An Etymological Dictionary of Astronomy and Astrophysics
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The → standard deviation of a velocity → distribution. It indicates how objects of the sample move relative to one another. Objects with similar velocities have a small velocity dispersion, whereas objects with very different velocities have a large velocity dispersion.

→ velocity; → dispersion.

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.

In the theory of → radiation-driven winds, an equation that describes the behavior of the → wind velocity of → hot stars as a function of distance from the star. This velocity β-law is given by the expression: v(r) = v_∞(1 - R_*/r)^β, where v_∞ is the → terminal velocity, R_* is the stellar radius, and r the distance from the center. For → O-type stars, the exponent is estimated to be β = 0.8.

→ velocity; → law.

A → physical constant which represents the ultimate speed limit for anything moving through space, according to the theory of → special relativity. It is the speed of propagation of → electromagnetic waves in a vacuum, equal to 299,792.458 km/s (nearly 3 x 10⁸ m/s). The velocity of light appears as the connecting link between mass and energy in the → mass-energy relation. Usually denoted by c, from L. celeritas "swiftness," from celer "swift," → acceleration.

→ velocity; → speed; → light.

→ dynamic pressure.

→ velocity; → pressure.

A plot of the fluid velocity as a function of position.

→ velocity; → profile.

Of a dynamical system, a three-dimensional space which consists of the set of values that the velocity can take (v_x, v_y, v_z). → phase space.

→ velocity; → space.

The linear relation wherein all galaxies are moving away from one another, with velocities that are greater with increasing distance of the galaxy. Same as → Hubble's law.

→ velocity; → distance; → relation.

In fluid mechanics, a measure of the rate of rotational spin in a fluid. Mathematically, vorticity is a vector field defined as the curl of the velocity field: ω = ∇ x v.
Meteo.: The rotation of air around a vertical axis.

From L. vortic-, from → vortex + → -ity.

Gerdšârigi, from gerdšâr→ vortex + -igi, → -ity.

The speed at which the → stellar wind is forced away from the star. Wind velocities of → hot stars are directly measured from → P Cygni profiles, which indicate velocities from several hundred to several thousand km s^-1. See also → escape velocity, → terminal velocity, → velocity law, → radiation-driven wind, → CAK model.

→ wind; → velocity.

In the → restricted three-body problem, a surface which limits the region of space in which a small body can move. In the expression for the → Jacobi integral, the left side value is always positive or nul; hence the particle motion is confined to the region where U ≤ C_J. The surface that limits this region, defined by U = C_J, is called the zero-velocity surface.

→ zero; → velocity; → surface.