FR I, FR II
FR I, FR IIAbbrev. for Fanaroff–Riley types I, II. A simple but powerful classification, made in 1974, of radio galaxies that contain hot spots and diffuse emission. If the distance between the hot spots is less than half the total extent of the source, the radio galaxy is of type I, if greater than half, it is of type II. The distinction is thought to arise from the speed of the jet powering the radio lobes. A jet that is always faster than the sound speed in the intergalactic medium will produce an FR II source, whereas a subsonic jet will produce an FR I. The speed of the jet is affected by entrainment. See also radio-source structure.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006