An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

The speed at which any fixed phase (individual wave) in a → wave packet travels. It is expressed as v_ph = ω/k, where ω is the → angular frequency and k the → wave number. See also the → group velocity.

→ phase; → velocity.

The → angular velocity of a star deduced from the → rotational broadening of its → spectral lines. It is expressed as v sini, where i is the → inclination of the rotational axis with respect to the normal to the → plane of the sky. The real equatorial rotational velocity can be determined only if the inclination of the rotational axis is known.

Projected, p.p. of → project; → rotational; → velocity.

The component of a three-dimensional velocity vector of an object directed along the line of sight. It is measured by examining the Doppler shift of lines in the spectrum of astronomical objects.

→ radial; → velocity.

A curve describing the variation of the radial velocity of a star, due to the Doppler effect, under the gravitational effect of a secondary body (companion or exoplanet). The amplitude of these variations depends upon the mass of the secondary and its distance from the star.

→ radial velocity; → curve.

The technique based on the analysis of the → radial velocity curve, used to detect the presence of an invisible secondary around a host star. This method holds the majority of exoplanet discoveries.

→ radial velocity; → method.

The velocity with which an object moves away from another object or a reference point.

→ recession; → velocity.

For two objects A and B, the velocity which B, supposing itself at rest, assigns to A.

→ relative; → velocity.

The velocity of a body when it is a significant fraction of the → speed of light.

→ relativistic; → velocity.

The velocity of a → rotational motion; same as → angular velocity.

→ rotational; → velocity.

The rate of change of the Sun's position with respect to the local standard of rest toward the → solar apex.

→ solar; → velocity.

The velocity of a star relative to the Sun.

→ space; → velocity.

1) The instantaneous linear velocity of a body moving in a circular path. It is equal to the → angular velocity multiplied by the radius: v_t = ωr.
3) The component of the velocity of an object, such as a star, that is at right-angles to the observer's → line of sight; also known as transverse velocity. See also → radial velocity.

→ tangential; → velocity.

1) The constant maximum velocity reached by a body falling under gravity through a liquid or gas, especially the atmosphere. The body ceases to accelerate downward because the force of gravity is equal to the opposing force of resistance by the medium.
2) The velocity acquired at the end of a body's motion.
3) The velocity attained by → stellar wind at very large distance from the star. The material that escapes from the outer layers of the stars is accelerated outward from a small radial velocity at the → photosphere of the star, to some high velocity at large distance from the star. The theory of → radiation-driven winds predicts that the terminal velocity scales with the → escape velocity as: v_∞  ≅ 2-3 v_esc. The winds of hot → O stars experience fast acceleration and reach 80% of their terminal velocity near the star (some 3 R_*). The terminal velocity ranges from about 10 km s^-1 for a cool → supergiant star to 3000 km s^-1 for a luminous hot star. Terminal velocities are quite accurately measured from the violet trough of saturated → P Cygni line profiles in the ultraviolet (mainly N V λ1239, C IV λ1548, Si IV λ1394).

→ terminal; → velocity.

Same as → tangential velocity.

→ transverse; → velocity.

Of a rotating body, a vector of magnitude ω (→ angular velocity) pointing in the direction of advance of a right-hand screw which is turned in the direction of rotation.

→ vector; → angular; → velocity.

The time rate of change of position in a given direction, measured as length per unit time. → speed.

L. velocitatem (nominative velocitas) "swiftness, speed," from velox (genitive velocis) "swift."

Tondâ, from tond "swift, rapid, brisk; fierce, severe" (Mid.Pers. tund "sharp, violent;" Sogdian tund "violent;" cf. Skt. tod- "to thrust, give a push," tudáti "he thrusts;" L. tundere "to thrust, to hit" (Fr. percer, E. pierce, ultimately from L. pertusus, from p.p. of pertundere "to thrust or bore through;" PIE base *(s)teud- "to thrust, to beat") + noun suffix -â.

A plot of the radial velocity of an object against time, derived from the Doppler shift of spectral lines.

→ velocity; → curve.

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.