An Etymological Dictionary of Astronomy and Astrophysics
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The temperature of a source calculated assuming that it behaves as a → blackbody that radiates with the same intensity at the same frequency. Compared to the → effective temperature, the radiation temperature is measured over a narrow region of the → electromagnetic spectrum.

→ radiation; → temperature.

→ radiative transfer.

→ radiation; → transfer.

→ radiative transfer equation.

→ radiation; → transfer; → equation.

An early epoch in the history of the → Universe when the radiation → density parameter was Ω_r≈ 1, while other density parameters had negligible contributions. A radiation-dominated Universe is characterized by R/R₀ ∝ t^1/2, where R is the → cosmic scale factor and t is time. According to the → Big Bang model, the radiation-dominated phase was followed by the → matter-dominated phase.

→ radiation; → dominate; → Universe.

A hydrodynamic process occurring in star forming regions where a neutral cloud (→ clump) is subjected to the intense ultraviolet radiation of a newly-born → massive star. The gas within the layer exposed to the radiation is ionized and forms an → ionization front at the front surface. The increased pressure due to temperature rise at the top layer drives an → isothermal  → shock front into the clump, which compresses the neutral gas ahead of it, below the surface. A density → gradient builds up leading rapidly to the formation of a condensed core. With further concentration of the gas, the hydrogen density in the center of the core increases drastically, reaching 10⁸ cm^-3 about 4 x 10⁵ years after the first impact of the ionizing radiation on the clump, according to current models (e.g. Bertoldi 1989, ApJ 346, 735; Miao et al. 2006, MNRAS 369, 143, and references therein). The core can develop further to form a → cometary globule or → collapse under its self-gravity, eventually giving rise to new → low-mass stars (→ triggered star formation). In the process, the whole clump accelerates away from the initial ionizing star. Indeed, the gas evaporated off the side of the clump facing the ionizing star can create a rocket effect accelerating the clump away from the star (with a velocity of up to 5 km s^-1), while losing part of its initial mass.

→ radiation; driven, p.p. of → drive; → implosion.

The → mass loss experienced by a → massive star due to the effect of → radiation-driven wind.

→ radiation; driven, p.p. of → drive; → mass; → loss.

The loss of matter from the → photosphere due to the acceleration imparted to the outer layers of the star by photons created inside the star. The coupling between radiation and matter creates a → radiative acceleration that may exceed the → gravity. This mechanism is particularly important in → massive stars, since the luminosity is high and therefore the number of energetic ultraviolet photons important. Same as → line-driven wind.

→ radiation; → drive; → wind.

A process in which a molecule relaxes without emitting a → photon.

→ radiation; → -less; → relaxation.

The acceleration imparted to matter by → radiation pressure.

→ radiative; → acceleration.

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.

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.

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

→ radio; → astronomy.

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.