The traditional Fanaroff Riley classification of radio galaxies (Fanaroff & Riley, 1974). FRI galaxies are "centre-bright", and FRII galaxies are "edge-brightened" (often with "hotspots" towards the edge). In this diagram, the FRII is drawn with a visible jet on one side, typical of galaxies whose jets have some orientation towards the observer. However, an FRII may have visible jets on both sides or neither.
The Fanaroff-Riley classification is a way of categorizing extragalactic radio sources, especially radio galaxies and radio-loud quasars, in terms of the separation between the brightest parts of their radio-emitting lobes; named after the South African astronomer Bernard Lewis Fanaroff (1947–) and the British astronomer Julia Margaret Riley (1947–) who published their results in 1974.
Fanaroff and Riley noticed that the relative positions of regions of high and low surface brightness in the lobes of extragalactic radio sources are correlated with their radio luminosity. Their conclusion was based on a set of 57 radio galaxies and quasars that were clearly resolved at 1.4 GHz or 5 GHz into two or more components. Fanaroff and Riley divided this sample into two classes using the ratio, RFR, of the distance between the regions of highest surface brightness on opposite sides of the central galaxy or quasar to the total extent of the source up to the lowest brightness contour in the map. Sources with RFR < 0.5 were placed in Class I and sources with RFR > 0.5 in Class II. Various properties of sources in the two classes are different, which is indicative of a direct link between luminosity and the way in which energy is transported from the central region and converted to radio emission in the outer parts.
Fanaroff-Riley Class I (FR-I) sources have their low brightness regions further from the central galaxy or quasar than their high brightness regions. The sources become fainter toward the outer extremities of the lobes and the spectra here are steepest, indicating that the radiating particles have aged the most. Jets are detected in 80% of FR-I galaxies, which also tend to be bright, large galaxies (D or cD) that have a flatter light distribution than an average elliptical galaxy and are often located in rich clusters of galaxies (those with many embers in a relatively region of space) with extreme X-ray emitting gas. As the galaxy moves through the cluster, the gas can sweep back and distort the radio structure through ram pressure, which explains why narrow-angle-tail or wide-angle-tail sources, say, appear to be derived from the FR-I class of objects.
Fanaroff-Riley Class II (FR-II) is made up of luminous radio sources with hotspots in their lobes at distances from the center such that RFR > 0.5. These sources are said to be edge-darkened, a particularly apt terminology when the angular resolution and dynamic range used in observing the classical sources were not always good enough to reveal the hotspots as distinct structures. In keeping with the overall high luminosity of this type of source, the cores and jets in them are also brighter than those in FR-I galaxies in absolute terms, but relative to the lobes these features are much fainter in FR-II galaxies. Jets are detected in less than 10% of luminous radio galaxies, but in nearly all quasars. FR-II sources are generally associated with galaxies that appear normal, except that they have nuclear and extended emission line regions. The galaxies are giant ellipticals, but not first-ranked cluster galaxies.