The present book is a translation of the original German edition (published in 1982) with some minor corrections and improvements. The guide to sup plementary and advanced literature given in the Appendix, however, has been brought up to date. This book is addressed primarily to students taking astronomy as a prin cipal or subsidiary subject, and to scientists of related fields, but amateur as tronomers should also be able to profit from it. For most chapters an elementary knowledge of mathematics and physics will be sufficient, however, Chaps. 5 and 6 impose somewhat greater requirements. In addition the reader should already be acquainted with the basic concepts of stellar physics as treated in introduc tory books, including the spectral types, the system of stellar magnitudes and colours, absolute magnitudes and luminosities, the Herlzsprung-Russell dia gram and its interpretation. A modem textbook should use SI units. On the other hand, the use of the cgs system is still the prevailing custom in astrophysics - together with the special units of astronomy: length is quoted in parsecs [pc], mass in solar masses [M0] and time in years [a]. We have therefore compromised and employed both cgs and SI units in this book, whichever was the appropriate choice in each instance. A table for conversion of cgs units into SI units and vice versa is given in the Appendix.

Die AktivitAt von Galaxien ist eine relativ seltene und offenbar kurzlebige Erscheinung, die sich vor allem durch eine verstArkte Abstrahlung, oft A1/4ber das ganze Spektrum hinweg, vom RAntgen- bis zum Radiobereich, bemerkbar macht. Dabei wird auf neue Befunde an Infrarotgalaxien eingegangen, die dafA1/4r sprechen, daA die gravitative Wechselwirkung zwischen Galaxien eine wesentliche Rolle spielt. Das Buch ist eine zusammenfassende Darstellung des heutigen Kenntnisstandes A1/4ber "Aktive Galaxien," ein Thema, das gegenwArtig im Zentrum des astronomischen Interesses und der aktuellen Forschung steht. Nach einem Aoeberblick A1/4ber die seit lAngerem bekannten PhAnomene und die Probleme ihrer Deutung wird auf neue, am Max-Planck-Institut fA1/4r Astronomie mit den Teleskopen der Calar Alto Sternwarte (SA1/4dspanien) gewonnene, Ergebnisse eingegangen.