MAY 1, 1925.
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. | |
| 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. (a) Ultimate lines. (b) Temperature classes. (c) Furnace experiments. (d) Conductivity of flames. Solar intensities as a test of ionization theory. | |
| VIII. OBSERVATIONAL MATERIAL FOR THE TEST OF IONIZATIONTHEORY | [116] |
| Measurement of line intensity. Method of standardization. Summary of results. Consistency of results. | |
| IX. THE IONIZATION TEMPERATURE SCALE | [133] |
| Consistency of the preliminary scale. Effect of pressure. Levels of origin of ultimate and subordinate lines. Influence of relative abundance. Method of determining effective partial pressure. The corrected temperature scale. | |
| X. THE IONIZATION TEMPERATURE SCALE | [133] |
| PART III | |
| ADDITIONAL DEDUCTIONS FROM IONIZATION THEORY | |
| XI. THE ASTROPHYSICAL EVALUATION OF PHYSICAL CONSTANTS | [155] |
| Spectroscopic constants (Plaskett). Critical potentials (Payne). Duration of atomic states (Milne). | |
| XII. SPECIAL PROBLEMS IN STELLAR ATMOSPHERES | [161] |
Class  stars.Class  stars.The Balmer lines. Classification of stars.Silicon and Strontium stars. Peculiar Class stars.c-stars. | |
| XIII. THE RELATIVE ABUNDANCE OF THE ELEMENTS | [177] |
| Terrestrial data. Astrophysical data. Uniformity of composition of stellar atmospheres. Marginal appearance. Comparison of stellar and terrestrial estimates. | |
| XIV. THE MEANING OF STELLAR CLASSIFICATION | [190] |
| Principles of classification. Object of the Draper Classification. Method of classifying. Finer Subdivisions of the Draper Classes. Implications of the Draper system. Homogeneity of the classes. Spectral differences between giants and dwarfs. | |
| XV. ON THE FUTURE OF THE PROBLEM | [199] |
| APPENDICES | |
| I. INDEX TO DEFINITIONS | [203] |
| II. SERIES RELATIONS IN LINE SPECTRA | [203] |
| III. LIST OF STARS USED IN CHAPTER VIII | [205] |
| IV. INTENSITY CHANGES OF LINES WITH UNKNOWN SERIESRELATIONS | [207] |
| V. MATERIAL ON A STARS, QUOTED IN CHAPTER XII | [208] |
| SUBJECT INDEX | [211] |
| NAME INDEX | [214] |
PART I
THE PHYSICAL GROUNDWORK
CHAPTER I
THE LABORATORY BASIS OF ASTROPHYSICS
THE application of physics in the domain of astronomy constitutes a line of investigation that seems to possess almost unbounded possibilities. In the stars we examine matter in quantities and under conditions unattainable in the laboratory. The increase in scope is counterbalanced, however, by a serious limitation—the stars are not accessible to experiment, only to observation, and there is no very direct way to establish the validity of laws, deduced in the laboratory, when they are extrapolated to stellar conditions.
The verification of physical laws is not, however, the primary object of the application of physics to the stars. The astrophysicist is generally obliged to assume their validity in applying them to stellar conditions. Ultimately it may be that the consistency of the findings in different branches of astrophysics will form a basis for a more general verification of physical laws than can be attained in the laboratory; but at present, terrestrial physics must be the groundwork of the study of stellar conditions. Hence it is necessary for the astrophysicist to have ready for application the latest data in every relevant branch of physical science, realizing which parts of modern physical theory are still in a tentative stage, and exercising due caution in applying these to cosmical problems.
The recent advance of astrophysics has been greatly assisted by the development, during the last decade, of atomic and radiation theory. The claim that it would have been possible to predict the existence, masses, temperatures, and luminosities of the stars from the laws of radiation, without recourse to stellar observations, represents the triumph of the theory of radiation. It is equally true that the main features of the spectra of the stars could be predicted from a knowledge of atomic structure and the origin of spectra. The theory of radiation has permitted an analysis of the central conditions of stars, while atomic theory enables us to analyze the only portion of the star that can be directly observed—the exceedingly tenuous atmosphere.
The present book is concerned with the second of these two problems, the analysis of the superficial layers, and it approaches the subject of the physical chemistry of stellar atmospheres by treating terrestrial physics as the basis of cosmical physics. From a brief working summary of useful physical data ([Chapter I]) and a synopsis of the conditions under which the application is to be made ([Chapters II] and [Chapter III]), we shall pass to an analysis of stellar atmospheres by means of modern spectrum theory. The standpoint adopted is primarily observational, and new data obtained by the writer in the course of the investigation will be presented as part of the discussion.
The first chapter contains a synopsis of the chief data which bear on atomic structure—the nuclear properties, and the disposition of the electrons around the nucleus. The origin of line spectra is discussed, and the ionization potentials corresponding to different atoms are tabulated. Lastly a brief summary is made of the effect of external conditions, such as temperature, pressure, and magnetic or electric fields, upon a line spectrum.