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Updated: June 16, 2025
Large as Thett, near a half million miles in diameter, its mass was very closely equal to that of our sun. Yet it was but the burned-out sweepings of the outermost photospheric layers of this giant sun, and the radioactive atoms that made a sun active were not here; it was a cold planet.
But this view, though countenanced by Ångström, and advocated by Hastings of Baltimore, and other authorities, is open to grave objections. In Faye's theory, sun-spots were regarded as simply breaks in the photospheric clouds, where the rising currents had strength to tear them asunder.
According to this view, the star is to be regarded as possessing an extensive atmosphere of hydrogen, which, during the maximum, is upheaved into enormous prominences, and the brilliance of the light from these prominences suffices to swamp the photospheric light, so that in the spectrum the hydrogen lines appear bright instead of dark.
From a series of experiments carefully conducted at Daramona, Ireland, with a delicate thermal balance, of the kind invented by Boys and designated a "radio-micrometer," Messrs. Wilson and Gray arrived in 1893, with the aid of Stefan's Law, at a photospheric temperature of 7,400° C., reduced by the first-named investigator in 1901 to 6,590°. Dr.
But in 1884, G. A. Hirn of Colmar, having regard to the enormous velocities of projection observed in the interim, fixed two million degrees Centigrade as the lowest internal temperature by which they could be accounted for; although admitting the photospheric condensations to be incompatible with a higher external temperature than 50,000° to 100,000° C.
In other spots subsequently examined by him, similar perspective effects were visible, and he proved in 1774, by strict geometrical reasoning, that they could only arise in vast photospheric excavations. It was not, indeed, the first time that such a view had been suggested.
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