An Etymological Dictionary of Astronomy and Astrophysics
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فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 18 Search : fusion
ambipolar diffusion
  پخش ِ اوبا قطبی   
paxš-e ubâqotbi

Fr.: diffusion ambipolaire   

A physical process which allows a → molecular cloud to decouple from → interstellar magnetic field in order to undergo → gravitational collapse. A cloud of pure molecular gas would form stars very fast through collapse since neutral matter does not respond to the magnetic field. However, the magnetic field holds up a collapse because the ions present in the cloud collide with the neutrals and tie them to the field. The collapse can then only proceed if the magnetic field can be separated from the gas. In denser molecular cores the ionization degree decreases substantially and therefore neutrals and ions decouple.

ambipolar; → diffusion.

atomic diffusion
  پخش ِ اتمی   
paxš-e atomi

Fr.: diffusion atomique   

element diffusion.

atomic; → diffusion.

confusion
  پشش   
pašeš

Fr.: confusion   

1) The act of confusing.
2) The state of being confused.
3) Disorder; upheaval; tumult; chaos (Dictionary.com).

Verbal noun of → confuse.

confusion limit
  حد ِ پشش   
hadd-e pašeš

Fr.: limite de confusion   

The → fluctuations of the → background  → sky brightness below which astronomical → sources cannot be → detected individually. The confusion limit is reached when the density of sources brighter than the → root mean square  → noise becomes high enough within the area of the resolution element.

confusion; → limit.

diffusion
  پخش   
paxš (#)

Fr.: diffusion   

1) Movement of a gas or liquid as a result of the random thermal motion of its atoms or molecules.
2) The random spreading out of a beam of radiation on reflection (→ diffuse reflection) or transmission through a translucent medium (such as amber, milk, or frosted glass).
3) In chemical thermodynamics, the process of attaining an equilibrium distribution of the → concentration within the phases. A result of diffusion at constant equilibrium is the equalization of the → chemical potentials.
Related terms: → diffraction; → dispersion; → distribution; → scatter; → scattering.

L. diffusionem, from stem of diffundere "scatter, pour out," from dif- "apart, in every direction," → dis-, + fundere "to melt, cast, pour out," from PIE *gheud-, from root *gheu- "to pour."

Paxš, verbal noun and stem of paxšidandiffuse.

diffusion coefficient
  همگر ِ پخش   
hamgar-e paxš

Fr.: coefficient de diffusion   

A factor of proportionality involved in the → diffusion equation. It may be defined as the amount of the quantity diffusing across a unit area through a unit concentration gradient in unit time. → magnetic diffusivity.

diffusion; → coefficient.

diffusion equation
  هموگش ِ پخش   
hamugeš-e paxš

Fr.: équation de diffusion   

An equation that expresses the time rate of change of a quantity in terms of the product of the diffusion coefficient and the → Laplacian operating on the quantity. For example the diffusion equation for temperature is: ∂T/∂t = D2T.

diffusion; → equation.

diffusion region
  ناحیه‌ی ِ پخش   
nâhiye-ye paxš

Fr.: région de diffusion   

A narrow boundary layer above the solar → photosphere, between two magnetic field lines, where the plasma becomes demagnetized or unfrozen. The presence of a localized magnetic region is necessary for → magnetic reconnection.

local; → -ize; → diffusion; → region.

eddy diffusion
  پخش ِ گیژاوی   
paxš-e gižâvi

Fr.: diffusion turbulente   

A macroscopic process that occurs in a → fluid because of the relative motions induced by the non-uniform → turbulent motions of the fluid. Also known as turbulent → diffusion. Eddy diffusion may occur in an atmosphere if it is unstable against turbulence. It dominates the atmosphere below the homopause. See also → molecular diffusion.

eddy; → diffusion.

