Extended radio sources – Fanaroff–Riley types

All galaxies are sources of radio emission – high energy electrons are accelerated in supernova remnants and these are dispersed throughout the interstellar medium where they radiate radio waves by synchrotron radiation. These are, however, very weak radio emitters indeed compared with what are conventionally referred to by the terms radio galaxy or radio quasar in which the radio luminosity can exceed that of our own Galaxy by factors of 108 or more. The big surprise was the discovery that the most luminous radio sources contain jets of relativistic material which give rise to a wide variety of large scale radio structures. The example of the brightest extragalactic radio source in the northern sky, Cygnus A, as observed by the Very Large Array, illustrates a number of the characteristic features of these sources (Fig. 21.1). Many more details are contained in the review by Carilli and Barthel (1996).