Our Astronomical Column

Abstract

THE NATURE OF SUN-SPOTS.—A useful summary of our knowledge of the phenomena presented by sun-spots is given by the Kev. A. L. Cortie in Science Progress for October. A spot is regarded as an up-rush of metallic vapours, which become cooled by rapid expansion, so that the spot appears dark by contrast with the bright solar surface. The umbra is considered to rise above the level of the photosphere, while the penumbra is built up by dark radial streams flowing from the umbra and seeking a level slightly lower than that of the photosphere. In round spots the penumbra is a shallow, saucer-like cavity, the lowest portion being due to the falling-in of the photospheric clouds caused by the initial uprush. This falling-in and heaping together of the photospheric clouds to fill the partial void produced by the ejection of the umbral vapours would account for the bright border which is generally seen to separate the umbra and penumbra. At a high level above the spot are the hvdrogen flocculi, the rotation of which gives rise to the appearance of the solar vortices. Friction of the gyrating gases and vapours is considered competent to generate electric currents, and the accompanying magnetic fields which produce the Zeeman effects in the spectra of spots. Father Cortie considers it doubt ful whether the umbra and penumbra of the spots themselves share this gyratory motion.