectrum with the bands of the compound.
It has been found, that, in some cases, the spectra of the elementary gases change with the temperature of the gas; and Lockyer thinks he has discovered conclusive evidence, in the spectra of the sun and stars, that many of the substances regarded as elementary are really resolved into simpler substances by the intense heat of the sun; in other words, that our so-called elements are really compounds.
162. The Solar Spectrum.—The solar spectrum is crossed transversely by a great number of fine dark lines, and hence it belongs to the class of reversed spectra.
These lines were first studied and mapped by Fraunhofer, and from him they have been called Fraunhofer's lines.
Fig. 179.
A reduced copy of Fraunhofer's map is shown in Fig. 179. A few of the most prominent of the dark solar lines are designated by the letters of the alphabet. The other lines are usually designated by the numbers at which they are found on the scale which accompanies the map. This scale is usually drawn at the top of the map, as will be seen in some of the following diagrams. The two most elaborate maps of the solar spectrum are those of Kirchhoff and Angström. The scale on Kirchhoff's map is an arbitrary one, while that of Angström is based upon the wave-lengths of the rays of light which would fall upon the lines in the spectrum.
Fig. 180.
The appearance of the spectrum varies greatly with the power of the spectroscope employed. Fig. 180 shows a portion of the spectrum as it appears in a spectroscope of a single prism: while Fig. 181 shows the b group of lines alone, as they appear in a powerful diffraction spectroscope.
Fig. 181.
163. The Telluric Lines.—There are many lines of the solar spectrum which vary considerably in intensity as the sun passes from the horizon to the meridian, being most intense when the sun is nearest the horizon, and when his rays are obliged to pass through the greatest depth of the earth's atmosphere. These lines are of atmospheric origin, and are due to the absorption of the aqueous vapor in our atmosphere. They are the same lines that are obtained when a candle or other artificial light is examined with a spectroscope through a long tube filled with steam. Since these lines are due to the absorption of our own atmosphere, they are called telluric lines. A map of these lines is shown in Fig. 182.
Fig. 182.
164. The Solar Lines.—After deducting the telluric lines, the remaining lines of the solar spectrum are of solar origin. They must be due to absorption which takes place in the sun's atmosphere. They are, in fact, the reversed spectra of the elements which exist in the solar atmosphere in the state of vapor: hence we conclude that the luminous surface of the sun is surrounded with an atmosphere of luminous vapors. The temperature of this atmosphere, at least near the surface of the sun, must be sufficient to enable all the elements known on the earth to exist in it as vapors.
Fig. 183.
165. Chemical Constitution of the Sun's Atmosphere.—To find whether any element which exists on the earth is present in the solar atmosphere, we have merely to ascertain whether the bright lines of its gaseous spectrum are matched by dark lines in the solar spectrum when the two spectra are placed side by side. In Fig. 183, we have in No. 1 a portion of the red end of the solar spectra, and in No. 2 the spectrum of sodium vapor, both as obtained in the same spectroscope by means of the comparison prism. It will be seen that the double sodium line is exactly matched by a double dark line of the solar spectrum: hence we conclude that sodium vapor is present in the sun's atmosphere. Fig. 184 shows the matching of a great number of the bright lines of iron vapor by dark lines in the solar spectrum. This matching of the iron lines establishes the fact that iron vapor is present in the solar atmosphere.
Fig. 184.
