It should be remembered that the interpretation of stellar spectra from the standpoint of thermal-ionization is new and the methods employed are as yet relatively primitive. We are only at the beginning of the astronomical application of the methods arising from the newer analyses of atoms. Hence we must expect (and endeavor to provide) that a study such as is presented here will promptly need revision and extension in many places. Nevertheless, as it stands, it shows the current state of the general problem, and will also serve, we hope, as a summary of past investigations and an indication of the direction to go in the immediate future. In the course of her investigation of stellar atmospheres, Miss Payne has had the advantage of conferences with Professors Russell and Stewart of Princeton University and Professor Saunders of Harvard University, as well as with various members of the Harvard Observatory staff. The book has been accepted as a thesis fulfilling the requirements for the degree of Doctor of Philosophy in Radcliffe College. H. S. MAY 1, 1925. [Pg vii] CONTENTS PART I THE PHYSICAL GROUNDWORK I. THE LABORATORY BASIS OF ASTROPHYSICS 3 Relation of physics to astrophysics. Properties of matter associated with nuclear structure. Arrangement of extra-nuclear electrons. Critical potentials. Duration of atomic states. Relative probabilities of atomic states. Effect on the spectrum of conditions at the source. (a) Temperature class. (b) Pressure effects. (c) Zeemann effect. (d) Stark effect. II. THE STELLAR TEMPERATURE SCALE 27 Definitions. The mean temperature scale. Temperatures of individual stars. Differences in temperature between giants and dwarfs The temperature scale based on ionization. III. PRESSURES IN STELLAR ATMOSPHERES 34 Range in stellar pressures. Measures of pressure in the reversing layer. (a) Pressure shifts of spectral lines. (b) Sharpness of lines. (c) Widths of lines. (d) Flash spectrum. (e) Equilibrium of outer layers of the sun. (f) Observed limit of the Balmer series. (g) Ionization phenomena. IV. THE SOURCE AND COMPOSITION OF THE STELLAR SPECTRUM 46 General appearance of the stellar spectrum. Descriptive definitions. The continuous background. The reversing layer. Emission lines. V. ELEMENTS AND COMPOUNDS IN STELLAR ATMOSPHERES 55 Identifications with laboratory spectra. Occurrence and behavior of known lines in stellar spectra. [Pg viii] PART II THEORY OF THERMAL IONIZATION VI. THE HIGH-TEMPERATURE ABSORPTION SPECTRUM OF A GAS 91 The schematic reversing layer. The absorption of radiation. Low temperature conditions. Ultimate lines. Ionization. Production of subordinate lines. Lines of ionized atoms. Summary. VII. CRITICAL DISCUSSION OF IONIZATION THEORY 105 Saha’s treatment—marginal appearance. Theoretical formulae. Physical constants required by the formulae. Assumptions necessary for the application. Laboratory evidence bearing on the theory.