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second approximation nazdineš-e dovom Fr.: deuxième approximation Math: In calculus, limiting an equation to its → second derivative, for example: e^{x}≅ 1 + x + x^{2}/2. Also called linear approximation. → first approximation. → second; → approximation. |
second collapse rombeš-e dovom Fr.: deuxième effondrement An early evolutionary period in the process of star formation which succeeds the → first collapse. When the mass of the → first core has increased by about a factor 2 and the radius has decreased by a similar factor, the central temperature of the core reaches about 2000 K. At this point the → molecular hydrogen begins to dissociate into atoms. This reduces the → adiabatic index (γ) below the critical value 4/3, with the result that the material at the center of the core becomes unstable and begins to collapse. Most of the gravitational energy generated by this collapse goes into the → dissociation of H_{2} molecules, so that the temperature rises only slowly with increasing density. In this second collapse phase, as in the first, the density distribution in the collapsing region becomes more and more sharply peaked at center, and the time scale becomes shorter and shorter with increasing central density. The central collapse of the core continues until the hydrogen molecules are nearly all dissociated and γ again rises above 4/3. The central pressure then rises rapidly and once again becomes sufficient to decelerate and stop the collapse at the center. A small core in the → hydrostatic equilibrium then arises, bounded by a shock front in which the surrounding infalling material is suddenly stopped. The initial mass and radius of the second core are about 3 x 10^{30} g (1.5 x 10^{-3}M_{sun}) and 9 x 10^{10} cm (1.3 R_{sun}) respectively, and the central density and temperature are about 2 x 10^{-2} g cm^{-3} and 2 x 10^{4} K, respectively. The second core will evolve into a → young stellar object (R. B. Larson, 1969, MNRAS 145, 271). |
second contact parmâs-e dovom Fr.: deuxième contact The beginning of the total phase of a solar eclipse when the leading edge of the Moon touches the eastern edge of the Sun completely obscuring the Sun. |
second core maqze-ye dovom Fr.: deuxième cœur A hydrostatic object predicted to result from the → second collapse of a → molecular cloud in an early stage of star formation. |
second derivative vâxane-ye dovom Fr.: dérivée seconde In → calculus, the → derivative of a → first derivative. It is usually written as f''(x), d^{2}y/d^{2}x, or y''. → second; → derivative. |
second derivative test âzmun-e vâxane-ye dovom Fr.: test de la dérivée seconde A method, used in → calculus, for determining whether a given → stationary point of a → function is a → local minimum or → local maximum. → second; → derivative; → test. |
second dredge-up borunkašid-e dovom Fr.: deuxième dragage A → dredge-up process that occurs after core helium burning, in which the convective envelope penetrates much more deeply, pushing hydrogen burning shell into close proximity with the helium burning shell (→ first dredge-up). This arrangement is unstable and leads to burning pulses. The reason is that the hydrogen shell burns out until there is enough helium for the helium combustion to occur and all the helium is rapidly burnt. Afterward the hydrogen shell again burns outward and the process repeats. |
second generation star setâre-ye âzâneš-e dovom Fr.: étoile de deuxième génération A star whose formation is induced by an older star itself formed previously in the same region. See also → stimulated star formation, → sequential star formation, → triggered star formation. → second; → generation; → star. |
second law of black-hole mechanics qânun-e dovom-e mekânik-e siyah-câl Fr.: deuxième loi de la mécanique des trous noirs The surface area of a black hole's horizon can never decrease. → second; → law; → black hole; → mechanics. |
second law of thermodynamics qânun-e dovom-e garmâtavânik Fr.: deuxième loi de la thermodynamique 1) Heat cannot be transferred from a colder to a hotter body without some other effect, i.e.
without → work being done. Expressed in terms of
→ entropy: the entropy of an
→ isolated system tends toward a maximum and its
available energy tends toward a minimum. → second; → law; → thermodynamics. |
second quantization kuântomeš-e dovom Fr.: deuxième quantification In quantum mechanics, the quantization of the field that replaces potential in Newtonian mechanics, whereby the field variables become operators from which the creation (of particle) operators and destruction operators can be constructed. → second; → quantization. |
second-order logic guyik-e râye-ye dovom Fr.: logique du seconde ordre An n extension of → first-order logic that quantifies not only → variables that range over → individuals, but also quantifies over → relations. |
secondary dovomân Fr.: secondaire 1) Derived or derivative; not primary or original. From → second + -ary a suffix occurring on adjectives (elementary; honorary; stationary) and nouns denoting objects, especially receptacles or places (library; rosary; glossary). Dovomân, from dovom, → second. |
secondary atmosphere javv-e dovomân, havâsepehr-e ~ Fr.: atmosphère secondaire An atmosphere of a planet that forms after primordial gases had been lost or had failed to accumulate. A secondary atmosphere develops from internal volcanic outgassing, or by accumulation of material from comet impacts. It is characteristic of terrestrial planets, such as Earth, Mercury, Venus, and Mars. → primordial atmosphere. → secondary; → atmosphere. |
secondary body jesm-e dovomân Fr.: corps secondaire A body that revolves around a more massive body
under the → gravitational attraction of the latter
is called the → primary body. |
secondary calibrator kabizande-ye dovomân Fr.: calibrateur secondaire An indicator of extragalactic distances that relies on → primary calibrators in our Galaxy. Secondary calibrators of the distance scale depend on statistical measures of the properties of a class of objects, such as the brightness of H II regions, globular clusters, red and blue stars, or the neutral hydrogen 21-cm line width or velocity dispersion (of spiral galaxies), etc. Same as secondary distance indicator. → secondary; → calibrator. |
secondary cell pil-e dovomân Fr.: An electric cell that can be charged by passing a current through it in reverse direction to its discharge. Same as → accumulator. See also → primary cell. |
secondary cosmic rays partowhâ-ye keyhâni-ye dovomân Fr.: rayons cosmiques secondaires A burst of secondary charged and neutral particles arising when → primary cosmic rays collide with the atmospheric oxygen or nitrogen nuclei in the upper atmosphere. The collision produces mostly → pions (π), along with some → kaons (K), → antiprotons, and → antineutrons. Neutral pions very quickly decay, usually into two → gamma rays. Charged pions also decay but after a longer time. Therefore, some of the pions may collide with yet another nucleus of the air before decaying, which would be into a → muon and a → neutrino. The fragments of the incoming nucleus also interact again, also producing new particles. |
secondary crater lâvak-e dovomân, kandâl-e ~ Fr.: cratère secondaire A crater formed by the relatively low-velocity impact of fragments ejected from a large primary crater. Secondary craters tend to cluster in a ring around the primary crater. |
secondary eclipse gereft-e dovomân Fr.: éclipse secondaire Of a transiting → exoplanet, the event and the interval of time during which the planet passes behind its host star. → primary eclipse. |
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