element diffusion
  پخش ِ بن‌پار   
paxš-e bonpâr

Fr.: diffusion des éléments   

An important physical process occurring in stars, which is the relative separation of the various → chemical elements. It is caused by → gravitational settling and → thermal diffusion, on the one hand, and → radiative levitation on the other. This process, which was described by Michaud (1970) to account for the abundance anomalies observed in → chemically peculiar  → A star, is now recognized as occuring in all types of stars. Its influence on the observed → chemical abundances is extremely variable, however, due to competing macroscopic motions like → convective  → mixing or rotation-induced → turbulence. In the Sun, no observable abundance anomalies are expected from element diffusion, as the time scale of the process is longer than the solar lifetime. However the small induced → depletion of → helium and → heavy elements by about 20% is detectable through → helioseismology. Such detections are more difficult in stars, as only global → oscillation modes can be detected, in contrast to the Sun, where local oscillations of the surface can be analyzed (Théado et al., 2005, A&A 437, 553).

element; → diffusion.

fusion
  ۱، ۲، ۳) ایوش؛ ۳) گداز   
1, 2, 3) iveš; 3) godâz (#)

Fr.: fusion   

1) The act or process of fusing; the state of being → fused; that which is fused; the result of fusing.
2) A → nuclear reaction between atomic nuclei (→ nucleus) as a result of which a heavier nucleus is formed and a large quantity of → nuclear energy is released. → proton-proton chain, → CNO cycle, → nucleosynthesis.
3) Change of the → state of a → substance from → solid to → liquid which occurs at a definite → temperature at a given applied → pressure. Same as melting.

From M.Fr. fusion, from L. fusionem, from fusus, p.p. of fundere "to pour, melt."

Verbal noun form of → fuse.

gaseous diffusion
  پخش ِ گازی   
paxš-e gâzi

Fr.: diffusion gazeuse   

An → isotope separation process using the different diffusion speeds of → atoms or → molecules for separation. This process is used to divide → uranium hexafluoride (UF6) into two separate streams of U-235 and U-238. Before processing by gaseous diffusion, uranium is first converted from → uranium oxide (U3O8) to UF6. The UF6 is heated and converted from a solid to a gas. The gas is then forced through a series of compressors and converters that contain porous barriers. Because uranium-235 has a slightly lighter isotopic mass than uranium-238, UF6 molecules made with uranium-235 diffuse through the barriers at a slightly higher rate than the molecules containing uranium-238. At the end of the process, there are two UF6 streams, with one stream having a higher concentration of uranium-235 than the other (EVS, a Division of Argonne National Laboratory).

gaseous; → diffusion.

hydrogen fusion
  ایوش ِ هیدروژن   
iveš-e hidrožen

Fr.: fusion de l'hydrogène   

A → nuclear reaction where hydrogen (H) nuclei combine to form helium (4He) nuclei. Same as the → proton-proton chain.

hydrogen; → fusion.

magnetic diffusion
  پخش ِ مغناتیسی   
paxš-e meqnâtisi

Fr.: diffusion magnétique   

The process whereby the magnetic field tends to diffuse across the plasma and to smooth out any local inhomogeneities under the influence of a finite resistance in the plasma. For a stationary plasma the → induction equation becomes a pure → diffusion equation: ∂B/∂t = Dm2B, where Dm = (μ0σ0)-1 is the → magnetic diffusivity.

magnetic; → diffusion.

molecular diffusion
  پخش ِ مولکولی   
paxš-e molekuli

Fr.: diffusion moléculaire   

A mixing process in a → fluid caused by the → random relative motions due to → Brownian motion of the individual particles. See also → eddy diffusion.

molecular; → diffusion.

nuclear fusion
  ایوش ِ هسته‌ای   
iveš-e haste-yi

Fr.: fusion nucléaire   

A → nuclear reaction between atomic nuclei as a result of which a heavier → atomic nucleus is formed, a small fraction of mass is lost, and a large quantity of energy is released. Nuclear fusion is the source of the energy of stars.

nuclear; → fusion.

radiative diffusion
  پخش ِ تابشی   
paxš-e tâbeši

Fr.: diffusion radiative   

A process of → radiative transfer in which photons are repeatedly absorbed and re-emitted by matter particles.

radiative; → diffusion.

thermal diffusion
  پخش ِ گرمایی   
paxš-e garmâyi

Fr.: diffusion thermique   

A physical process resulting from → temperature gradients in stellar interiors, whereby more highly charged and more massive chemical species are concentrated toward the hottest region of the star, its center. Therefore, thermal diffusion and → gravitational settling tend to make heavier species sink relative to the light ones.

thermal; → diffusion.