X-ray emission from clusters of galaxies and galactic nuclei.

Observational data on compact X-ray sources in quasars and active galactic nuclei are reviewed, along with X-ray data on clusters of galaxies. Attention is given to general characteristics of X-ray quasars and galactic nuclei, model-independent limits on compact-source parameters, and X-ray emission mechanisms, including thermal bremsstrahlung, Comptonization, relativistic Compton scattering, and accretion onto massive black holes. Implications of the data on clusters of galaxies are examined in relation to the present state of knowledge about cluster sources and the evolution of the gas content of clusters. Other possible source classes that may be discovered by HEAO-B and the next generation of X-ray telescopes are considered, in particular, Compton scattering in remote radio sources, supernovae in young galaxies, and unknown sources of the 2-6-keV X-ray background.