An Etymological Dictionary of Astronomy and Astrophysics
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A physical process which is caused by the free electrons that are energized either by → photoionization or → collisional ionization. Collisional excitation puts ions, atoms, and molecules into excited states from which they may decay radiatively. Collisional excitation is important in the → interstellar medium.

→ collisional; → excitation.

The transition to a higher → energy level than → ground state undergone by an atomic nucleus when a → charged particle of appropriate energy moves past it.

→ coulomb; → excitation.

Transition from an excited energy state to a lower energy level, as in spectral line formation or particle emission from an atomic nucleus.

From → de- + → excitation.

1) The addition of → energy to an → atomic or → molecular system, → transferring it from its → ground state to an → excited state.
2) A property of an → emission nebula usually expressed by the ratio [O III] 5007Å/Hβ.

Verbal noun of → excite; → -tion.

Amount of energy (usually measured in → electron-volts) required to bring an electron from its → ground state to a given → excited state.

→ excitation; → energy.

In quantum mechanics, the energy that is necessary to change a system from a → ground state to a given → excited state; also called excitation energy.

→ excitation; → potential.

Of a gas or plasma, the temperature deduced from the → populations of atomic → excited states, as expressed by the Boltzmann formula: N_u/N_l = (g_u/g_l) exp (-ΔE/kT_ex), where N_u and N_l are the upper level and lower level populations respectively, g_u and g_l the statistical weights, ΔE = hν the energy difference between the states, k is → Boltzmann's constant, and h→ Planck's constant. The higher the energy of the occupied states, the higher the excitation temperature.

→ excitation; → temperature.

A rare class of → H II regions in the → Magellanic Clouds. In contrast to the typical H II regions of the Magellanic Clouds, which are extended structures (sizes of several arc minutes corresponding to more than 50 pc, powered by a large number of exciting stars), HEBs are very dense and small regions (~ 4" to 10" in diameter corresponding to ~ 1-3 pc). They have a higher degree of → excitation ([O III] 5007Å /Hβ) with respect to the typical H II regions, and are, in general, heavily affected by local → dust. They are powered by a relatively smaller number of → massive stars.

→ high; → excitation; → blob.

The mechanism of raising an electron to higher energies by photon absorption, when the energy of the photon is too low to cause photoionization.

→ photo- + → excitation.

The mechanism arising from turbulent convection in the → convective zone of stars, which is responsible for the driving of stellar → pulsation modes. In stars cooler than typically ~ 7 500 K (→ F-type stars and cooler), the stochastic excitation occurs in the convection envelope. In massive stars, it may develop either in the → convective core or in the convective layer beneath the → photosphere. Recent studies suggest that in → Be stars stochastic excitation takes place in the convective core. The stochastic waves can transport → angular momentum from the core to the surface. Fast rotation, as in Be stars, amplifies the stochastic excitation.

→ stochastic; → mode.

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

→ thermal; → excitation.