The following table (given by Professor Young) contains a list of all the elements which have, up to the present time, been detected with certainty in the sun's atmosphere. It also gives the number of bright lines in the spectrum of each element, and the number of those lines which have been matched by dark lines in the solar spectrum:—
| Elements. | Bright Lines. | Lines Reversed. | Observer. |
|---|---|---|---|
| 1. Iron | 600 | 460 | Kirchhoff. |
| 2. Titanium | 206 | 118 | Thalen. |
| 3. Calcium | 89 | 75 | Kirchhoff. |
| 4. Manganese | 75 | 57 | Angström. |
| 5. Nickel | 51 | 33 | Kirchhoff. |
| 6. Cobalt | 86 | 19 | Thalen. |
| 7. Chromium | 71 | 18 | Kirchhoff. |
| 8. Barium | 26 | 11 | Kirchhoff. |
| 9. Sodium | 9 | 9 | Kirchhoff. |
| 10. Magnesium | 7 | 7 | Kirchhoff. |
| 11. Copper? | 15 | 7? | Kirchhoff. |
| 12. Hydrogen | 5 | 5 | Angström. |
| 13. Palladium | 29 | 5 | Lockyer. |
| 14. Vanadium | 54 | 4 | Lockyer. |
| 15. Molybdenum | 27 | 4 | Lockyer. |
| 16. Strontium | 74 | 4 | Lockyer. |
| 17. Lead | 41 | 3 | Lockyer. |
| 18. Uranium | 21 | 3 | Lockyer. |
| 19. Aluminium | 14 | 2 | Angström. |
| 20. Cerium | 64 | 2 | Lockyer. |
| 21. Cadmium | 20 | 2 | Lockyer. |
| 22. Oxygen a | 42 | 12 ± bright | H. Draper. |
| Oxygen b | 4 | 4? | Schuster. |
In addition to the above elements, it is probable that several other elements are present in the sun's atmosphere; since at least one of their bright lines has been found to coincide with dark lines of the solar spectrum. There are, however, a large number of elements, no traces of which have yet been detected; and, in the cases of the elements whose presence in the solar atmosphere has been established, the matching of the lines is far from complete in the majority of the cases, as will be seen from the above table. This want of complete coincidence of the lines is undoubtedly due to the very high temperature of the solar atmosphere. We have already seen that the lines of the spectrum change with the temperature; and, as the temperature of the sun is far higher than any that we can produce by artificial means, we might reasonably expect that it would cause the disappearance from the spectrum of many lines which we find to be present at our highest temperature.
Lockyer maintains that the reason why no trace of the spectral lines of certain of our so-called elements is found in the solar atmosphere is, that these substances are not really elementary, and that the intense heat of the sun resolves them into simpler constituents.
166. Change of Pitch caused by Motion of Sounding Body.—When a sounding body is moving rapidly towards us, the pitch of its note becomes somewhat higher than when the body is stationary; and, when such a body is moving rapidly from us, the pitch of its note is lowered somewhat. We have a good illustration of this change of pitch at a country railway station on the passage of an express-train. The pitch of the locomotive whistle is considerably higher when the train is approaching the station than when it is leaving it.
167. Explanation of the Change of Pitch produced by Motion.—The pitch of sound depends upon the rapidity with which the pulsations of sound beat upon the drum of the ear. The more rapidly the pulsations follow each other, the higher is the pitch: hence the shorter the sound-waves (provided the sound is all the while travelling at the same rate), the higher the pitch of the sound. Any thing, then, which tends to shorten the waves of sound tends also to raise its pitch, and any thing which tends to lengthen these waves tends to lower its pitch.
When a sounding body is moving rapidly forward, the sound-waves are crowded together a little, and therefore shortened; when it is moving backward, the sound-waves are drawn out, or lengthened a little.
The effect of the motion of a sounding body upon the length of its sonorous waves will be readily seen from the following illustration: Suppose a number of persons stationed at equal intervals in a line on a long platform capable of moving backward and forward. Suppose the men are four feet apart, and all walking forward at the same rate, and that the platform is stationary, and that, as the men leave the platform, they keep on walking at the same rate: the men will evidently be four feet apart in the line in front of the platform, as well as on it. Suppose next, that the platform is moving forward at the rate of one foot in the interval between two men's leaving the platform, and that the men continue to walk as before: it is evident that the men will then be three feet apart in the line after they have left the platform. The forward motion of the platform has the effect of crowding the men together a little. Were the platform moving backward at the same rate, the men would be five feet apart after they had left the platform. The backward motion of the platform has the effect of separating the men from one another.
The distance between the men in this illustration corresponds to the length of the sound-wave, or the distance between its two ends. Were a person to stand beside the line, and count the men that passed him in the three cases given above, he would find that more persons would pass him in the same time when the platform is moving forward than when it is stationary, and fewer persons would pass him in the same time when the platform is moving backward than when it is stationary. In the same way, when a sounding body is moving rapidly forward, the sound-waves beat more rapidly upon the ear of a person who is standing still than when the body is at rest, and less rapidly when the sounding body is moving rapidly backward.
Were the platform stationary, and were the person who is counting the men to be walking along the line, either towards or away from the platform, the effect upon the number of men passing him in a given time would be precisely the same as it would be were the person stationary, and the platform moving either towards or away from him at the same rate. So the change in the rapidity with which pulsations of sound beat upon the ear is precisely the same whether the ear is stationary and the sounding body moving, or the sounding body is stationary and the ear moving.
