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

A collision between charged particles in which part of the kinetic energy is converted into electromagnetic radiation.

→ radiative; → collision.

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

→ radiative; → diffusion.

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.

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.

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.

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

→ radio; → continuum; → emission.

→ Electromagnetic radiation carried by → radio waves.

→ radio; → emission.

A → recombination line whose wavelength lies in the radio range of the electromagnetic spectrum. Radio recombination lines are the result of electronic transitions between high energy levels (n > 50) in an atom or ion.

→ radio; → recombination line.

A radioactive → nuclide.

→ radio + → nuclide.

Same as → Larmor radius.

→ radius; → gyration.

The agitated motion of molecular, atomic, or → subatomic particles in all possible directions at any temperature, except at → absolute zero, where → thermal motion would cease.

→ random; → thermal; → motion.

Arrangement of data in such a way as to simulate chance occurrence.

Verbal noun of → randomize.

Hydrodynamics → conservation laws (which can be extended to → magnetohydrodynamics, MHD) which describe the physical conditions of material across a → shock front. A fluid is completely described by its velocity, density, pressure, specific heat ratio, and magnetic field (in the MHD case). Mass, momentum, and energy fluxes are conserved in the shock, leading to the Rankine-Hugoniot relations. Also called Rankine-Hugoniot jump conditions. See also → jump condition.

Named after William John Rankine, → Rankine scale, and Pierre Henri Hugoniot, → Hugoniot curve; → condition.

The state of being rarefied, less dense.
An instantaneous reduction in density of a gas resulting from passage a sound wave; opposite of → compression.

M.E. rarefien, from M.Fr. rarefier, from L. rarefacere "make rare," from rarus "loose, wide apart, thin, infrequent."

Verbal noun from âlar present stem of âlaridan→ rarefy + -š, a suffix.

A pressure wave in a fluid generated by rarefaction. It travels in the opposite direction to that of a shock wave in the medium.

→ rarefaction; → wave.

1) Having or exercising the ability to → reason.
2) Consistent with or based on reason; logical; in accordance with sound reasoning.
3) Math.: Capable of being expressed as a quotient of integers.

M.E. racional, from O.Fr. racionel, from L. rationalis "of or belonging to reason, reasonable," from ratio (genitive rationis) "reckoning, calculation, reason," from rat-, pp. stem of reri "to reckon, calculate; consider, think."

1) Râyani, of or pertaining to râyan, → reason.
2) Xeradâné, xeradmandâné, of or pertaining to xerad "understanding, judgement, intellect, wisdom;" Mid.Pers. xrad "reason, intellect, intelligence, wisdom, understanding;" O.Pers. xraθu- "wisdom;" Av. xratu- "intelligence, undestanding, wisdom; will, purpose, council;" cf. Skt. krátu- "power, will-power;" Gk. kratos "power, strength."
3) vâbari, of or pertaining to vâbar, → ratio.

Any number that can be expressed as a ratio of two integers, providing the second number is not zero.

→ rational; → number.

1) The fundamental reason or reasons serving to account for something.
2) A statement of reasons.
3) A reasoned exposition of principles (Dictionary.com).

L. neuter of rationalis.

Râyanâl, from râyan, → reason, + -âl, → -al.

A philosophical doctrine that holds that → reason alone, unaided by experience, can arrive at basic truth regarding the world.

From → rational + -ism a Gk. suffix used in the formation of nouns denoting action or practice, state or condition, principles, doctrines, and so forth.

Râyan, → reason; xerad, → rational; bâvari, from bâvar "belief" (Mid.Pers. wâbar "beleif;" Proto-Iranian *uar- "to choose; to convince; to believe;" cf. Av. var- "to choose; to convince" varəna-, varana- "conviction, faith;" O.Pers. v(a)r- "to choose; to convince;" Skt. vr- "to choose," vara- "choosing").