An Etymological Dictionary of Astronomy and Astrophysics
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The energy radiated from an object in the form of → electromagnetic waves as a result of its → temperature. Thermal radiation ranges in → wavelength from the longest → infrared radiation through the → visible light spectrum to the shortest → ultraviolet rays. In opposition, → non-thermal radiation is caused by energetic particles.

→ thermal; → radiation.

Stresses induced in a material because of rapid temperature change or a → thermal gradient .

→ thermal; → shock.

A → transient → rise in → temperature above the normal level in a medium.

→ thermal; → spike.

In star formation models, the gas pressure that counters the collapsing pull of gravity.

→ thermal; → support.

1) The process by which a system reaches → thermal equilibrium. Thermalization results from energy exchange between the components constituting the system and their exchange with the outside medium. In a gas at a given temperature, molecules move with different velocities. The gas temperature corresponds to the mean velocity of the molecules, but individual molecules may deviate largely from the mean velocity. Some move very fast others slowly and change velocity upon collisions. Collisions reduce the energy of fast moving molecules and increase that of slow ones. In the process of thermalization → matter and → radiation are in constant interaction such that their → temperatures become identical. The process goes on until energy distribution reaches → equilibrium. The system is said to be → thermalized.
2) The condition where in an → atomic or → molecular → transition the → Boltzmann factor for the two → levels of transition takes on the value it would have in → thermodynamic equilibrium.

Verbal noun of → thermalize.

To bring neutrons into → thermal equilibrium with their surroundings; to produce → thermal neutrons.

From → thermal + → -ize.

Yekgarmâyidan, literally "equal warming," from yek-, → one, + garmâyidan, infinitive from garmâ, → thermo-.

A collisionally excited spectral line formed in high density condition well above the → critical density. At such densities the → excitation temperature is at (or very near) the → kinetic temperature of the gas. At low densities, below the critical density, the excitation temperature will be only slightly above the radiation temperature and the emission line will be practically invisible.

Thermalized, p.p. of → thermalize; → line.

An electron that has been emitted from a heated body such as the hot cathode of an electron tube.

From therm- variant of → thermo- before a vowel + → ion.

Electrons gaining enough thermal energy to escape spontaneously from the cathode or dynodes and mimic photoelectrons.

thermion; → emission.

A combining form meaning "heat, hot," used in the formation of compound words. Also therm- before a vowel.

From Gk. therme "heat," thermos "hot;" cf. L. fornax "oven, kiln," related to fornus, furnus "oven," and to formus "warm;" cognate with Pers. garm "warm," as below; P.Gmc. *warmaz (O.E. wearm; E. warm; O.H.G., Ger. warm).

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."

A layer in a large body of water, such as a lake, in which temperature changes more rapidly with depth than it does in the layers above or below.

→ thermo- + → -cline.

Electrical circuit consisting of two dissimilar metals, in which an electromotive force is produced when the two junctions are at different temperatures.

→ thermo- + → couple.

Of or pertaining to → thermodynamics.

→ thermo-; → dynamic.

The condition of a → thermodynamic system in which the available → energy is distributed uniformly among all the possible forms of energy. Furthermore, all → thermodynamic process es must be exactly balanced by their reverse processes. For example, inside a star there will be as many → ionizations of helium per second as there are → recombinations of free electrons and helium ions. Se also → local thermodynamic equilibrium (LTE).

→ thermodynamic; → equilibrium.

The loci of various changes between two → states through which a → thermodynamic system passes during a → thermodynamic process.

→ thermodynamic; → path.

A measure of the energy level of a → thermodynamic system. It represents the amount of → work obtainable when the system undergoes a → change. The main types of thermodynamic potential are: → internal energy, → enthalpy, the → Helmholtz free energy, and the → Gibbs free energy.

→ thermodynamic; → potential.

An ordered set of → equilibrium states undergone by a → thermodynamic system. Thermodynamics processes have various types: → cyclic process, → reversible process, and → irreversible process, → isothermal process, → adiabatic process, → isentropic process.

→ thermodynamic; → process.

A quantity of substance or a working machine which in a well-defined way is set apart from its → environment. The boundary between the system and its surroundings can be real or an imaginary mathematical envelope. A thermodynamic system is not necessarily bound to a predefined geometry. Thermodynamic systems can be divided into three types: → open systems, → closed systems, and → isomated systems.

→ thermodynamic; → system.

A temperature scale, measured in → kelvin (K), that is related to the energy possessed by matter; it was formerly known as → absolute temperature. The zero point on the scale (0 K) is absolute zero. Thermodynamic temperature can be converted to temperature on the → Celsius scale by subtracting 273.15.

→ thermodynamic; → temperature.

A branch of physics concerned with the relations between heat and other forms of energy and how these affect temperature, pressure, volume, mechanical action, and work.

→ thermo-; → dynamics, coined by the Scottish physicist William Thomson (Lord Kelvin, 1824-1907), in 1849.