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Bohr's postulate farâvas-e Bohr Fr.: postulat de Bohr One of the three postulates advanced in the → Bohr model which led to the correct prediction of the observed line spectrum of hydrogen atom. See also → Bohr's first postulate, → Bohr's second postulate, → Bohr's third postulate, |
Bohr's second postulate farâvas-e dovom-e Bohr Fr.: deuxième postulat de Bohr One of the postulates used in the → Bohr model, whereby when an atom is in the steady state an electron travelling in a circular orbit should have → quantized values of the → angular momentum which comply with the condition p = n(h/2π), where p is the angular momentum of the electron, h is → Planck's constant, and n is a positive integer called → quantum number. |
Bohr's third postulate farâvas-e sevom-e Bohr Fr.: troisième postulat de Bohr One of the postulates used in the → Bohr model, whereby the atom emits (absorbs) a quantum of electromagnetic energy (→ photon) when the electron passes from an orbit with a greater (lesser) n value to one with a lesser (greater) value. The energy of the quantum is equal to the difference between the energies of the electron on its orbits before and after the transition or "jump": hν = ε_{1} - ε_{2}, where h is the → Planck's constant and ν the frequency of the transition. |
boiling point noqte-ye juš (#) Fr.: point d'ébullition The temperature at which a liquid changes to a gas (vapor) at normal atmospheric pressure. In other words, the temperature at which the vapor pressure of a liquid is equal to the external pressure. M.E. boillen; O.Fr. boillir, from L. bullire "to bubble, seethe," from bulla "a bubble, knob;" → point. Noqté, → point; juš "boiling," present stem of jušidan "to boil;" Khotanese jis- "to boil;" Av. yaēšiiant- "boiling;" cf. Skt. yas- "to boil, become hot," yasyati "boils, seethes;" Gk. zein "to bubble, boil, cook;" O.H.G. jesan "to ferment, foam;" Ger. Gischt "foam, froth," gären "to ferment;" O.E. gist; E. yeast. |
Bok globule guyce-ye Bok Fr.: globule de Bok A small, roughly spherical cloud of → interstellar dust and gas that appears as a dark compact globule when viewed against the background of an → H II region. Bok globules range in mass from about 1 to 1,000 or more → solar masses, and in size from about 10,000 → astronomical units to 3 → light-years. They typically have temperatures of around 10 → Kelvin. Bok globules are thought to represent a stage in the collapse of a dense fragment of → molecular clouds that are in the process of forming new stars. → elephant trunk. In honor of Bart Jan Bok (1906-1983), the Dutch-American astronomer, who first observed these objects. In 1947, in collaboration with Edith F. Reilly, he put forward the hypothesis that these globules were undergoing → gravitational collapse to form new stars (Bok & Reilly, 1947, ApJ 105, 255); → globule. |
bolide garzin Fr.: bolide A → meteor which is extremely bright, particularly one that breaks up during its passage through the → atmosphere. Also called → fireball. Bolide, Fr., from L. bolis, bolidis, from Gk. bolis, bolidos "missile, flash of lightning," from ballein "to throw;" PIE *g^{w}elH_{1}- "to throw;" → ballistics. Garzin "arrow;" cf. Tâleši ger "meteor" (from Proto-Iranian *garH- "to throw"), cognate with Gk. ballein, as above; → ballistics. |
bolometer tâvsanj Fr.: bolomètre 1) An instrument for measuring the intensity of radiant energy
in amounts as small as one millionth of an erg.
It uses the change in resistance of a thin conductor caused by
the heating effect of the radiation.
