The theory of stellar atmospheres is one of the most important branches of modern astrophysics. It is first of all a major tool for understanding all aspects of stars. As the physical properties of their outer layers can now be found with high precision, firm conclusions can be drawn about the internal structure and evolution of stars. Moreover, improvements in our knowledge of the chemical composition of stars is shedding new light on the chemical evolution of galaxies and of the Universe as a whole. Because the outer layers of stars are among the best-understood astrophysical objects, the theory of stellar atmospheres plays an important role in the study of many other types of objects. These include planetary nebulae, H II regions, interstellar matter, and objects of interest in high-energy astrophysics, such as accretion disks (close binaries, dwarf novae, cataclysmic variables, quasars, active galactic nuclei), pulsar magnetospheres, and Seyfert galaxies. Finally, as stars provide a laboratory in which plasmas can be studied under more extreme conditions than on earth, the study of stellar atmospheres has strong connections with modern physics. Astronomical observations provided a vital stimulus in the early stages of quantum theory and atomic physics; even today topics such as low-temperature dielectronic recombination develop hand in hand with the interpretation of stellar and nebular spectra. Early work on MHD was similiarly motivated. Many such connections remain to be explored.

The theory of stellar atmospheres is one of the most important branches of modern astrophysics. It is first of all a major tool for understanding all aspects of stars. As the physical properties of their outer layers can now be found with high precision, firm conclusions can be drawn about the internal structure and evolution of stars. Moreover, improvements in our knowledge of the chemical composition of stars is shedding new light on the chemical evolution of galaxies and of the Universe as a whole. Because the outer layers of stars are among the best-understood astrophysical objects, the theory of stellar atmospheres plays an important role in the study of many other types of objects. These include planetary nebulae, H II regions, interstellar matter, and objects of interest in high-energy astrophysics, such as accretion disks (close binaries, dwarf novae, cataclysmic variables, quasars, active galactic nuclei), pulsar magnetospheres, and Seyfert galaxies. Finally, as stars provide a laboratory in which plasmas can be studied under more extreme conditions than on earth, the study of stellar atmospheres has strong connections with modern physics. Astronomical observations provided a vital stimulus in the early stages of quantum theory and atomic physics; even today topics such as low-temperature dielectronic recombination develop hand in hand with the interpretation of stellar and nebular spectra. Early work on MHD was similiarly motivated. Many such connections remain to be explored.

The publication in English of this monograph seems to me to indicate the ever increasing interest of astrophysicists in the physical and dynamical problems of planetary nebulae-one of the most interesting and fruitful branches of theoretical astrophysics. Their interest in part arises from the fact that the methods of identify ing the physical processes occurring in planetary nebulae, as well as the many theo retical results, are now acquiring a degree of uni versality as their sphere of application increases. Finally, the special cosmic significance of planetary nebulae is becoming apparent. The English edition of Planetary Nebulae differs considerably from the Russian version published in 1962, primarily because of the new results included in it, but also because of numerous editorial revisions. The problems of magnetic fields and hydrodynamics in planetary nebulae are beginning to occupy an important place in the study of the dynamics of these objects. Recent studies by D. H. Menzel confirm the idea advanced in the present mono graph as to the existence of magnetic fields in planetary nebulae. New light is being cast on the dynamics of planetary nebulae by the hydrodynamic investigations of F. D. Kahn, W. G. Mathews and others. Unfortunately I was not able to include these and other interesting results in the present edition."