An Etymological Dictionary of Astronomy and AstrophysicsEnglish-French-Persian

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 9 Search : Jeans
 Jeans   جینز   JeansFr.: Jeans   Named after Sir James Hopwood Jeans (1877-1946), English mathematical physicist, astrophysicist, and popularizer of science. He made important contributions to theoretical astrophysics, especially to the theory of stellar formation. Jeans instability   ناپایداری ِ جینز   nâpâydâri-ye JeansFr.: instabilité de Jeans   An instability that occurs in a → self-gravitating  → interstellar cloud which is in → hydrostatic equilibrium. Density fluctuations caused by a perturbation may condense the material leading to the domination of gravitational force and the cloud collapse. The advent of instability involves a threshold called the → Jeans length or the → Jeans mass.→ Jeans; → instability. Jeans length   درازای ِ جینز   derâzâ-ye Jeans (#)Fr.: longueur de Jeans   The critical size of a homogeneous and isothermal interstellar cloud above which the cloud is unstable and must collapse under its own gravity. Below this size the cloud's internal pressure is sufficient to resist collapse. The Jeans length is defined by: λJ = (π cs2/Gρ)1/2 = 0.2 pc (T/10 K)1/2(nH2/104 cm-3)-1/2, where cs is the → sound speed, G is the → gravitational constant, ρ is the gas density, T is the gas temperature, and nH2 is the molecular hydrogen density.→ Jeans; → length. Jeans mass   جرم ِ جینز   jerm-e Jeans (#)Fr.: masse de Jeans   The → minimum mass for an → interstellar cloud below which the → thermal pressure of the gas prevents its → collapse under the force of its own → gravity. It is given by the formula MJ = (π5/2 / 6) G -3/2ρ0-1/2cs3, where G is the → gravitational constant, ρ0 the initial → density, and cs the isothermal → sound speed. It can be approximated to MJ ~ 45 (TK) 3/2 (ncm-3) -1/2 in units of solar masses, where TK is the temperature in → Kelvin, and ncm-3 the gas density per cm3. High density favors collapse, while high temperature favors larger Jeans mass. See also: → thermal Jeans mass, → turbulent Jeans mass.→ Jeans; → mass. Jeans scale   مرپل ِ جینز   marpel-e JeansFr.: échelle de Jeans   Same as → Jeans length.→ Jeans; → scale. Rayleigh-Jeans law   قانون ِ ریلی-جینز   qânun-e Rayleigh-Jeans(#)Fr.: loi de Rayleigh-Jeans   A classical law approximately describing the intensity of radiation emitted by a → blackbody. It states that this intensity is proportional to the temperature divided by the fourth power of the wavelength (8πkT/λ4). The Rayleigh-Jeans law is a good approximation to the experimentally verified Planck radiation formula only at long wavelengths. At short wavelengths it runs into a paradox named the → ultraviolet catastrophe.→ Rayleigh; → Jeans; → law. Rayleigh-Jeans spectrum   بیناب ِ ریلی-جینز   binâb-e Rayleigh-JeansFr.: spectre Rayleigh-Jeans   The part of → electromagnetic spectrum approximated by the → Rayleigh-Jeans law.→ Rayleigh; → Jeans; → spectrum. thermal Jeans mass   جرم ِ جینز ِ گرمایی   jerme-e Jeans-e garmâyiFr.: masse de Jeans thermique   The → Jeans mass when → turbulence is insignificant.→ thermal; → Jeans; → mass. turbulent Jeans mass   جرم ِ جینز ِ آشوبناک   jerm-e Jeans-e âšubnâkFr.: masse de Jeans turbulente   The characteristic mass for → cloud fragmentation in a → turbulent medium. While the standard → Jeans mass depends simply on the gas mean → density and → temperature, and fragmentation is purely gravitational, turbulent Jeans mass depends strongly also on the → Mach number (Chabrier et al. 2014, arXiv:1409.8466).→ turbulent; → Jeans; → mass.