Fig. 26.—The great sun-spot of October 9, 1903. Taken with the photo-heliograph of Greenwich in the usual manner. The spot is shown at mean level of the calcium faculæ. The two following photographs show a lower-level and a higher-level section through the calcium faculæ
Sometimes the matter of the prominences seems to fall back upon the surface of the sun between the smaller flames of fire which we have likened to blades of grass (Fig. 21). In most cases, however, the prominences appear slowly to dissolve. When their brilliant glow fades owing to their intense radiation, they can no longer be observed. The quiet prominences, which seem to float at heights of about 50,000 km. and at still greater heights, must there be almost in a vacuum. Their particles cannot be supported by any surrounding gases, after the manner of the drops of water in terrestrial clouds. In order that they may remain floating they must be pushed away from the sun by a peculiar force—the radiation pressure (see [Chapter IV].).
Fig. 27.—The great sun-spot of October 9, 1903. Photograph of the low-level calcium faculæ with the aid of the light of the calcium line H. The spot is not obscured by the faculæ—at least, not so much as in the following illustrations
The faculæ can be studied in the same way as the prominences, and of late Deslandres and Hale have used for this purpose a special instrument, the heliograph (compare Figs. 26 to 29). When the faculæ approach the limb of the sun they appear particularly brilliant by comparison with their surroundings. That seems to indicate that they are lying at a great altitude, and that their light is hence not weakened by the superposed hazy stratum. When they reach the sun’s limb they appear to us like raised portions of the photosphere. The clouds which form these faculæ are carried upward by powerful ascending streams of gas whose expansion is due to the diminution of the gaseous pressure.
Fig. 28.—The great sun-spot of October 9, 1903. Photograph of the higher-level calcium faculæ, taken with the light of the central portion of the line H (calcium). The higher-level faculæ hide the spot, indicating that the faculæ spread considerably during their ascent
Fig. 29.—The great sun-spot of October 9, 1903. Photograph of the hydrogen faculæ, taken with the light of the spectral line F (hydrogen). Only the darkest portions of the spot are visible. The other portions are obscured by masses of the hydrogen, which were evidently in a restless state
Sun-spots display many peculiarities in their spectra (Figs. 24 and 25). Very prominent is always the helium line; prominent likewise the dark sodium lines, which are markedly widened and which show in their middle portions a bright line—the so-called reversal of lines (Fig. 24). This occurrence indicates that the metal is lying in a deeper stratum. In the red portion of the spectrum we find bands, just as in the spectra of the red stars. These bands, which appear to be resolved into crowds of lines by the aid of powerful instruments, indicate the presence of chemical compounds. Since the spot is comparatively of feeble intensity, its spectrum appears superposed like a less bright ribbon upon the background of the spectrum of the more luminous photosphere. The violet end of the sun-spot spectrum is particularly weakened. Although the spot has the appearance of a pit in the photosphere, and when on the sun’s limb makes it look as if a piece had been cut out of the edge, it yet does not appear darker than the sun’s edge. That points to the conclusion that the light emitted by the spot emanates chiefly from its upper, cold portions.