An Etymological Dictionary of Astronomy and Astrophysics
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The process when the energy difference between the excited and non excited states of an atom is taken away by radiation.

→ radiative; → decay.

A process of → radiative transfer in which photons are repeatedly absorbed and re-emitted by matter particles.

→ radiative; → diffusion.

A → radiative zone occupying the outer parts of a star.

→ radiative; → envelope.

The balance between radiative emission and radiative absorption in a specified system.

→ radiative; → equilibrium.

The radiative energy put back to the environment through an astrophysical process. For example, in the process of → star formation  → accretion disks form around → protostars. The material in the disk spirals inward and on to the protostar, provided that there is an efficient mechanism to redistribute → angular momentum outward in the disk. During this process → gravitational energy is transformed into radiation due to → viscous dissipation in the disk and at the → accretion shock around the protostar. This radiation heats the region around the protostar and may → suppress subsequent → fragmentation and further star formation. Thus, radiative feedback plays a critical role in regulating the stellar → initial mass function.

→ radiative; → feedback.

The radiative energy per unit time and unit area.

→ radiative; → flux.

The process by which temperature increases due to an excess of absorbed radiation over emitted radiation.

→ radiative; → heating.

A physical process occurring in → stellar atmospheres whereby → radiation pressure selectively pushes certain → chemical elements outward, leading to an atmospheric overabundance of such elements. See also → gravitational settling.

→ radiative; → levitation.

For a → supernova remnant (SNR), same as the → snowplow phase.

→ radiative; → phase.

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.