An Etymological Dictionary of Astronomy and Astrophysics
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In evolutionary models of → low-mass and → intermediate-mass stars, the occurrence of a → helium shell flash just at the moment when the star is leaving the → asymptotic giant branch phase.

→ AGB; → final; → thermal; → pulse.

The quantity kT in the → Maxwell-Boltzmann distribution law, where k is → Boltzmann's constant and T the gas temperature. See also → thermal energy.

→ characteristic; → thermal; → energy

The period during the → early Universe before the → recombination era when the photons were hot enough to ionize hydrogen. The density was so high that the interactions between → matter and → radiation were very numerous. Therefore, matter and photons were in constant contact and their → temperatures were the same. As a result, the radiation became → thermalized, i.e. the → electromagnetic spectrum of the radiation became that of a → blackbody, a process called → thermalization. Since the time of recombination the photons of → cosmic background radiation have been free to travel uninhibited by interactions with matter. Thus, their distribution of energy is a perfect → blackbody curve, as predicted by the → Big Bang theory and shown by several observations, such as → Cosmic Background Explorer (COBE), → Wilkinson Microwave Anisotropy Probe (WMAP), and → Planck Satellite.

→ epoch; → thermalization.

Geology: Relating to or caused by high temperature underground water or gas heated by natural processes.

→ hydro-; → thermal.

1) Relating to or having a constant temperature.
2) Pertaining to an → isotherm.

Isothermal, from → iso- + therm, from Gk. therme "heat" thermos "warm;" cognate with Pers. garm "warm" (garmâ "heat, warmth," from Mid.Pers. garmâg; O.Pers./Av. garəma- "hot, warm;" cf. Skt. gharmah "heat;" L. formus "warm," fornax "oven;" P.Gmc. *warmaz; O.E. wearm; E. warm; O.H.G., Ger. warm; PIE *ghworm-/*ghwerm- "warm") + → -al; → process.

Izodamâ, from → iso- + damâ, → temperature.

A → thermodynamic process that takes place at → constant  → temperature. For the temperature of a system to remain strictly constant, the changes in other coordinates (pressure and volume) must be carried out slowly, and → heat must enter or leave the system to maintain a constant temperature. → polytropic process. Astrophysical examples include the → collapse of a → protostar down the → Hayashi track, and the collapse of a star at the end of its life to become a → white dwarf.

→ isothermal; → process.

A → stellar wind in which the gas is subject to only two forces: the inward directed gravity and the outward directed gradient of the gas pressure.

→ isothermal; → wind.

In evolutionary models of → low-mass and → intermediate-mass stars, the occurrence of a → helium shell flash on the → horizontal branch of the → post-AGB track, while → hydrogen shell burning is still going on.

→ late; → thermal; → pulse.

The nature of a → non-thermal radiation.

→ non-; → thermal.

→ non-thermal radiation.

→ non-thermal; → emission.

Any of many long and slender structures visible in → radio continuum images of the inner hundred parsecs of the → Galactic Center. NTFs are typically tens of parsecs long and only a fraction of parsec wide. They may occur in isolation or in bundles, such as those comprising the linear portion of the prominent → radio Arc. Their → non-thermal spectrum and strong → linear polarization indicate → synchrotron radiation. The magnetic fields in the NTFs have been estimated from various means. Early estimates centered on the radio Arc, and focused on a comparison between the → magnetic pressure and the estimated → ram pressure from nearby → molecular cloud interactions, indicated magnetic field strengths as high as 1 mG (Morris and Yusef-Zadeh 1985). More recent observations, however, have pointed to significantly weaker magnetic fields among the population of NTFs. Synchrotron models of the radio spectrum imply equipartition magnetic fields between 50-200 μG. Theoretically, it has been challenging to understand the nature of these filaments that resemble extragalactic → radio jets but are not accompanied with any obvious source of acceleration of charged particles to high energy → relativistic energies. Although a number of detailed models have been considered, there is no consensus as to the origin of the NTFs. These models suggest that molecular and ionized gas clouds, mass-losing stars, → Galactic winds, magnetic activity of the → supermassive black hole at the Galactic center, and → lepton production due to → dark matter annihilation play a role in the processes that lead to the production of the NTFs (Linden et al. 2011, ApJ 741,95, and references therein). In most models, the magnetic field is strong and its global geometry in the central region of the Galaxy is considered to be → poloidal and static. However, some recent models have argued that the magnetic field is local and dynamic.

→ non-thermal; → filament.

The electromagnetic radiation whose characteristics do not depend on the temperature of the emitting source. In contrast to → thermal radiation, it has a different spectrum from that of → blackbody radiation. The three common types of non-thermal radiation in astronomy are: → synchrotron radiation, → bremsstrahlung radiation, and → maser  → stimulated emission.

→ non-thermal; → radiation.

A radio emission with a negative → spectral index. In this type of emission the intensity of the emitted radiation increases with wavelength.

→ non-thermal; → spectrum.

See also → non-thermal.

→ non-; → thermal.

Same as → suprathermal escape.

→ non-; → thermal; → escape.

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.

In models of star formation, an isothermal sphere in which the density distribution in the static or nearly static outer envelope obeys an r^-2 power law. In the limit of infinite central concentration, the unstable equilibrium approaches the singular isothermal sphere which has the density and mass distributions ρ(r) = (a²/2πG)r^-2 and M(r) = (2a²/G)r, where a is the isothermal → sound speed inside the cloud, G is the → gravitational constant, and r the distance from the center (F. H. Shu, 1977, ApJ 214, 488).

→ singular; → isothermal; → sphere.

An → atmospheric escape mechanism that occurs where individual atoms or molecules in the atmosphere are raised to → escape velocity because of chemical reactions or ionic interactions. Same as → nonthermal escape (see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to Space, pp. 129-167. Cambridge University Press).

→ supra-; → thermal; → escape.

Of, pertaining to, or caused by heat or temperature.

From M.Fr. thermal, from Gk. therme "heat," cognate with Pers. garm "warm," as below.

Garmâyi, adj. of garmâ "heat, warmth," from Mid.Pers. garmâg; O.Pers./Av. garəma- "hot, warm;" cf. Skt. gharmah "heat;" cognate with Gk. therme, thermos, as above; PIE *ghworm-/*ghwerm- "warm."

1) The random movement of the molecules of a substance, the energy of which is, by kinetic theory, synonymous with the heat content of the substance.
2) Solid state physics: The random motion of free electrons in a conductor due to heat energy.

→ thermal; → agitation.