168. Change of Refrangibility due to the Motion of a Luminous Body.—Refrangibility in light corresponds to pitch in sound, and depends upon the length of the luminous waves. The shorter the luminous waves, the greater the refrangibility of the waves. Very rapid motion of a luminous body has the same effect upon the length of the luminous waves that motion of a sounding body has upon the length of the sonorous waves. When a luminous body is moving very rapidly towards us, its luminous waves are shortened a little, and its light becomes a little more refrangible; when the luminous body is moving rapidly from us, its luminous waves are lengthened a little, and its light becomes a little less refrangible.
Fig. 185.
169. Displacement of Spectral Lines.—In examining the spectra of the stars, we often find that certain of the dark lines are displaced somewhat, either towards the red or the violet end of the spectrum. As the dark lines are in the same position as the bright lines of the absorbing vapor would be, a displacement of the lines towards the red end of the spectrum indicates a lowering of the refrangibility of the rays, due to a motion of the luminous vapor away from us; and a displacement of the lines towards the violet end of the spectrum indicates an increase of refrangibility, due to a motion of the luminous vapor towards us. From the amount of the displacement of the lines, it is possible to calculate the velocity at which the luminous gas is moving. In Fig. 185 is shown the displacement of the F line in the spectrum of Sirius. This is one of the hydrogen lines. RV is the spectrum, R being the red, and V the violet end. The long vertical line is the bright F line of hydrogen, and the short dark line to the left of it is the position of the F line in the spectrum of Sirius. It is seen that this line is displaced somewhat towards the red end of the spectrum. This indicates that Sirius must be moving from us; and the amount of the displacement indicates that the star must be moving at the rate of some twenty-five or thirty miles a second.
Fig. 186.
170. Contortion of Lines on the Disk of the Sun.—Certain of the dark lines seen on the centre of the sun's disk often appear more or less distorted, as shown in Fig. 186, which represents the contortion of the hydrogen line as seen at various times. 1 and 2 indicate a rapid motion of hydrogen away from us, or a down-rush at the sun; 3 and 4 (in which the line at the centre is dark on one side, and bent towards the red end of the spectrum, and bright on the other side with a distortion towards the violet end of the spectrum) indicate a down-rush of cool hydrogen side by side with an up-rush of hot and bright hydrogen; 5 indicates local down-rushes associated with quiescent hydrogen.
The contorted lines, which indicate a violently agitated state of the sun's atmosphere, appear in the midst of other lines which indicate a quiescent state. This is owing to the fact that the absorption which produces the dark lines takes place at various depths in the solar atmosphere. There may be violent commotion in the lower layers of the sun's atmosphere, and comparative quiet in the upper layers. In this case, the lines which are due to absorption in the lower layers would indicate this disturbance by their contortions; while the lines produced by absorption in the upper layers would be free from contortion.
It often happens, too, that the contortions are confined to one set of lines of an element, while other lines of the same element are entirely free from contortions. This is undoubtedly due to the fact that different layers of the solar atmosphere differ greatly in temperature; so that the same element would give one set of lines at one depth, and another set at another depth: hence commotion in the solar atmosphere at any particular depth would be indicated by the contortion of those lines of the element only which are produced by the temperature at that particular depth.
A remarkable case of contortion witnessed by Professor Young is shown in Fig. 187. Three successive appearances of the C line are shown. The second view was taken three minutes after the first, and the third five minutes after the second. The contortion in this case indicated a velocity ranging from two hundred to three hundred miles a second.
Fig. 187.
171. Contortion of Lines on the Sun's Limb.—When the spectroscope is directed to the centre of the sun's disk, the distortion of the lines indicates only vertical motion in the sun's atmosphere; but, when the spectroscope is directed to the limb of the sun, displacements of the lines indicate horizontal motions in the sun's atmosphere. When a powerful spectroscope is directed to the margin of the sun's disk, so that the slit of the collimator tube shall be perpendicular to the sun's limb, one or more of the dark lines on the disk are seen to be prolonged by a bright line, as shown in Fig. 188. But this prolongation, instead of being straight and narrow, as shown in the figure, is often widened and distorted in various ways, as shown in Fig. 189. In the left-hand portion of the diagram, the line is deflected towards the red end of the spectrum; this indicates a violent wind on the sun's surface blowing away from us. In the right-hand portion of the diagram, the line is deflected towards the violet end of the spectrum; this indicates a violent wind blowing towards us. In the middle portion of the figure, the line is seen to be bent both ways; this indicates a cyclone, on one side of which the wind would be blowing from us, and on the other side towards us.
