An Etymological Dictionary of Astronomy and Astrophysics
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The emission of electromagnetic radiation from high temperature plasma, produced as electrons are deviated by positive ions. Same as → free-free emission

→ thermal; → bremsstrahlung.

A process that occurs in a medium where a → temperature gradient exists: dQ = -κ(dT/dx)dA.dt, where dQ is the amount of heat passing through the time dt across an area dA in the direction of the normal x to this area and toward the reduction in temperature, κ is the → thermal conductivity, and (dT/dx) the temperature gradient.

→ thermal; → conduction.

In → thermal conduction, the amount of heat passing across unit area per unit time and per unit → temperature gradient.

→ thermal; → conductivity.

A detector that senses the change of temperature due to the absorption of photons.

→ thermal; → detector.

A physical process resulting from → temperature gradients in stellar interiors, whereby more highly charged and more massive chemical species are concentrated toward the hottest region of the star, its center. Therefore, thermal diffusion and → gravitational settling tend to make heavier species sink relative to the light ones.

→ thermal; → diffusion.

→ thermal radiation.

→ thermal; → emission.

1) The energy in the form of heat emitted by an object by virtue of its temperature.
2) The total potential and kinetic energies associated with the random motions of the particles of a material. The quantity of thermal energy possessed by a body determines its temperature. The thermal energy which is absorbed, given up, or transferred from one material to another is heat.
3) The characteristic energy of → thermal neutrons at room temperature, about 0.025 eV.

→ thermal; → energy.

In thermodynamics, the state of a system all parts of which have attained a uniform temperature and no net heat exchange is taking place between it and its surroundings. If two bodies are in thermal equilibrium, they have the same temperature. Thermal equilibrium is the central criterion of the → zeroth law of thermodynamics. See also → local thermodynamic equilibrium (LTE).

→ thermal; → equilibrium.

An → atmospheric escape that occurs when irradiation from a parent star (or a very high heat flux from a planet interior) heats a planetary atmosphere, causing its molecules to escape to space. In basic models, the theory assumes neutral species with a → Maxwell-Boltzmann distribution of velocities, which occurs when collisions between molecules are frequent. Thermal escape has two types: → Jeans' escape and → hydrodynamic escape (see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to Space, pp. 129-167. Cambridge University Press).

→ thermal; → escape.

A process in which collisions that occur between particles cause atoms or molecules to obtain additional kinetic energy.

→ thermal; → excitation.

The change in dimensions of a material resulting from a change in temperature.

→ thermal; → expansion.

A vector quantity that depends on the distribution of temperature in three dimensions with respect to a given point. The magnitude and orientation of the maximum thermal gradient are given by: ∇T = (∂T/∂x)i + (∂T/∂y)j + (∂T/∂z)k, where T is the temperature distribution function in three dimensions, and i, j, and k are the unit vectors along the x, y, and z axes defining the temperature field. Same as → temperature gradient.

→ thermal; → gradient.

A mechanism for the → transport of → electrons which occurs when the → Fermi level lies below a low but wide energy → barrier. The → tunneling probability across the barrier is considerably suppressed due to the width of the barrier. However, at higher temperatures, the electron can raise its energy with the assistance of a vibrational mode. The electron is said to hop from one side of the barrier to the other side via an intermediate state.

→ thermal; → hop; → -ing.

The tendency of a body to resist a change in temperature. A body with a low thermal inertia requires very few calories to change its surface temperature. A low thermal inertia material tends to be thermally insulating, so that the surface temperature changes readily, but those changes are not conducted to depth within the material (Ellis et al., 2007, Planetary Ring Systems, Springer).

→ thermal; → inertia.

The → Jeans mass when → turbulence is insignificant.

→ thermal; → Jeans; → mass.

The random motions and collisions of molecules, atoms, electrons, or other subatomic particles constituting an object at all temperatures above → absolute zero. The thermal motion of particles rises with the temperature of those particles and is governed by the laws of → thermodynamics. The most convincing experimental proof of thermal motion → Brownian motion.

→ thermal; → motion.

A neutron of very slow speed and consequently of low energy. The energy of thermal neutrons is of the same order as the → thermal energy of the atoms and molecules of the substance through which they are passing.

→ thermal; → neutron.

Electric noise signals that are produced by the random thermal motion of charges in circuit.

→ thermal; → noise.

The ordinary pressure in a gas that is due to motions of particles and can be attributed to the object's → temperature.

→ thermal; → pressure.

Repeated instabilities of the He burning shell which ignites in sudden burst during the final phases of the → AGB evolution.

→ thermal; → pulse.