An Etymological Dictionary of Astronomy and Astrophysics
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An equation that expresses the energy radiated per unit area per unit time per unit wavelength range by a blackbody as a function of temperature. It is expressed by → Planck's blackbody formula.

→ Planck; → radiation; → law.

The emission of any → rays, → waves, or → particles from a source; usually applied to the → emission of → electromagnetic energy.

Verbal noun of → radiate.

A ring-shaped region in the → magnetosphere of a planet in which charged particles are trapped by the planet's magnetic field. The radiation belts surrounding Earth are known as the → Van Allen belts.

→ radiation; → belt.

Same as → radiation density constant.

→ radiation; → constant.

Damping of a system which loses energy by → electromagnetic radiation.

→ radiation; → damping.

The constant related to the total energy radiated by a → blackbody and defined as: a = 4σ/c, where σ is the → Stefan-Boltzmann constant and c the → speed of light. Its value is a = 7.5657 x 10^-15 erg cm^-3 K^-4. Same as → radiation constant.

→ radiation; → density; → constant.

The epoch in the history of the Universe, lasting from the → Big Bang until about 400,000 years later, when the temperature had dropped to 10⁹ K and the rate of electron-positron → pair annihilation exceeded the rate of their production, leaving radiation the dominant constituent of the Universe. The radiation era was followed by the → matter era.

→ radiation; → era.

1) The portion of an → electromagnetic field outside the → induction field where there is a power flow of both → magnetic and → electric components in a well-defined relationship.
2) → interstellar radiation field

→ radiation; → field.

The mean distance traveled by a photon or particle in a given medium before its energy is reduced by a factor e due to its interaction with matter.

→ radiation; → length.

Same as → antenna pattern.

→ radiation; → pattern.

The → momentum carried by → photons to a surface exposed to → electromagnetic radiation. Stellar radiation pressure on big and massive objects is insignificant, but it has considerable effects on → gas and → dust particles. Radiation pressure is particularly important for → massive stars. See, for example, → Eddington limit, → radiation-driven wind , and → radiation-driven implosion. The → solar radiation pressure is also at the origin of various physical phenomena, e.g. → gas tails in → comets and → Poynting-Robertson effect.

→ radiation; → pressure.

An illness resulting from excessive exposure to ionizing radiation. The earliest symptoms are nausea, vomiting, and diarrhea, which may be followed by loss of hair, hemorrhage, inflammation of the mouth and throat, and general loss of energy.

→ radiation; sickness, M.E. siknesse, seknesse; O.E. sēocnesse, from seoc + suffix -ness.

Bimâri "sickness, infirmity, disease," from bimâr "sick, infirm, afflicted;" Mid.Pers. vêmâr "sick, ill;" maybe by corruption of Proto-Iranian *amavayā-bara- "bearing illness;" cf. Av. amavayā- "pain, suffering, affliction;" Skt. ámīvā- "pain, grief, distress" + *bara- "bearing;" cf. Av. bar- "to bear, carry;" Mod.Pers. bar-, bordan "to bear, carry, lead." Alternatively, from *vi-mar-, prefixed *mar- "to die;" cf. Av. mar- "to die;" Mod.Pers. mir-, mordan "to die;" Skt. mar- "to die;" cognate with Gk. emorten "died;" L. morior "to die;" tâbeši related to tâbeš, → radiation.

The components of radiation arranged in order of their wavelengths, frequencies, or quantum energies. For particle radiation they are arranged in order of their kinetic energies.

→ radiation; → spectrum.

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.