[346] Menzel, H. C. 258, 1924. [347] Kiess, Pub. Obs. Mich., 3, 106, 1923. [348] Mitchell, Ap. J., 38, 407, 1913. [349] Gousmid, Naturwiss., 41, 851, 1924. [350] A. N., 191, 393, 1912. [351] A. N., 192, 82, 1912. [352] A. N., 192, 266, 1912. [353] Chapter I, p. 5. [354] Abbot, The Sun, 92, 1911. [Pg 89] PART II THEORY OF THERMAL IONIZATION [Pg 90] [Pg 91] CHAPTER VI THE HIGH-TEMPERATURE ABSORPTION SPECTRUM OF A GAS IT is certain that the conditions of which we see the integrated result in the stellar spectrum are exceedingly complicated. Unfortunately, the superficial portion of the star about which direct observational evidence can be obtained is far less tractable to theory than is the interior. Progress is only made possible by treating at the outset a simplified case, by aiming merely at approximate results, and in particular by limiting the preliminary discussion to the factors which are numerically the most effective. As an introduction to the theory of thermal ionization, the present chapter aims at the reconstruction and interpretation of a stellar spectrum by applying known physical laws under very simple conditions. The stellar reversing layer may be represented by an optically thin layer of gas, at a pressure of the order of one ten thousandth of an atmosphere; it lies between the observer and a photosphere which radiates as a black body. The observer receives the radiation from both reversing layer and photosphere, which are regarded, in the present descriptive section, as independent. A more complete treatment would take account of the temperature and pressure gradients in the atmosphere of the star, the flux of energy, and the consequent intimate connection between reversing layer and photosphere. Actually they grade imperceptibly one into the other. The photosphere is that level in the atmosphere at which the general opacity cuts off the direct light from the interior;[355] in the case discussed the reversing layer is considered to be optically so thin that the general opacity is negligible. The selective opacity, depending on the natural absorption frequencies of the atoms present in the gas, gives rise [Pg 92] to the line absorption spectrum which we are about to consider; the region of sensible general opacity, represented by the photosphere, gives rise to a continuous spectrum corresponding to the continuous background in the star. THE ABSORPTION OF RADIATION