Fig. 2.—Group of Solar Spots observed in 1864 by Nasmith.
Fig. 3.—Another Solar Spot observed by Nasmyth.
We have just said that the sun has an atmosphere. Such is the conclusion to which the careful examination of the great star has led.
From the earliest times at which the sun was observed, a theory of its constitution was formulated, which was perpetuated down to the present age, without receiving authoritative contradiction. In the eighteenth century the astronomers Herschel and Wilson developed this theory, which was popularized in our time by the writings of Humboldt and Arago.
According to this theory, the sun is composed of a dark nucleus, and a burning atmosphere, which is the only source of the light proper to this star. Arago and Humboldt called the incandescent atmosphere of the sun, the photosphere. Heat and light would not, therefore, come to us from the nucleus, but only from the photosphere.
The spots are explained, according to this theory, by admitting that they are openings accidentally formed in the sun's atmosphere by gases discharged from volcanic craters, or in some other way. Through these openings the dark nucleus of the sun is seen. The penumbra of the spots are formed by the lower parts of the atmosphere of the sun, which is either hot or luminous. This lower portion of the atmosphere, reflecting the light emitted by the upper portion or photosphere, is slightly warm, and only partially illumined.
This theory of the constitution of the sun, and of the solar spots, seemed for a long time satisfactory. A similar explanation, that is to say, by partial eruptions of gas from volcanic craters, was assigned to the kind of black dotted appearance observed on the surface of the solar disc, and which is exactly reproduced in the two figures here given.
The brilliant spots scattered over the surface of the sun, which touched here and there with points of intense luminosity, are called faculæ. These brilliant points are said to proceed from local accidents, which cause an escape of light and heat from certain parts of the solar atmosphere.
Thus, according to this theory, the sun would be a solid body, opaque and dark like the planets, surrounded by an atmospheric layer, which would prevent any heat in the nucleus. Outside that layer would be a second atmosphere, the photosphere, which only would be luminous, and capable of emitting light and heat. Dark nucleus, dark atmosphere, luminous photosphere, such would be the constituent elements of the sun, according to Wilson, William Herschel, Humboldt, and Arago. To any who hold this theory, it is not impossible to believe that the sun may be inhabited by beings who differ but slightly to man, or who are endowed with an organization similar to that of the inhabitants of the earth. If the body of the sun be preserved by the interposition of a cold, and but slightly conducting atmosphere from the rays of the photosphere which burns at an immense distance, we can believe that creatures organized almost like ourselves could live within it. The heat of the burning photosphere can reach it through the thickness of the lower atmosphere with only the degree of heat necessary to maintain life. The light thus filtered is brilliant, but not dazzling, and admits of the existence of beings of organization similar to those who live on the earth.