The Temperature of Reversing Layers of Stars

Abstract

FOLLOWING the well-known treatment of the problem by Schwarzchild (1906), it has become conventional to estimate the temperature, T, of the outer atmosphere of a star as of the order T₁/2^¼, or about 0.85 T₁, where T₁ is the effective temperature of the surface (practically, the surface of the photosphere) as given by application of Stefan's law to the total energy radiation. This result presupposes that the stellar atmosphere can be treated as “grey”, that is, as having an absorption coefficient which is the same for all wave-lengths. Milne has published (Mon. Not. R.A.S., 82, 368, 1922) an approximate mathematical discussion which indicates that when the atmospheric absorption-coefficient varies with the wave-length, the value of T may fall as low as ½T₁, or rise as high as T₁. The equation upon which he bases his treatment postulates that the outflowing radiation may be taken as approximately black. For stellar reversing layers this assumption seems unjustified, in consideration of the sharply selective opacity indicated by the existence of Fraunhofer lines. The radiation is indeed deficient in the ivery wave-lengths in which the gas may be expected to absorb and radiate most strongly. That the general, as compared with the sharply selective, opacity of the upper reversing layer in the sun is negligible, is shown by the existence of the bright line flash spectrum.