An Etymological Dictionary of Astronomy and Astrophysics
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In the → Fanaroff-Riley classification, sources with R_FR < 0.5. Fanaroff and Riley (1974) found that nearly all sources with luminosity L(178MHz) ≤ 2 × 10²⁵h₁₀₀^-2 W Hz^-1 sr^-1 were of class I. FR-I → radio jets are thought to be → subsonic, possibly due to mass entrainment, which makes them amenable to distortions in the interaction with the ambient medium.

→ Fanaroff-Riley classification; → class.

In the → Fanaroff-Riley classification, → radio sources with hotspots in their lobes at distances from the center which are such that R_FR > 0.5. The → radio jets in FR-II sources are expected to be highly → supersonic, allowing them to travel large distances.

→ Fanaroff-Riley classification; → class.

A classification scheme for distinguishing a → radio galaxy from an → active galaxy based on their → radio frequency  and → luminosity and their kpc-scale appearance. Analyzing a sample of 57 radio galaxies from the → 3CR catalogue, which were clearly resolved at 1.4 GHz or 5 GHz, Fanaroff & Riley (1974) discovered 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. They divided the sample into two classes using the ratio R_FR 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. → Fanaroff-Riley Class I (FR-I) , → Fanaroff-Riley Class II (FR-II). The boundary between the two classes is not very sharp, and there is some overlap in the luminosities of sources classified as FR-I or FR-II on the basis of their structures. The physical cause of the FR-I/II dichotomy probably lies in the type of flow in the → radio jets.

Bernard L. Fanaroff and Julia M. Riley, 1974, MNRAS 167, 31P; → classification.

Away from a place.

→ off; cognate with Av. and O.Pers. apā "away from, from," as below.

Ap-, from apâ-, from Av. and O.Pers. apā "away from, from;" cf. Skt. apa "away, off;" L. ab- "from, away;" Hittite appa; Gothic af-; Ger. ab-; E. of, as above; PIE base *apo- "off, away"

An → optical system in which the → optical axis of the → aperture is not coincident with the mechanical center of the aperture.

→ off; → axis; → optical; → system.

Of computers, operating independently of, or disconnected from, an associated computer.

→ off; → line.

An observation when the telescope is pointed away from the source in order to measure the sky background contribution.

→ off-; → source; → observation.