An Etymological Dictionary of Astronomy and AstrophysicsEnglish-French-Persian

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 27 Search : parameter
 acceleration parameter   پارامون ِ شتاب   pârâmun-e šetâbFr.: paramètre d'accéleration   A measure of the departure from a constant rate of the acceleration of the Universe, expressed by: q(t) = R(t)R ..(t)/R .2(t), where R(t) represents the size of the Universe at time t. Traditionally, a negative sign is inserted in the above equation for the → deceleration parameter.→ acceleration; → parameter. angular momentum parameter   پارامون ِ جنباک ِ زاویه‌ای   pârâmun-e jonbâk-e zâviye-yiFr.: paramètre de moment angulaire   The ratio J/M, where J is the → angular momentum of a → rotating black hole and M the mass of the black hole.→ angular; → momentum; → parameter. deceleration parameter   پارامون ِ واشتاب   pârâmun-e vâšetâbFr.: paramètre de décéleration   A parameter designating the rate at which the expansion of the Universe would slow down owing to the braking gravitational effect of the matter content of the Universe. It is expressed by: q(t) = -R(t)R ..(t)/R .2(t), where R(t) represents the size of the Universe at time t. See also → expansion parameter; compare with → acceleration parameter.→ deceleration; → parameter. density parameter   پارامون ِ چگالی   pârâmun-e cagâliFr.: paramètre de densité   One of the four terms that describe an arranged version of the → Friedmann equations. They are all time dependent. 1) For matter: Ωm = 8πGρm/(3H2), where G is the → gravitational constant, ρm is the mean matter density, and H the → Hubble parameter. The matter density parameter is also expressed as Ωm = ρm/ρcrit, where ρcrit is the → critical density. 2) For radiation: Ωr = 8πGρr/(3H2), where ρr is the radiation equivalent of matter density. This parameter is also expressed as Ωr = ρr/ρcrit. 3) For the → cosmological constant: ΩΛ = Λc2/(3H2). Similarly, ΩΛ = ρΛ/ρcrit, where &rhoΛ = Λc2/(8πG) is sometimes referred to as the density of → dark energy. 4) For the → curvature of space-timeΩk = -kc2/(R2H2), where k is the → curvature constant and R the → cosmic scale factor. Note that: Ωm + Ωr + ΩΛ + Ωk = 1, and Ωtotal = Ωm + Ωr + ΩΛ = 1 - Ωk.→ density; → parameter. Eddington parameter   پارامون ِ ادینگتون   pârâmun-e EddingtonFr.: paramètre d'Eddington   A → dimensionless parameter indicating the degree to which a star is close to the → Eddington limit. It is expressed as Γ = L / LEdd = κ L / (4πGMc), where L and M are the star luminosity and mass respectively, κ is the opacity, c the speed of light, and G the → gravitational constant. At the Eddington limit, Γ = 1, the star would become unbound. Because stellar luminosity generally scales with a high power of the stellar mass (L∝ M3-4), → massive stars with M larger than 10 Msun generally have electron Eddington parameters of order Γ ≅ 0.1-1.After Arthur Stanley Eddington (1882-1944), prominent British astrophysicist; → parameter. effective Eddington parameter   پارامون ِ ادینگتون ِ اسکرمند   pârâmun-e Eddington-e oskarmandFr.: paramètre d'Eddington effectif   The effective value of the → Eddington parameter in a non-homogeneous system (porous opacity).→ effective; → Eddington limit; → parameter. equation of state parameter   پارامون ِ هموگش ِ حالت   pârâmun-e hamugeš-e hâlatFr.: paramètre de l'équation d'état   In cosmology, a → dimensionless parameter introduced by the → equation of state representing the ratio of the pressure to the energy density of a fluid, such as the → dark energy: w = p/ρ. The → deceleration or → acceleration of an → expanding Universe depends on this parameter (→ accelerating Universe). A number of numerical values of this parameter are as follows: for the → cosmological constant: w = -1, for → non-relativistic matter (present-day → baryons): w = 0, and for → relativistic matter (photons, neutrinos): w = +1/3. Together with Ω(dark energy) and Ω(matter), w provides a three-parameter description of the dark energy. The simplest parametrization of the dark energy is w = constant, although w might depend on → redshift.→ equation; → state; → parameter. expansion parameter   پارامون ِ سپانش   pârâmun-e sopânešFr.: paramètre d'expansion   A → scale factor that relates the size of the Universe R = R(t) at time t to the size of the Universe R0 = R(t0) at time t0 by R = aR0. The expansion parameter represents the history of expansion of the Universe.