No CrossRef data available.
Published online by Cambridge University Press: 15 February 2018
There is a growing body of evidence that the non-thermal extra-galactic sources in the Universe- -the Radio Galaxies, Seyfert nuclei, QSO's, BL Lacertae objects and the X-ray galaxies- -are all powered ultimately by collapsed objects at the centers of galaxies. Moreover, there is no reason at this stage to doubt that the central energy source is probably the same in all cases. At this point, five years before the Space Telescope is to be launched, the fundamental problem is to identify the nature of the central engine and then to understand in detail the variety of observed phenomena.