The book is written primarily for theorists, although individual questions should be of interest to experimenters. In the most general terms, the presentation is constructed according to the scheme "from physics to problems - to mathematics and then again to physics" .. As for the place assigned to mathematics, there is a possibility of criticism from two sides .. Some readers will probably not be satisfied with the rigor of the presentation, while others, on the contrary, will seem unjustified excessive attention to purely mathematical issues. It seems most correct to say that the book provides a solution to a number of problems of applied mathematical physics at the physical level of rigor. The selection of the material was largely determined by the author's personal interests. Great place is given to the results obtained in recent years by a group of astrophysicists-theoreticians of Leningrad University .. A significant part of the material is published for the first time.