Theoretical Astrophysics in the Byurakan Observatory (1946 - 2021)

The purpose of this review is to give a general idea of the results on theoretical astrophysics investigations carried out at the Byurakan Observatory during the period of its existence. A special place in the review is given to the research work in the Department of "Theoretical Astrophysics", founded in 1965. The review consists of two parts. The first part is mainly devoted to works on the theory of radiation transfer somehow related to Ambartsumian's ideas developed in his fundamental studies of the 40-50s of the last century. The second part presents, on the one hand, the results which are of importance for theoretical physics in general and, on the other hand, gives their application to the interpretation of observational data of various cosmic objects. In order to get an idea of the importance of each result, the background and prerequisites of the research are given, indicating its significance from the point of view of astrophysics. Naturally, the most important results are presented in more detail. At the same time, the review in one way or another includes almost all of the most important results obtained by various members of the department at different times. It should be noted that works on some other areas of theoretical astrophysics are not included in the review, since they are presented in other reviews of this series.