An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



<< < Alf rad vel > >>

Number of Results: 43 Search : velocity
Alfven velocity
  تندای ِ آلفون   
tondâ-ye Alfvén

Fr.: vitesse d'Alfvén   

same as → Alfven speed.

Alfven wave; → velocity.

angular velocity
  تندا‌ی ِ زاویه‌ای   
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.

angular; → velocity.

asymptotic velocity
  تندای ِ ناهمساوی   
tondâ-ye nâhamsâvi

Fr.: vitesse asymptotique   

For → stellar winds, same as → terminal velocity.

asymptotic; → velocity.

average velocity
  تندای ِ میانگین   
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.

average; → velocity.

break-up velocity
  تندای ِ گسست   
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.

break + up; M.E.; O.E. up, uppe, → hyper-; → velocity.

compact high-velocity clouds (CHVCs)
  ابرهای ِ همپک ِ تندرو   
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
  تندای ِ پرژنی   
tondâ-ye paržani

Fr.: vitesse critique   

1) Velocity of → fluid through a pipe at which the motion changes from → laminar to → turbulent flow.
2) Same as → break-up velocity.

critical; → velocity.

drift velocity
  تندای ِ دلک   
tond-ye delek

Fr.: vitesse de dérive   

The average velocity of a charged particle in a plasma in response to an applied electric field.

drift; → velocity.

escape velocity
  تندای ِ گریز   
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: Ve = (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.

escape; → velocity.

group velocity
  تندای ِ گروه   
tondâ-ye goruh

Fr.: vitesse de groupe   

The velocity at which the envelope of a → wave packet propagates, vgr = dω/dk, at k0 (the central value of k). The group velocity can be equal to, larger, or smaller than the → phase velocity.

group; → velocity.

high-velocity clouds (HVCs)
  ابرهای ِ تندرو   
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 (vLSR> 100 km/sec). They have substantial neutral column densities (> 1019 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.

high; → velocity; → cloud.

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)
  ستاره‌ی ِ هیپرتند   
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:
1) → Tidal disruption of → close binary stars by the central SMBH of the Milky Way. In this process one star is captured by the SMBH while the other is ejected at high speed via the → gravitational slingshot mechanism.
2) Exchange encounters in other dense stellar environments between hard binaries (→ hard binary) and → massive stars may cause stars to be ejected and escape our Galaxy.
3) Disruption of close binaries via → supernova explosions. The → runaway velocities of both ejected stars can reach large values when asymmetric supernovae are considered, i.e. when the newborn → neutron star receives a momentum kick at birth.
(see, e.g., T. M. Tauris, 2014, and references therein, arXiv:1412.0657).

hyper-; → velocity; → star.

instantaneous velocity
  تندای ِ لحظه‌ای   
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.

instant; → velocity.

Keplerian angular velocity
  تندای ِ زاویه‌ای ِ کپلری   
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: ΩK = (GM/r3)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.

Keplerian; → angular; → velocity.

orbital velocity
  تندایِ مداری   
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.

orbital; → velocity.

parabolic velocity
  تندای ِ سهمی   
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.

parabolic; → velocity.

peculiar velocity
  تندا‌ی ِ اَفد   
tondâ-ye afd

Fr.: vitesse particulière   

1) Velocity with respect to the Local Standard of Rest.
2) Any velocity a galaxy has with respect to us that is not a Hubble law velocity due to the expansion of space.

peculiar; → velocity.

phase velocity
  تندای ِ فاز   
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 vph = ω/k, where ω is the → angular frequency and k the → wave number. See also the → group velocity.

phase; → velocity.

projected rotational velocity
  تندای چرخشی ِ فراشانده   
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
  تندای ِ شعاعی   
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.

radial; → velocity.

<< < Alf rad vel > >>