An Etymological Dictionary of Astronomy and Astrophysics
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→ mixing process.

→ chemical; → mixing.

Any → mixing process that utilizes the → kinetic energy of relative → fluid motion.

→ mechanical; → mixing.

1) General: The process or result of irregular fluctuations in fluid motions on all scales from the molecular to large eddies.
2) Relating to → stellar interior, → mixing process.

Verbal noun of → mix.

In a → turbulent flow, the average distance traveled by a → convective cell before it dissolves into its surroundings and deposits its energy. The mixing length is of the order of the → pressure scale height (H_P), l = αH_P, where α is the → mixing length parameter. See also → mixing length theory.

→ mixing; → length.

In the → mixing length theory, a parameter, α, that relates the → mixing length, l, to the → pressure scale height: α = l/H_P. It is usually supposed that α is of order unity. Changes in α correspond to variations in the efficiency of the → convection, hence the transfer of heat.

→ mixing; → length; → parameter.

A theory dealing with heat transport by → turbulence which includes an elementary treatment of → convection. The central idea is that an unbalanced → buoyancy force drives a → convective cell to move through a distance, called the → mixing length, before the cell dissolves and joins the ambient medium. In this theory an adjustable → mixing length parameter  α is used. The theory, originally due to L. Prandtl (1925), was first applied to the Sun by L. Biermann (1932, Z. Astrophys. 5, 117).

→ mixing; → length; → theory.

A process whereby → angular momentum and chemical species are transported from layer to layer within a star. The main mixing processes include: → convection, → overshooting, → rotation, and → turbulence. The extent to which the interiors of stars are mixed strongly influences their evolution, age, chemical content, and the relationship between their internal and surface → chemical abundances.

→ mixing; → process.

Mass of water vapor per mass of dry air; expressed as grams per kilogram. → humidity

→ mixing; → ratio.

A consequence of → stellar rotation that deforms the star, triggers instabilities (→ shear turbulence and → meridional currents) leading to → transport of chemical species in the star. The efficiency of rotational mixing (measured for instance by the degree of surface → enrichments at a given → evolutionary stage) increases when the initial mass and rotation increase. This efficiency increases also when the initial → metallicity decreases. This is due to the fact that when the metallicity is lower, the stars are more compact. This makes the → gradients of the → angular velocity steeper in the stellar interiors. Steeper gradients produce stronger shear turbulence and thus more mixing. Rotational mixing can bring to the surface heavy elements newly synthesized in the stellar core. Rotation thus produces an increase of the → opacity of the outer layers and activates strong → mass loss through → radiatively driven winds. This effect may be responsible for the loss of large fractions of the initial mass of the star (Meynet et al. 2007, arXiv:0709.2275).

→ rotational; → mixing.

In stars, an instability phenomenon, reminiscent of the → thermohaline convection in the oceans, that takes place when layers of higher molecular weight occur above a region of lower molecular weight. A situation of heavier material being above lighter gas in a star can occur during the → helium flash when → helium burning does not start in the center but in the shell. Similarly, in → close binary systems it may happen that helium-rich material is transferred to a → main sequence star. Then a helium-rich outer layer is formed and the instability occurs at the interface between that layer and the original stellar material. This process can explain several surface abundance variations in stars. First discussed by S. Kato (1966, Publ. Astron. Soc. Japan 18, 374).

→ thermohaline; → mixing.