The only source of our information on stars is their radiation. From stellar spectra we judge the structure of stellar atmospheres, their chemical composition, and physical processes taking place there. Spectrograms are interpreted on the basis of the theory of stellar spectra, wherein lies its enormous value for astrophysics. Until recently astronomers could observe stellar radiation only in a very small range of frequencies making up the visible region of the spectrum. However, about 20 years ago radio astronomy came into being which permits investigating the radiation of celestial bodies in a completely different spectral region.

Quite recently, in connection with the launching of satellites and rockets, there arose the theoretical possibility of obtaining stellar spectra in any frequency range. So far, highly useful spectrograms of stars and the sun have been obtained in the so-called rocket ultraviolet. It is obvious that this broadening of observational data will even further increase the significance of the theory of stellar spectra. At the same time it is necessary to improve and extend this theory.

The surface layers of stars, out of which their spectra arise, represent
strongly ionized gases, i.e., plasma. Plasma studies are also being carried out in physics laboratories, having increased in intensity of late. Methods used by physicists in studying plasmas are in many respects similar to methods used by astrophysicists in studying stellar atmospheres. Therefore, the theory of stellar spectra is of interest not only to astrophysicists but also to physicists.

An excellent example of the broad interest in the theory of stellar spectra is the summer seminary on problems of this theory, organized by the
Astronomical Council of the Academy of Sciences of the USSR and the Leningrad University and held in Leningrad in June 1964* About 150 young astrophysicists and physicists of the Soviet Union participated in the sessions. This book was written on the basis of the lectures given at that time.
The first part of the book examines atomic processes associated with the
formation of spectra, with special emphasis on calculation of the energy levels of the atom and the probability of transitions between these levels. The second part deals with the theory of radiation transfer, which forms an important aspect of the theory of stellar spectra. The next two parts discuss the most essential problems in the formation of spectra of different types of stars and nebulae. The last part, devoted to ultraviolet spectra of celestial bodies, mainly gives a review of observational data and their qualitative interpretation (since, as yet, no quantitative theory of these spectra has been established).
The diversity of the problems of the modern theory of stellar spectra makes it impossible to present them with sufficient completeness in a single monograph.
The authors of this book have endeavored to acquaint the reader with the most important of these problems.

You can get the book here and here

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Preface …………………………………………… iii
PART I. PHYSICAL PROCESSES CONNECTED WITH THE FORMATION OF SPECTRA ………… 1
Plasma Spectroscopy (S.E. Frish) …………………….. 1
Calculations of Atomic Energy Levels (A.P. Yutsis and Ya.I. Vizbarayte) …………… 23
Theory of Atomic Transitions (G.F. Drukarev) ……….. 35

PART II. THEORY OF RADIATIVE TRANSFER ……………………… 64
Certain Nonlinear Problems of the Theory of Radiative Transfer (V.A. Ambartsumyan) ……. 64
Radiative Diffusion in Gases (V.V. Sobolev) …………. 75
Determination of the Populations of Excited Levels in an Optically Thick Gas Layer (V.V. Ivanov) ……….. 92
Nonstationary Radiation Field (I.N. Minin) …………. 116
Randomized Problem of Diffuse Reflection (R.V. Ambartsumyan) ………… 135

PART III. SPECTRA OF FIXED STARS ……………………….. 140
Models of Stellar Atmospheres (V.V. Sobolev) ………. 140
Continuous Spectra of Hot White Dwarfs (A.K. Kolesov) ……. 147
Model Atmospheres of Main-Sequence Stars of Class M (V.G. Buslavskiy) …….. 152
Determination of the Chemical Composition of Stellar Atmospheres (A.A. Boyarchuk) ……….. 160

PART IV. SPECTRA OF NONSTATIONARY STARS AND INTERSTELLAR MATTER ………… 170
Spectra of Nonstationary (Variable) Stars (V.G. Gorbatskiy) ……….. 170
Analysis of the Emission Spectra of Nonstationary Stars (A.A. Boyarchuk) ………… 194
Spectra of Interstellar Matter (S.A. Kaplan) ………… 203
Radio Observations of Planetary Nebulae (Yu. N. Pariyskiy) ………. 216

PART V. SPECTRA OF CELESTIAL BODIES IN THE FAR ULTRAVIOLET REGION ……….. 220
Spectra of Celestial Bodies in the Far Ultraviolet Region (G.A. Gurzadyan) ………. 220