The extra-galactic sources can be conveniently divided into two groups: the extended or transparent sources, and the compact or opaque sources. The observed properties of the two groups are discussed in more detail in the following pages.
Quite surprisingly there is no simple relation between the dimensions of the radio-emitting region and the dimensions of the optical galaxy or QSO, and it must be emphasized that the division into compact and extended radio sources in no way separates the quasars from the radio galaxies. In fact, insofar as we know for any individual source, the quasars are indistinguishable from the radio galaxies on the basis of their radio properties alone. There are, however, clear statistical differences; the majority of the compact sources are identified with quasars or with galaxies that have bright nuclei, such as N-type or Seyfert galaxies. However, many also appear in normal-looking elliptical galaxies as well. Likewise the extended radio sources are not identified only with galaxies, but are frequently associated with quasars showing no visible optical extent. Because the compact sources are all affected by self-absorption (see Section 12.5.4), their spectra are flat and they are therefore most easily detected by radio surveys made at short wavelengths. The total power radiated at radio frequencies extends from about 1038 ergs/sec from so-called normal galaxies such as our own Galaxy and other spirals, to 1041 ergs/sec for the weaker radio elliptical galaxies such as M87 or Centaurus A, and up to 1045 ergs/sec for the most luminous radio galaxies such as Cygnus A and 3C 295 and many quasars. In many numerical examples Cygnus A is often discussed as a "typical source," whereas in fact it is a very outstanding source since less than one source in 10 million in any given volume of space approaches this luminosity.