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fingering instability nâpâydâri-ye angoštvâr Fr.: instabilité à traines A type of instability that often occurs in fluids which are thermally stably stratified, but have an inhomogeneous composition. A well-known example, found in upper layers of the Earth's oceans, is → salt fingers. Similar fingering instabilities can occur in any other thermally stably stratified solution, provided the concentration of the slower-diffusing solute increases with height. The saturated state of this instability, → fingering convection, takes the form of tightly-packed, vertically-elongated plumes of sinking dense fluid and rising light fluid, and significantly enhances the vertical transport of both heat and chemical composition. The fingering instability occurs in stars within radiation zones that have an unstable mean → molecular weight → gradient (μ gradient). This situation is often found as a result of material accretion onto a star by anything from a single or multiple planets, to material from a dust-enriched or debris accretion disk, or material from a more evolved companion. It also naturally arises in the vicinity of the → hydrogen shell burning in → red giant branch (RGB) stars, and in thin element-rich layers near the surface of intermediate-mass stars. The fingering instability initially takes the form of thin tubes, hence the name "finger," within which the fluid moves vertically. The tubes rapidly break down, however, as a result of parasitic shear instabilities that develop inbetween them, and the fingering instability eventually saturates into a state of homogeneous fingering convection where the typical aspect ratio of the eddies is closer to one (P. Garaud et al., 2015, arXiv:1505.07759). → finger; → -ing; → instability. |
fingers of God angoštân-e Xodâ Fr.: doigts de Dieu A → redshift space distortion which causes the dense central regions of → galaxy clusters to appear elongated along the → line of sight. This effect is attributed to random velocities in clusters of galaxies deviating from pure → Hubble flow. For an observer galaxies with peculiar velocity perpendicular to the line of sight do not change the redshift, which is given just by the normal Hubble expansion. On the contrary, galaxies with peculiar velocity along the line of sight appear with a different redshift, resulting from the Hubble expansion velocity plus the peculiar velocity. Since this affects only redshift and not position on the sky, the stretching occurs only radially, toward the observer. See also → Kaiser effect, → peculiar velocity. |
finite karânmand (#) Fr.: fini 1) Math: The opposite of → infinite. From L. finitus, p.p. of finire "to limit, set bounds, end." Karânmand, from karân "boundary, side, end, coast" + -mand adjective suffix. Karân, variants karâné, kenâr, from Mid.Pers. karân, karânak, kenâr "edge, limit, boundary," Av. karana- "side, boundary, end." |
finite population porineš-e karânmand Fr.: population finie A → statistical population consisting of individuals or items which are finite in number. → finite; → population. |
finite series seri-ye karânmand (#) Fr.: série finie A sum a_{1} + a_{2} + a_{3} + · · · + a_{N}, where the a_{i}'s are real numbers. In terms of Σ-notation, it is written as a_{1} + a_{2} + a_{3} + · · · + a_{N} = Σ (n = 1 to N). See also → infinite series. |
finite set hangard-e karânmand Fr.: ensemble fini A → set whose elements can be numbered from 1 to n, for some positive integer n. |
fire âtaš(#), taš (#), âzar (#) Fr.: feu A state, process, or instance of combustion in which a substance combines with oxygen producing heat, light, and flame. O..E. fyr, from P.Gmc. *fuir (cf. O.N. fürr, M.Du. vuur, Ger. Feuer), from PIE *paewr-; cf. Mod.Pers. Lori porpor "blazing charcoal," Gilaki bur, biur "smokeless red fire" (Lori perisk, periska "spark," Kurd. biriske "spark," Lârestâni pelita "spark"); Tokharian por, puwar "fire;" Gk. pyr "fire;" Hitt. pahhur "fire;" Skt. pū- "to cleanse." Âtaš, variants âzar, taš, from Mid.Pers. âtaxš, âtur "fire;" Av. ātar-, āθr- "fire," singular nominative ātarš-; O.Pers. ātar- "fire;" Av. āθaurvan- "fire priest;" Skt. átharvan- "fire priest;" cf. L. ater "black" ("blackened by fire"); Arm. airem "burns;" Serb. vatra "fire;" PIE base *āter- "fire." |
fireball tašguy (#), âzarguy (#) Fr.: boule de feu A → meteor that is brighter than the brightest planets, i.e. with an apparent magnitude of -5 or greater. Fireballs are often followed by → meteorite falls. Also called → bolide. From → fire + ball, from O.E., from O.N. bollr "ball," from P.Gmc. *balluz (cf. O.H.G. ballo, Ger. Ball), from PIE base *bhel- "to swell." Tašguy, from taš "fire," variant of âtaš→ fire + guy "ball, sphere," variants golulé, gullé, goruk, gulu, gudé (cf. Skt. guda- "ball, mouthful, lump, tumour," Pali gula- "ball," Gk. gloutos "rump," L. glomus "ball," globus "globe," Ger. Kugel, E. clot; PIE *gel- "to make into a ball"). |
first naxost (#), naxostin (#), yekom (#), âqâz (#) Fr.: premier Being before all others with respect to time, order, importance, etc., used as the ordinal number of one. O.E. fyrst "foremost," superlative of fore, from P.Gmc. *furisto (cf. O.H.G. furist, O.N. fyrstr, Dan. første, M.Du. vorste "first," Ger. Fürst "prince"), superlative of *fur-/*for-, from PIE *pro- (cf. Av. pouruua- "first," fra- "forward, forth;" Skt. pūrva- "first," pra- "before, formerly," Gk. pro; L. pro; E. fore). Naxost, from Mid.Pers. naxust "the first," Parthian Mid.Pers.
