A category in a classification of extragalactic radio sources, typically radio galaxies or quasars, based on the separation between the brightest regions in their radio-emitting lobes. In Class I (FRI) sources, the bright regions are separated by less than half the overall extent of the radio lobes. Hence these regions are closer to the nucleus than in Class II (FRII) sources, where they are separated by more than half the total length of the source. Class II sources are in general considerably more powerful than Class I. The scheme is named after the South African astronomer Bernard Lewis Fanaroff (1947– ) and the British astronomer Julia Margaret Riley (1947– ).
Subjects: Astronomy and Astrophysics.