An Etymological Dictionary of Astronomy and Astrophysics
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An process in which an excited state loses its absorbed energy by emission of radiation. → non-radiative process.

→ radiative; → process.

The process by which an ionized atom binds a free electron in a → plasma to produce a new atomic state with the subsequent radiation of photons.

→ radiative; → recombination.

A → shock wave in which the → time-scale for → cooling is much shorter than the appropriate → dynamical or → evolutionary time-scale of the system that drives the shock. Radiative shock waves are believed to play a key role in a variety of different astrophysical environments, including → magnetic cataclysmic variables, → jets from → young stellar objects, → accretion in → T Tauri stars, → colliding stellar winds, and → supernova remnants.

→ radiative; → shock.

The process by which the → electromagnetic radiation passes through a medium that may contain any combination of → scatterers, → absorbers, and → emitters.

→ radiative; → transfer.

The equation that describes the → radiative transfer. It states that the → specific intensity of radiation I_σ during its propagation in a medium is subject to losses due to → extinction and to → gains due to → emission: dI_σ/dx = - μ_σ . I_σ + ρ . j_σ, where x is the coordinate along the → optical path, μ_σ is the → extinction coefficient, ρ is the mass → density, and j_σ is the → emission coefficient per unit mass.

→ radiative; → transfer; → equation.

A transition between two states of an atomic or molecular entity, the energy difference being emitted or absorbed as photons.

→ radiative; → transition.

The region of a star in which the energy generated by → nuclear fusion in the core is transferred outward by → electromagnetic radiation and not by → convection. Such zones occur in the interior of low-mass stars, like the Sun, and the envelopes of → massive star. The radiative zone of the Sun starts at the edge of the core of the Sun, about 0.2 solar radii, and extends up to about 0.7 radii, just below the → convective zone.

→ radiative; → zone.

Same as → radiation-driven wind

→ radiative; → drive; → wind.

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.