nxwšt, from naxu, Manichean Parthian nwx
"beginning" + -ist superlative suffix, Av. -išta-,
cf. Skt. -istha-, Gk. -istos, O.H.G.
-isto, -osto, O.E. -st, -est, -ost; naxostin,
from naxost + suffix -in. |
first approximation nazdin-e naxost Fr.: première approximation 1) Generally, an expression to indicate that a comment or result is
only approximate. → first; → approximation. |
first collapse rombeš-e naxost Fr.: premier effondrement An early phase in the process of star formation which begins when the mass of a → molecular cloud → clump exceeds the → Jeans mass. The collapse is initially → optically thin to the thermal emission from → dust grains, and the compressional heating rate is much smaller than the cooling rate by the → thermal radiation. The collapse proceeds → isothermally. The isothermal condition is broken when the central density reaches about 10^{-13} g cm^{-3} and a small region at the center of the cloud starts to become → opaque. The heat generated by the collapse in this region is no longer freely radiated away, and the compression becomes approximately → adiabatic. The central temperature and pressure then begin to rise rapidly, soon becoming sufficient to decelerate and stop the collapse at the center. There then arises a small central core, called the → first core, in which the material has stopped collapsing and is approaching → hydrostatic equilibrium. Outside this core, the material is still nearly isothermal and continues to fall inward almost in → free fall. Consequently a shock front arises at the boundary of the core, where the infalling material is suddenly stopped. The initial mass and radius of the core are about 10^{31} g and 6 x 10^{13} cm, respectively, and the central density and temperature are about 2 x 10^{-10} g cm^{-3} and 170 K, respectively. As the collapse proceeds, the core grows in mass due to the infall of the surrounding material; at the same time, however, the core radius decreases because of radiative energy losses from the outer layers of the core. The process leads to the → second collapse (R. B. Larson, 1969, MNRAS 145, 271). |
first contact parmâs-e naxost Fr.: premier contact 1) The beginning of a → solar eclipse when the eastern part of
the lunar limb touches the western limb of the Sun, marking
the beginning of an eclipse. |
first core maqze-ye naxost Fr.: premier cœur A first object in → hydrostatic equilibrium predicted to form during early dynamical contraction of a → molecular cloud → clump in the course of the → first collapse. |
first degree equation hamugeš-e daraje-ye yekom Fr.: équiation du premier degré A equation in which the highest → exponent of the → variable is 1. Same as → linear equation. |
first derivative vâxane-ye naxost Fr.: dérivée première The simplest mathematical → differentiation of one quantity relative to another; df(x)/dx. It is the same as → derivative, contrasted with the → second derivative or the nth derivative. → first; → derivative. |
first derivative test âzmun-e vâxane-ye naxost Fr.: teste de la dérivée première In → calculus, a method for determining whether an → inflection point is a → local minimum, → local maximum, or neither. → first; → derivative; → test. |
first dredge-up borunkašid-e naxost Fr.: premier dragage The → dredge-up occurring after core hydrogen burning as the core contracts before helium burning ignites (on the ascending giant branch). The hydrogen envelope becomes convective and this convective zone penetrates deep into the core dredging up material that has been processed by the central nuclear reactions. As a result the abundances of helium and nitrogen are boosted. |
first law of thermodynamics qânun-e naxost-e garâtavânik Fr.: première loi de la thermodynamique The total energy of a → closed system is constant. This means that energy can be changed from one form to another, or transferred from one system to another, but it cannot be created or destroyed. A mathematical formulation of the first law is: δQ = δU + δW, where δQ is the heat transferred to the system, δU the change in internal energy (resulting in a rise or fall of temperature), and δW is the work done by the system. → first; → law; → thermodynamics. |
first light naxostin foruq, ~ nur Fr.: première lumière The first astronomical observation done with a major newly built telescope. |
First Point of Aries noqte-ye âqâz-e barré (#) Fr.: premier point de Bélier One of the two points on the celestial sphere where the ecliptic and the celestial equator crossed one another at → vernal equinox several thousands years ago. |
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