An Etymological Dictionary of Astronomy and Astrophysics
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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.

Of or pertaining to radiation.

Adjective of → radiation.

The acceleration imparted to matter by → radiation pressure.

→ radiative; → acceleration.

The slowing down of a star's rotation due to radiative momentum transfer caused by emission of electromagnetic radiation.

→ radiative; → braking.

Capture of a free electron by an ion with the subsequent emission of photons; also called → radiative recombination.

→ radiative; → capture.

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

→ radiative; → collision.

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

→ radiative; → cooling.

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.