An Etymological Dictionary of Astronomy and Astrophysics
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The temperature gradient defining the → radiative equilibrium condition in a region. It is expressed as: dT/dr = (1 - 1/ γ)((T / P)(dP / dr), where T and P are temperature and pressure, dT / dr and dP / dr temperature and pressure gradients respectively, and γ = C_P / C_V. For radiative equilibrium to be stable against → convection, the actual temperature gradient must be less than the adiabatic temperature gradient, i.e. |dT /dr|_rad < |dT /dr|_ad. See also → Schwarzschild's criterion.

→ adiabatic; → temperature; → gradient.

1) General: Degree of slope.
2) Physics: Change in the value of a quantity (as temperature, pressure) with change in a given variable.
3) Math.: A differential → operator (symbol → nabla, ∇) that, operating upon a function (f) of several variables, creates a → vector whose coordinates are the → partial derivatives of the function: ∇f = (∂f/∂x)i + (∂f/∂y)j + (∂f/∂z)k. The gradient of a → scalar function is a vector function.

From L. gradient-, gradiens, pr.p. of gradi "to walk, go," from grad- "walk" + -i- thematic vowel + -ent suffix of conjugation.

Ziné "ladder, steps, stair," may be related to ciné, from cidan "to place (something) above/upon (another similar thing);" cf. Lori râ-zina, Yazdi râ-cina "stairs," Nâyini orcen "stairs, ladder;" the phoneme change -c- into -z-, as in gozidan, gozin-/cidan, cin- both deriving from Proto-Ir. *cai- "to heap up, gather, collect."

The decrease in the → abundances of → heavy elements in a → disk galaxy as a function of distance from the center. Radial metallicity gradients are observed in many galaxies, including the → Milky Way and other galaxies of the → Local Group. In the case of the Milky Way, several objects can be used to determine the gradients: → H II regions, → B stars, → Cepheids, → open clusters, and → planetary nebulae. The main diagnostic elements are oxygen, sulphur, neon, and argon in photoionized nebulae, and iron and other elements in Cepheids, open clusters, and stars. Cepheids are probably the most accurate indicators of abundance gradients in the Milky Way. They are bright enough to be observed at large distances, so that accurate distances and spectroscopic abundances of several elements can be obtained. Average abundance gradients are generally between -0.03 → dex/kpc and -0.10 dex/kpc, with a a flattening out of the gradients at large galactocentric distances (≥ 10 kpc). The existence of these gradients offers the opportunity to test models of → chemical evolution of galaxies and stellar → nucleosynthesis.

→ metallicity; → gradient.

At a point, the rate of change of potential V, with distance x, measured in the direction in which the variation is a maximum. The intensity F of the field is proportional to the potential gradient, but is oppositely directed: F = -dV/dx.

→ potential; → gradient.

The pressure difference between two adjacent regions of a fluid that results in a force being exerted from the high pressure region toward the low pressure region.

→ pressure; → gradient.

A force resulting from → pressure gradient that is directed from high to low pressure.

→ pressure; → gradient; → force.

A condition in which there is an excess of the actual temperature gradient over the → adiabatic temperature gradient corresponding to the same pressure gradient. A region with superadiabatic temperature gradient is convectively unstable. → Hayashi forbidden zone.

→ super-; → adiabatic; → temperature; → gradient.

A physical quantity that describes the rate of change of temperature with displacement in a given direction from a given reference point. Same as → thermal gradient.

→ temperature; → gradient.

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.

Fluid Mechanics: The rate at which the velocity changes with the distance across the flow. When a fluid flows past a stationary wall, the fluid right close to the wall does not move. However, away from the wall the flow speed is not zero. Therefore a velocity gradient exists, which is due to adhesive, cohesive, and frictional forces. The amount of the velocity gradient is characteristic of the fluid.

→ velocity; → gradient.