→ actinometer, → photometer, →
pyrheliometer, → pyrometer,
radiometer. From Gk. bole "stroke, beam of light," from ballein "to throw" + middle suffix -o- + → -meter.. Tâvsanj, from tâv "light, brightness, heat, warmth" (from tâbidan "to radiate") + sanj, → -meter. |
bolometric tâvsanji, tâvsanjik Fr.: bolométrique Of or relating to or measured by a → bolometer. |
bolometric correction aršâyeš-e tâvsanji, ~ tâvsanjik Fr.: correction bolométrique The difference between the → visual magnitude and → bolometric magnitude. → bolometric; → correction. |
bolometric luminosity tâbandegi-ye tâvsanji, ~ tâvsanjik Fr.: luminosité bolométrique The total rate of energy output of an object integrated over all wavelengths. → bolometric; → luminosity. |
bolometric magnitude borz-e tâvsanji, ~ tâvsanjik Fr.: magnitude bolométrique The magnitude of an astronomical object for the entire range of its electromagnetic spectrum. → bolometric; → magnitude. |
Boltzmann constant pâyâ-ye Boltzmann Fr.: constante de Boltzmann |
Boltzmann factor karvand-e Boltzmannn Fr.: facteur de Boltzmann The factor e^{-E/kT} involved in the probability for atoms having an excitation energy E and temperature T, where k is Boltzmann's constant. → Boltzmann's constant; → factor. |
Boltzmann's constant pâyâ-ye Boltzmann Fr.: constante de Boltzmann The physical constant, noted by k, relating the mean → kinetic energy of → molecules in an → ideal gas to their → absolute temperature. It is given by the ratio of the → gas constant to → Avogadro's number. Its value is about 1.380 x 10^{-16}erg K^{-1}. Named after the Austrian physicist Ludwig Boltzmann (1844-1906), who made important contributions to the theory of statistical mechanics; → constant. |
Boltzmann's entropy formula disul-e dargâšt-e Boltzmann Fr.: formule d'entropie de Boltzmann In → statistical thermodynamics, a probability equation relating the → entropy S of an → ideal gas to the quantity Ω, which is the number of → microstates corresponding to a given → macrostate: S = k. ln Ω. Same as → Boltzmann's relation. → Boltzmann's constant; → entropy; → formula. |
Boltzmann's equation hamugeš-e Boltzmann Fr.: équation de Boltzmann 1) An equation that expresses the relative number (per unit volume) of → excited atoms in different states as a function of the temperature for a gas in → thermal equilibrium: N_{u}/N_{l} = (g_{u}/g_{l}) exp (-ΔE/kT_{ex}), where N_{u} and N_{l} are the upper level and lower level populations respectively, g_{u} and g_{l} the → statistical weights, ΔE = hν the energy difference between the states, k is → Boltzmann's constant, and h → Planck's constant. → Boltzmann's constant; → equation. |
Boltzmann's relation bâzâneš-e Boltzmann Fr.: relation de Boltzmann A relation between the → entropy of a given → state of a → thermodynamic system and the → probability of the state: S = k . ln Ω where S is the entropy of the system, k is → Boltzmann's constant, and Ω the thermodynamic probability of the state. Boltzmann's relation connects → statistical mechanics and → thermodynamics. Ω is the number of possible → microstates of the system, and it represents the → randomness of the system. The relation also describes the statistical meaning of the → second law of thermodynamics. This expression has been carved above Boltzmann's name on his tombstone in Zentralfreihof in Vienna. Same as → Boltzmann's entropy formula. → Boltzmann's constant; → relation. |
bond band (#) Fr.: lien The attractive force that holds together neighboring atoms in molecules. Bond, variant of band, from M.E. bende, O.E. bend, from O.Fr. bande, bende, PIE *bendh- "to bind" (cf. Goth bandi "that which binds;" Av./O.Pers. band- "to bind, fetter," banda- "band, tie," Skt. bandh- "to bind, tie, fasten," bandhah "a tying, bandage"). Band "band, tie," from Mid.Pers., O.Pers./Av. band- "to bind," banda- "band, tie." |
Bond albedo sepidâ-ye Bond Fr.: albedo de Bond The fraction of the total amount of electromagnetic radiation falling upon a non-luminous spherical body that is reflected in all directions by that body. The bond albedo takes into account all wavelengths at all → phase angles. Compare with → geometric albedo. Named after the American astronomer George Phillips Bond (1825-1865), who proposed it; → albedo. |
Bondi-Hoyle accretion farbâl-e Bondi-Hoyle Fr.: accrétion de Bondi-Hoyle The → accretion of mass by a star (assumed as point particle) moving at a steady speed through an infinite, uniform gas cloud. It is directly proportional to the star mass (M) and the medium density (ρ) and inversely proportional to the relative star/gas velocity (v). In its classical expression: 4πρ(G M)^{2} / v^{3}, where G is the → gravitational constant. See Bondi & Hoyle (1944, MNRAS 104, 273) and Bondi (1952, MNRAS 112, 195). For a recent treatment of accretion in a turbulent medium see Krumholtz et al. 2006 (ApJ 638, 369). Named after Hermann Bondi (1919-2005), an Anglo-Austrian mathematician and cosmologist and Fred Hoyle (1915-2001), British mathematician and astronomer best known as the foremost proponent and defender of the steady-state theory of the universe; → accretion. |
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