Theoretical astrophysics is the science which studies and explains, using the laws of physics, the physical processes occurring in the heavenly bodies. In doing so, wide use is made of mathematical methods, but these play only a subsidiary part.Although theoretical astrophysics is a young and very rapidly deve­ loping science, its results are already of great importance in all branches of astronomy and in many branches of physics. The subject has been developed particularly in the USSR.Soviet scientists who work in the field of theoretical astrophysics are guided by the method of dialectical materialism, and always link their work to reality, using mathematical and physical methods as an important and powerful means of studying the heavenly bodies.In the field of theoretical astrophysics, Soviet scientists now occupy the leading place in the world. The authors of this book have made by their investigations considerable contributions to the branches of theoretical astrophysics which they discuss. The treatment of many topics in the present work is therefore completely original, and sometimes differs sharply from anything that has been published in scientific lite­ rature. This fact should be kept in mind in studying the book.The course is designed for university students, graduates, and scientific workers. In a new branch of science, errors are inevitable, but it is hoped that this book contains as few as possible.