This third volume of "A Course in Astrophysics and Stellar Astronomy" deals with observational results and their interpretation. It is not the aimof this book either to present all existing astrophysical theories and hypo­ theses or to discuss systematically general problems in the modern theories of cosmogony and cosmology. Such subjects have been treated in special textbooks and monographs.* However, in all the astronomical literatureso far there has been no book which gives a systematic presentation of observational results together with methods for their interpretation. This fact has determined the nature and the scope of this text. It mainly presents facts, and the principal theoretical studies related to an interpretation ofthe observational data are given only briefly (they are often just mentionedin the references). As far as possible, we have tried to avoid controversial theoretical questions for which no definite answers have yet been found,since this book does not represent a survey of astrophysics and stellar astronomy in the sense of the volumes edited by Kuiper on the sun and the solar system. Rather, it constitutes a textbook for young astronomers,both graduates and undergraduates. For this reason, it was impossible to include a systematic discussion of general and complex problems related to cosmology and cosmogony, the proper place for which is in more specialized monographs.For the teaching of astrophysics in a university, this book should be used in parallel with a course in theoretical astrophysics. Consequently, in our presentation it is assumed that the reader is already familiar with the rudiments of theory.This volume. Volume III, is divided into two parts: 1) The Sun, and 2) The Planetary System (planets, comets, and meteors).Part I mainly stresses so-called solar activity, that is, the processes occurring on the surface of the sun. These processes are inevitably interrelated, since they represent different stages in the development of large-scale motions on the solar surface. These motions produce so-called active regions, which in a certain sense may be compared with terrestrial cyclones and anticyclones.