→ expansion; → parameter. force multiplier parameter   پارامون ِ بستاگر ِ نیرو   pârâmun-e bastâgar-e niruFr.: paramètre de multiplicateur de force   One of the three quantities (k, α, and δ) which are used in the → radiation-driven wind theory to express the radiation pressure due to spectral lines. These coefficients parametrize the radiation acceleration as: grad≅ k(dv/dr)αge, where ge = σeL/4πcR*2 is the radiative acceleration by electron scattering. The parameter k is dependent on the number of lines that produce the radiation pressure. The parameter α depends on the optical depth of the driving lines and varies between 0 (optically thin) and 1 (optically thick). The parameter δ describes the dependence of k on the density with k ≅ ρδ. The velocity law of radiation-driven winds depends on α and δ, but not on k. The → mass loss rate depends on k, α, and δ (Castor et al. 1975, ApJ 195, 157; Lamers et al., 1995, ApJ 455, 269 and references therein).→ force; → multiplier; → parameter. Fried parameter   پارامون ِ فرید   pârâmun-e FriedFr.: paramètre de Fried   One of the parameters that characterize atmospheric → seeing. It is the diameter of the largest aperture that can be used before → turbulence starts to degrade the image quality. As the turbulence gets stronger, the Fried parameter, denoted r0, becomes smaller. The Fried parameter is wavelength dependent: r0 ∝ λ6/5. On best astronomical mountain tops it ranges between 20 and 30 cm for λ = 5000 A.Named after David L. Fried, who defined the parameter 10 1966; → parameter. Hubble parameter   پارامون ِ هابل   pârânmun-e HubbleFr.: paramètre de Hubble   → Hubble; → parameter. Hubble-Lemaitre parameter   پارامون ِ هابل-لومتر   pârânmun-e Hubble-LemaîtreFr.: paramètre de Hubble-Lemaître   The rate pf change of the → cosmic scale factor: H(t) = (dR/dt)/R. The Hubble parameter is a time-dependent quantity and therefore is not constant. The → Hubble-Lemaitre constant is the Hubble-Lemaître parameter measured today.→ Hubble-Lemaitre law; → parameter. impact parameter   پارامون ِ برخورد   pârâmun-e barxordFr.: paramètre d'impact   1) A measure of the distance by which a collision fails being frontal. 2) In → gravitational lensing, the distance of closest approach of the light path to the → lensing object. 3) In → rainbows, the displacement of the → incident from an axis that passes through the center of the water droplet.→ impact; → parameter ionization parameter   پارامون ِ یونش   pârâmun-e yonešFr.: paramètre d'ionisation   A ratio representing the number of ionizing photons to the number of electrons in a nebular emitting region.→ ionization; → parameter. mixing length parameter   پارامون ِ درازای ِ آمیزش   pârâmun-e derâzâ-ye âmizešFr.: paramètre de la longueur de mémange   In the → mixing length theory, a parameter, α, that relates the → mixing length, l, to the → pressure scale height: α = l/HP. It is usually supposed that α is of order unity. Changes in α correspond to variations in the efficiency of the → convection, hence the transfer of heat.→ mixing; → length; → parameter. normalized Hubble parameter   پارامون ِ هابل ِ هنجاروریده   pârâmun-e Hubble-e hanjârvaridéFr.: paramètre Hubble normalisé   A dimensionless parameter expressed by h(z) = H(z)/H0, where H(z) is the → Hubble parameter at → redshift z and H0 is the → Hubble constant.→ normalized; → Hubble; → parameter. orbital parameter   پارامون ِ مداری   pârâmun-e madâriFr.: paramètre orbital   → orbital; → parameter. parameter   پارامون   pârâmunFr.: paramètre   General: Any of a set of physical properties whose values determine the characteristics or behavior of something. → impact parameter; → ionization parameter. Math.: A constant or variable term in a function that determines the specific form of the function but not its general nature, as a in f(x) = kx, where k determines only the slope of the line described by f(x).Mod.L. parametrum, from Gk. → para- + metron "measure," → meter.Parâmun, from parâ-, → para-, + mun/mân "measure," as in Pers. terms pirâmun "perimeter," âzmun "test, trial," peymân "measuring, agreement," peymâné "a measure; a cup, bowl," from O.Pers./Av. mā(y)- "to measure;" cf. Skt. mati "measures," matra- "measure;" Gk. metron "measure;" L. metrum; PIE base *me- "to measure." physical parameter   پارامون ِ فیزیکی   pârâmun-e fizikiFr.: paramètre physique   Any of a set of physical properties whose values determine the characteristics or behavior of a system; for example, → mass, → size, → temperature, → luminosity, etc.→ physical; → parameter. reddening parameter   پارامون ِ سرخش   pârâmun-e sorxešFr.: paramètre de rougissement   A dimensionless quantity characterizing the → interstellar extinction, defined by the total-to-selective extinction ratio: RV = AV/E(B-V). The typical value found for the reddening parameter in the Milky Way is RV ~ 3.1, but it is known to vary from one line of sight to another, from values as 2 to as large as 6. Very large → dust grains would produce extinction with RV → ∞.→ reddening; → parameter.

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