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Bohr magneton magneton-e Bohr (#) Fr.: magnéton de Bohr A fundamental constant, first calculated by Bohr, for the intrinsic → spin magnetic moment of the electron. It is given by: μ_{B} = eħ/2m_{e} = 9.27 x 10^{-24} joule/tesla = 5.79 x 10^{-5} eV/tesla, representing the minimum amount of magnetism which can be caused by the revolution of an electron around an atomic nucleus. It serves as a unit for measuring the magnetic moments of atomic particles. |
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. |
bolometric correction aršâyeš-e tafsanji, ~ tafsanjik Fr.: correction bolométrique The difference between the → visual magnitude and → bolometric magnitude. → bolometric; → correction. |
Boltzmann constant pâyâ-ye Boltzmann Fr.: constante de Boltzmann |
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 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" (see below); 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," also present stem of bastan "to bind, shut," → shutter. |
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. |
Bondi-Hoyle accretion radius šo'â'-e farbâl-e Bondi-Hoyle Fr.: rayon de l'accrétion de Bondi-Hoyle In the → Bondi-Hoyle accretion process, the radius where the gravitational energy owing to star is larger than the kinetic energy and, therefore, at which material is bound to star. The Bondi-Hoyle accretion radius is given by R_{BH} = 2 GM / (v^{2} + c_{s}^{2}) where G is the gravitational constant, M is the stellar mass, v the gas/star relative velocity, and c_{s} is the sound speed. → Bondi-Hoyle accretion; → radius. |
Bonner Durchmusterung (BD) Bonner Durchmusterung Fr.: Bonner Durchmusterung A catalog of 324,188 stars in the → declination zones +89 to -01 degrees. The goal of the survey was to obtain a → position and estimated → visual magnitude for every star visible with the 78 mm → refracting telescope at Bonn. Actual → magnitude estimates were made and reported to 0.1 mag for all stars down to 9.5 mag. Positions are given to the nearest 0.1 sec in → right ascension and 0.1 arcmin in declination. The survey was carried out by Friedrich W. Argelander (1799-1875) and his assistants in the years 1852-1861. The Ger. name means Bonn Survey. |
Bonnor-Ebert mass jerm-e Bonnor-Ebert Fr.: masse de Bonnor-Ebert The largest gravitationally stable mass of the → Bonnor-Ebert sphere. After W.B. Bonnor (1956) and R. Ebert (1955); → mass. |
Bonnor-Ebert sphere epehr-e Bonnor-Ebert, kore-ye ~ Fr.: sphère de Bonnor-Ebert A sphere of interstellar gas at uniform temperature in equilibrium under its own gravitation and an external pressure. The pressure of a hotter surrounding medium causes the sphere to collapse. → Bonnor-Ebert mass. → Bonnor-Ebert mass; → sphere. |
boron bor (#) Fr.: bore A soft, brown, nonmetallic chemical element; symbol B. → Atomic number 5; → atomic weight 10.81; → melting point about 2,300°C; → specific gravity 2.3 at 25°C; → valence +3. Boron occurs as borax and boric acid. It is used for hardening steel and for producing enamels and glasses. Since it absorbs slow neutrons, it is used in steel alloys for making control rods in nuclear reactors. Boron was separated in 1808 by Joseph Louis Gay Lussac (1778-1850) and Louis Jacques Thénard (1777-1857) and independently by Sir Humphry Davy (1778-1829). From bor(ax), from M.Fr. boras, from M.L. borax, from Ar. buraq, from Pers. burah "borax, nitre, used in soldering gold" + (car)bon. Bor, loan from Fr., as above. |
Bose-Einstein condensate (BEC) cagâlâk-e Bose-Einstein Fr.: condensat de Bose-Einstein A state of matter in which a group of atoms or subatomic particles,
cooled to within → absolute zero,
coalesce into a single quantum mechanical entity
that can be described by a → wave function.
When a group of atoms are cooled down to very near
absolute zero, the atoms hardly move relative to each other, because
they have almost no free energy
to do so. Hence the atoms clump together and enter
the same → ground energy states.
They become identical and the whole group starts behaving as though it were a
single atom. A Bose-Einstein condensate results from a
→ quantum transition phase
called the → Bose-Einstein condensation.
This form of matter was predicted in 1924 by Albert Einstein on
the basis of the quantum formulations of the Indian physicist
Satyendra Nath Bose. → boson; → Einstein; → condensate. |
Bose-Einstein condensation (BEC) cagâleš-e Bose-Einstein Fr.: condensation de Bose-Einstein A → quantum phase transition during which the → bosons constituting a sufficiently cooled boson gas are all clustered in the → ground energy state. The phase transition results in a → Bose-Einstein condensate. This phenomenon occurs when the temperature becomes smaller than a critical value given by: T_{c} = (2πħ^{2} / km)(n / 2.612)^{2/3}, where m is mass of each boson, ħ is the → reduced Planck's constant, k is → Boltzmann's constant, and n is the particle number density. When T ≤ T_{c}, the → de Broglie wavelength of bosons becomes comparable to the distance between bosons. → boson; → Einstein; → condensation. |
Bose-Einstein distribution vâbâžeš-e Bose-Einstein Fr.: distribution de Bose-Einstein For a → population of independent → bosons, a function that specifies the number of particles in each of the allowed → energy states. → boson; → Einstein; → distribution. |
boson boson (#) Fr.: boson Any of a class of particles (such as the → photon, → pion, or → alpha particle) that have zero or integral → spin and do not obey the → Pauli exclusion principle. The energy distribution of bosons is described by → Bose-Einstein statistics. See also: → gauge boson, → Higgs boson, → W boson, → Z boson, → intermediate boson. Boson, in honor of the Indian-American physicist Satyendra Nath Bose (1894-1974). |
bottom-up structure formation diseš-e sâxtâr az pâyin bé
bâlâ Fr.: formation des structures du bas vers le haut A → structure formation scenario in which small galaxies form first, and larger structures are then formed in due course. Contrary to → top-down structure formation. |
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