Fig. 188.
Fig. 189.
The distortions of the solar lines indicate that the wind at the surface of the sun often blows with a velocity of from one hundred to three hundred miles a second. The most violent wind known on the earth has velocity of a hundred miles an hour.
Fig. 190.
172. The Granulation of the Photosphere.—When the surface of the sun is examined with a good telescope under favorable atmospheric conditions, it is seen to be composed of minute grains of intense brilliancy and of irregular form, floating in a darker medium, and arranged in streaks and groups, as shown in Fig. 190. With a rather low power, the general effect of the surface is much like that of rough drawing-paper, or of curdled milk seen from a little distance. With a high power and excellent atmospheric conditions, the grains are seen to be irregular, rounded masses, some hundreds of miles in diameter, sprinkled upon a less brilliant background, and appearing somewhat like snow-flakes sparsely scattered over a grayish cloth. Fig. 191 is a representation of these grains according to Secchi.
Fig. 191.
With a very powerful telescope and the very best atmospheric conditions, the grains themselves are resolved into granules, or little luminous dots, not more than a hundred miles or so in diameter, which, by their aggregation, make up the grains, just as they, in their turn, make up the coarser masses of the solar surface. Professor Langley estimates that these granules constitute about one-fifth of the sun's surface, while they emit at least three-fourths of its light.
173. Shape of the Grains.—The grains differ considerably in shape at different times and on different parts of the sun's surface. Nasmyth, in 1861, described them as willow-leaves in shape, several thousand miles in length, but narrow and with pointed ends. He figured the surface of the sun as a sort of basket-work formed by the interweaving of such filaments. To others they have appeared to have the form of rice-grains. On portions of the sun's disk the elementary structure is often composed of long, narrow, blunt-ended filaments, not so much like willow-leaves as like bits of straw lying roughly parallel to each other,—a thatch-straw formation, as it has been called. This is specially common in the immediate neighborhood of the spots.
174. Nature of the Grains.—The grains are, undoubtedly, incandescent clouds floating in the sun's atmosphere, and composed of partially condensed metallic vapors, just as the clouds of our atmosphere are composed of partially condensed aqueous vapor. Rain on the sun is composed of white-hot drops of molten iron and other metals; and these drops are often driven with the wind with a velocity of over a hundred miles a second.
As to the forms of the grains, Professor Young says, "If one were to speculate as to the explanation of the grains and thatch-straws, it might be that the grains are the upper ends of long filaments of luminous cloud, which, over most of the sun's surface, stand approximately vertical, but in the neighborhood of a spot are inclined so as to lie nearly horizontal. This is not certain, though: it may be that the cloud-masses over the more quiet portions of the solar surface are really, as they seem, nearly globular, while near the spots they are drawn out into filamentary forms by atmospheric currents."
175. Faculæ.—The faculæ are irregular streaks of greater brightness than the general surface, looking much like the flecks of foam on the surface of a stream below a waterfall. They are sometimes from five to twenty thousand miles in length, covering areas immensely larger than a terrestrial continent.
These faculæ are elevated regions of the solar surface, ridges and crests of luminous matter, which rise above the general level of the sun's surface, and protrude through the denser portions of the solar atmosphere. When one of these passes over the edge of the sun's disk, it can be seen to project, like a little tooth. Any elevation on the sun to be perceptible at all must measure at least half a second of an arc, or two hundred and twenty-five miles.
The faculæ are most numerous in the neighborhood of the spots, and much more conspicuous near the limb of the sun than near the centre of the disk. Fig. 192 gives the general appearance of the faculæ, and the darkening of the limb of the sun. Near the spots, the faculæ often undergo very rapid change of form, while elsewhere on the disk they change rather slowly, sometimes undergoing little apparent alteration for several days.
Fig. 192.
