An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

1) Math.: The indicated root of a quantity, as denoted by an expression written under the → radical sign.
2) Chemistry: A group of atoms present in a series of compounds which conserves its identity in the course of chemical changes which affect the rest of the molecule. For example, the → cyano radical and → hydroxyl group.

M.E., from L.L. radicalis "of or having roots," from → radix "root."

→ root.

Of two circles, the straight line containing all points P such that the lengths of the tangents from P to the two circles are equal.

→ radical; → axis.

The symbol √ placed before a number or quantity to indicate the extraction of the square root. The value of a higher (the n-th) root is indicated by a raised positive digit (n) in front of the symbol, as in ³√ (cube root). The first known occurrence of this symbol was in the book Die Cross, published in 1525, by the German mathematician Christoff Rudolff.

→ radical; → symbol.

Plural form of → radius.

→ radius.

1) An apparatus for receiving or transmitting radio broadcasts.
2) Pertaining to the → radio waves of the → electromagnetic radiation.
3) As a prefix, indicating → radition or → radioactivity.

1); 2) Short from radiophone and radio-telegraphy.
3) Combining form of → radiation.

Râdio, loan from Fr., as above; partow→ ray.

A large number of narrow filaments in → radio continuum occurring toward the → Galactic Center, about 15 to 20 arc-minutes (some 50 parsecs in projection) north of → Sgr A*. The radio Arc is the prototype of → non-thermal filaments (NTFs) and resolves into a set of more than a dozen vertical filaments with lengths of about 30 pc distributed symmetrically with respect to the → Galactic equator (Yusef-Zadeh et al. 1984, Nature 310, 557). Among more than 100 NTFs found in the Galactic center region, the Arc is the only one known to show inverted spectrum with a → spectral index α = +0.3 (Law et al. 2008, ApJS 177, 515, and references therein). This implies a very hard energy spectrum of particles for a source of → synchrotron radiation.

→ radio; → arc.

The branch of astronomy that deals with the study of the Universe by means of → radio waves.

→ radio; → astronomy.

A burst of emission in the radio frequencies of the electromagnetic spectrum.

→ radio; → burst.

A → continuum emission with frequencies in the radio range of the electromagnetic spectrum.

→ radio; → continuum; → emission.

The representation in → radio wavelengths of an astronomical object that has emission in other parts of the → electromagnetic spectrum.

→ radio; → counterpart.

→ Electromagnetic radiation carried by → radio waves.

→ radio; → emission.

Total radiation in radio wavelengths going out from the 2π solid angles of a hemisphere. → flux.

→ radio; → flux.

The flux of → radio waves that falls on a → detector per → unit → surface area of the detector per unit → bandwidth of the radiation per unit → time. The unit is → jansky.

→ radio; → flux; → density.

The → electromagnetic radiation with the frequency range between 3 → kiloherz (kHz) to 300 → gigahertz (GHz). See also → radio wave.

→ radio; → frequency.

A galaxy that is extremely luminous at radio wavelengths between 10 MHz and 100 GHz. The radio luminosity of a strong radio galaxy (10³⁷-10³⁹ watts) can be up to a million times greater than the radio output of an ordinary galaxy and up to a hundred times greater than the optical luminosity of a galaxy such as the Milky Way. The optical counterparts of radio galaxies are usually an → elliptical galaxy. Radio galaxies often exhibit jet structure from a compact nucleus. They typically display two → radio lobes that are often approximately aligned with the jets observed in the optical and that may extend for millions of → light-years.

→ radio; → galaxy.

An → interferometer designed for radio frequencies.

→ radio; → interferometer.

An → astrophysical jet appearing in the radio wavelengths of the electromagnetic spectrum.

→ radio; → jet.

An extended region of diffuse radio emission, often dumbbell shaped, that surrounds a radio galaxy.

→ radio; → lobe.

The electromagnetic noise at radio wavelengths.

→ radio + → noise.

A → pulsar that emits → pulses in → radio waves. The bulk of discovered pulsars are radio pulsars. There is a small number of pulsars that emit at optical wavelengths, X-ray wavelengths, and gamma-ray wavelengths.

→ radio; → pulsar.