Cosmic Clouds of Calcium and Sodium

Abstract

THE most probable explanation I know of the existence of cosmic clouds of calcium and sodium, which reveal themselves in the stationary lines H and K in the spectrum of many helium stars, wras proposed by Prof. Saha in NATURE of June 16, 1921, p. 488, on the ground of his theory of thermal ionisation. Ionised vapours of calcium, the resonance lines of which appear as H and K, are repelled by the radiation pressure of B-type stars, the radiation of which is particularly great in the region of those lines; in this way at a sufficient distance from the star there is an equilibrium between gravitation and selective radiation pressure, the conditions being specially favourable for Ca +, which forms extensive layers near the star.