176. Why the Faculæ are most Conspicuous near the Limb of the Sun.—The reason why the faculæ are most conspicuous near the limb of the sun is this: The luminous surface of the sun is covered with an atmosphere, which, though not very thick compared with the diameter of the sun, is still sufficient to absorb a good deal of light. Light coming from the centre of the sun's disk penetrates this atmosphere under the most favorable conditions, and is but slightly reduced in amount. The edges of the disk, on the other hand, are seen through a much greater thickness of atmosphere; and the light is reduced by absorption some seventy-five per cent. Suppose, now, a facula were sufficiently elevated to penetrate quite through this atmosphere. Its light would be undimmed by absorption on any part of the sun's disk; but at the centre of the disk it would be seen against a background nearly as bright as itself, while at the margin it would be seen against one only a quarter as bright. It is evident that the light of any facula, owing to the elevation, would be reduced less rapidly as we approach the edge of the disk than that of the general surface of the sun, which lies at a lower level.
177. General Appearance of Sun-Spots.—The general appearance of a well-formed sun-spot is shown in Fig. 193. The spot consists of a very dark central portion of irregular shape, called the umbra, which is surrounded by a less dark fringe, called the penumbra. The penumbra is made up, for the most part, of filaments directed radially inward.
Fig. 193.
There is great variety in the details of form in different sun-spots; but they are generally nearly circular during the middle period of their existence. During the period of their development and of their disappearance they are much more irregular in form.
There is nothing like a gradual shading-off of the penumbra, either towards the umbra on the one side, or towards the photosphere on the other. The penumbra is separated from both the umbra and the photosphere by a sharp line of demarcation. The umbra is much brighter on the inner than on the outer edge, and frequently the photosphere is excessively bright at the margin of the penumbra. The brightness of the inner penumbra seems to be due to the crowding together of the penumbral filaments where they overhang the edge of the umbra.
There is a general antithesis between the irregularities of the outer and inner edges of the penumbra. Where an angle of the penumbral matter crowds in upon the umbra, it is generally matched by a corresponding outward extension into the photosphere, and vice versa.
The umbra of the spot is far from being uniformly dark. Many of the penumbral filaments terminate in little detached grains of luminous matter; and there are also fainter veils of a substance less brilliant, but sometimes rose-colored, which seem to float above the umbra. The umbra itself is made up of masses of clouds which are really intensely brilliant, and which appear dark only by contrast with the intenser brightness of the solar surface. Among these clouds are often seen one or more minute circular spots much darker than the rest of the umbra. These darker portions are called nuclei. They seem to be the mouths of tubular orifices penetrating to unknown depths. The faint veils mentioned above continually melt away, and are replaced by others in some different position. The bright granules at the tips of the penumbral filaments seem to sink and dissolve, while fresh portions break off to replace them. There is a continual indraught of luminous matter over the whole extent of the penumbra.
At times, though very rarely, patches of intense brightness suddenly break out, remain visible for a few minutes, and move over the spot with velocities as great as a hundred miles a second.
The spots change their form and size quite perceptibly from day to day, and sometimes even from hour to hour.
178. Duration of Sun-Spots.—The average life of a sun-spot is two or three months: the longest on record is that of a spot observed in 1840 and 1841, which lasted eighteen months. There are cases, however, where the disappearance of a spot is very soon followed by the appearance of another at the same point; and sometimes this alternate disappearance and re-appearance is several times repeated. While some spots are thus long-lived, others endure only a day or two, and sometimes only a few hours.
179. Groups of Spots.—The spots usually appear not singly, but in groups. A large spot is often followed by a train of smaller ones to the east of it, many of which are apt to be irregular in form and very imperfect in structure, sometimes with no umbra at all, often with a penumbra only on one side. In such cases, when any considerable change of form or structure shows itself in the principal spot, it seems to rush westward over the solar surface, leaving its attendants trailing behind. When a large spot divides into two or more, as often happens, the parts usually seem to repel each other, and fly apart with great velocity.
180. Size of the Spots.—The spots are sometimes of enormous size. Groups have often been observed covering areas of more than a hundred thousand miles square, and single spots occasionally measure from forty to fifty thousand miles in diameter, the umbra being twenty-five or thirty thousand miles across. A spot, however, measuring thirty thousand miles over all, may be considered a large one. Such a spot can easily be seen without a telescope when the brightness of the sun's surface is reduced by clouds or nearness to the horizon, or by the use of colored glass. During the years 1871 and 1872 spots were visible to the naked eye for a considerable portion of the time. The largest spot yet recorded was observed in 1858. It had a breadth of more than a hundred and forty-three thousand miles, or nearly eighteen times the diameter of the earth, and covered about a thirty-sixth of the whole surface of the sun.