Alfven velocity tondâ-ye Alfvén Fr.: vitesse d'Alfvén same as → Alfven speed. → Alfven wave; → velocity. |
angular velocity tondâ-ye zâviye-yi Fr.: vitesse angulaire A measure of the angular displacement per unit time. Of a particle traveling on a circular path or a rotating body, the ratio of the angle traversed to the amount of time it takes to traverse that angle: ω = dθ/dt. For a rigid body, all lines in it rotate through the same angle in the same time, and the angular velocity is the characteristic of the body as a whole. The angular velocity is related to the linear velocity by the equation v = rω, where r is the distance of the point from the rotation axis. → vector angular velocity. |
asymptotic velocity tondâ-ye nâhamsâvi Fr.: vitesse asymptotique For → stellar winds, same as → terminal velocity. → asymptotic; → velocity. |
average velocity tondâ-ye miyângin Fr.: vitesse moyenne The ratio of the displacement (Δx) of a particle, as it moves from point A to point B, to the corresponding time interval: v = Δx/Δt. |
break-up velocity tondâ-ye gosast Fr.: vitesse de rupture The velocity of a → rotating star at which the → centrifugal force equals the → gravitational force. Also known as → critical velocity. The simplest expression of the break-up velocity for an OB star, ignoring the → Eddington luminosity, is given by the relation: v = (GM / R)^{1/2}, where M and R are the mass and radius of the star respectively, and G the → gravitational constant. A more realistic expression takes into account not only the → radiation pressure, but also the non-uniformity of the brightness over the stellar surface, as indicated by → von Zeipel theorem. With these conditions, the break-up velocity has a more complicated formula, corresponding to the velocity reached when somewhere on the star the → total gravity becomes zero. |
compact high-velocity clouds (CHVCs) abrhâ-ye hampak-e tondrow Fr.: nuages compacts à grande vitesse A population of relatively small (typically < 2°) → high-velocity clouds, which are spatially and kinematically isolated from the gas distribution in their environment. They are thought to be located in the → intergalactic medium of the → Local Group. → compact; → high-velocity cloud. |
critical velocity tondâ-ye paržani Fr.: vitesse critique 1) Velocity of → fluid through a pipe at which the motion
changes from → laminar to
→ turbulent flow. |
drift velocity tond-ye delek Fr.: vitesse de dérive The average velocity of a charged particle in a plasma in response to an applied electric field. |
escape velocity tondâ-ye goriz Fr.: vitesse d'échapement The speed an object must attain in order to free itself from the gravitational influence of an astronomical body. It is the minimum velocity for the object to enter a parabolic trajectory. The escape velocity is given by: V_{e} = (2GM/r)^{1/2}, where G is the → gravitational constant, M is the mass of the astronomical body, and r is its radius. The escape velocity of the Earth is about 11.2 km s^{-1}, that of the Moon is 2.4 km s^{-1}, and that of the Sun about 618 km s^{-1}. |
group velocity tondâ-ye goruh Fr.: vitesse de groupe The velocity at which the envelope of a → wave packet propagates, v_{gr} = dω/dk, at k_{0} (the central value of k). The group velocity can be equal to, larger, or smaller than the → phase velocity. |
high-velocity clouds (HVCs) abrhâ-ye tondrow Fr.: nuages à grande vitesse A population of neutral or partly ionized gas clouds in the → Galactic halo which are seen as high-altitude structures in the atomic hydrogen 21 cm emission at high radial velocities (v_{LSR}> 100 km/sec). They have substantial neutral column densities (> 10^{19} cm^{-2}) and their metallicities range from 0.1 to about 1.0 times solar. The distances to the majority of them remain unknown. They may represent the continuing infall of matter onto the → Local Group. See also → compact high-velocity clouds. Abr, → cloud; tondrow "fast moving," from tond "fast," → speed; row, present stem of raftan "to go, walk; to flow" (Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- "to go; to attack"). |
hypervelocity star (HVS) setâre-ye hipertond Fr.: étoile hypervéloce A star whose velocity is so great that it will escape the
→ gravitational potential of our
→ Galaxy. Depending on the location and direction of
motion, this criterion typically corresponds to a stellar velocity in
the Galactic → rest frame larger than
400 km s^{-1}, and up to about 1200 km s^{-1}.
The nature of the HVSs spans a wide range of types from
→ OB stars, to metal-poor
→ F-type stars and G/K dwarfs. While there is evidence from many
late-type B HVSs in the → halo
to originate from the Galactic
→ supermassive black hole (SMBH),
other HVSs seem to originate from the → galactic disk.
HVSs can obtain their large velocities from a number of different processes: |
instantaneous velocity tondâ-ye lahze-yi Fr.: vitesse instantanée The velocity of a particle at some one instant of time, or at some one point of its path. It can be defined as the limiting value of the average velocity when the second point is taken closer and closer to the first point. |
Keplerian angular velocity tondâ-ye zâviye-yi-ye Kepleri Fr.: vitesse angulaire keplérienne The angular velocity of a point in a circular orbit around a central mass. It is given by: Ω_{K} = (GM/r^{3})^{1/2}, where G is the → gravitational constant, M is the mass of the gravitating object, and r is the radius of the orbit of the point around the object. |
orbital velocity tondâ-ye madâri Fr.: vitesse orbitale The velocity of an object in a given orbit around a gravitating mass. For a perfect circular orbit, the velocity is described by the formula V =√[G(M + m)/r], where G is the gravitational constant, M the mass of the primary gravitating body, m the mass of the orbiting object, and r the radius of the orbit. |
parabolic velocity tondâ-ye sahmi Fr.: vitesse parabolique The speed necessary to form a parabolic orbit around a gravitational center. It is also the minimum speed necessary to escape from the gravitational pull of a body. |
peculiar velocity tondâ-ye afd Fr.: vitesse particulière 1) Velocity with respect to the Local Standard of Rest. |
phase velocity tondâ-ye fâz Fr.: vitesse de phase 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. |
projected rotational velocity tondâ-ye carxeši-ye farâšândé Fr.: vitesse rotationnelle projetée 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. |
radial velocity tondâ-ye šo'â'i Fr.: vitesse radiale 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. |