The Heavens Above: A Popular Handbook of Astronomy

by J. A. Gillet; W. J. Rolfe

Professor Young also suggests that the spots may be depressions in the photosphere caused "by the diminution of upward pressure from below, in consequence of eruptions in the neighborhood; the spots thus being, so to speak, sinks in the photosphere. Undoubtedly the photosphere is not a strictly continuous shell or crust; but it is heavy as compared with the uncondensed vapors in which it lies, just as a rain-cloud in our terrestrial atmosphere is heavier than the air; and it is probably continuous enough to have its upper level affected by any diminution of pressure below. The gaseous mass below the photosphere supports its weight and the weight of the products of condensation, which must always be descending in an inconceivable rain and snow of molten and crystallized material. To all intents and purposes, though nothing but a layer of clouds, the photosphere thus forms a constricting shell, and the gases beneath are imprisoned and compressed. Moreover, at a high temperature the viscosity of gases is vastly increased, so that quite probably the matter of the solar nucleus resembles pitch or tar in its consistency more than what we usually think of as a gas. Consequently, any sudden diminution of pressure would propagate itself slowly from the point where it occurred. Putting these things together, it would seem, that, whenever a free outlet is obtained through the photosphere at any point, thus decreasing the inward pressure, the result would be the sinking of a portion of the photosphere somewhere in the immediate neighborhood, to restore the equilibrium; and, if the eruption were kept up for any length of time, the depression in the photosphere would continue till the eruption ceased. This depression, filled with the overlying gases, would constitute a spot. Moreover, the line of fracture (if we may call it so) at the edges of the sink would be a region of weakness in the photosphere, so that we should expect a series of eruptions all around the spot. For a time the disturbance, therefore, would grow, and the spot would enlarge and deepen, until, in spite of the viscosity of the internal gases, the equilibrium of pressure was gradually restored beneath. So far as we know the spectroscopic and visual phenomena, none of them contradict this hypothesis. There is nothing in it, however, to account for the distribution of the spots in solar latitudes, nor for their periodicity."

IV. THE CHROMOSPHERE AND PROMINENCES.

193. The Sun's Outer Atmosphere.—What we see of the sun under ordinary circumstances is but a fraction of his total bulk. While by far the greater portion of the solar mass is included within the photosphere, the larger portion of his volume lies without, and constitutes a gaseous envelope whose diameter is at least double, and its bulk therefore sevenfold, that of the central globe.

This outer envelope, though mainly gaseous, is not spherical, but has an exceedingly irregular and variable outline. It seems to be made up, not of regular strata of different density, like our atmosphere, but rather of flames, beams, and streamers, as transient and unstable as those of the aurora borealis. It is divided into two portions by a boundary as definite, though not so regular, as that which separates them both from the photosphere. The outer and far more extensive portion, which in texture and rarity seems to resemble the tails of comets, is known as the coronal atmosphere, since to it is chiefly due the corona, or glory, which surrounds the darkened sun during an eclipse.

194. The Chromosphere.—At the base of the coronal atmosphere, and in contact with the photosphere, is what resembles a sheet of scarlet fire. It appears as if countless jets of heated gas were issuing through vents over the whole surface, clothing it with flame, which heaves and tosses like the blaze of a conflagration. This is the chromosphere, or color-sphere. It owes its vivid redness to the predominance of hydrogen in the flames. The average depth of the chromosphere is not far from ten or twelve seconds, or five thousand or six thousand miles.

195. The Prominences.—Here and there masses of this hydrogen, mixed with other substances, rise far above the general level into the coronal regions, where they float like clouds, or are torn to pieces by conflicting currents. These cloud-masses are known as solar prominences, or protuberances.

196. Magnitude and Distribution of the Prominences.—The prominences differ greatly in magnitude. Of the 2,767 observed by Secchi, 1,964 attained an altitude of eighteen thousand miles; 751, or nearly a fourth of the whole, reached a height of twenty-eight thousand miles; several exceeded eighty-four thousand miles. In rare instances they reach elevations as great as a hundred thousand miles. A few have been seen which exceeded a hundred and fifty thousand miles; and Secchi has recorded one of three hundred thousand miles.

Fig. 214.

The irregular lines on the right-hand side of Fig. 214 show the proportion of the prominences observed by Secchi, that were seen in different parts of the sun's surface. The outer line shows the distribution of the smaller prominences, and the inner dotted line that of the larger prominences. By comparing these lines with those on the opposite side of the circle, which show the distribution of the spots, it will be seen, that, while the spots are confined mainly to two belts, the prominences are seen in all latitudes.