<< < -sc Sag sam sat sca sca Sch sco sec sec sec sei sel sem sep set sha She sho sid sig SIM sim Sin ske sle smo soc sol sol sol Sol Som sou spa spa spe spe spe sph spi spl spu sta sta sta sta ste ste ste Sto str str sub sub sub suf sun sup sup sup sup sur syl syn sys > >>
scattering of stars parâkaneš-e setâregân Fr.: diffusion des étoiles The progressive increase of random motions of → disk stars with increasing stellar → ages. While some initial random motion seems likely in the disturbed conditions of disks when the oldest stars formed, the observation is generally attributed to scattering processes. Both massive gas → clumps and → spiral waves are considered as scattering agents (J. A. Sellwood & J. J. Binney, 2002, astro-ph/0203510 and references therein). → scattering; → star. |
Schönberg-Chandrasekhar limit hadd-e Schönberg-Chandrasekhar Fr.: limite de Schönberg-Chandrasekhar During the → main sequence stage, a star burns the hydrogen in its core and transforms it into helium. When the helium mass amounts to about 10% of the initial stellar mass, the star can no longer maintain the → hydrostatic equilibrium in its core; the star increases its volume and leaves the main sequence in order to become a → red giant. Named after the Brazilian astrophysicist Mario Schönberg (1914-1990) and Subramahmanyan Chandrasekhar, → Chandrasekhar limit, who were the first to point out this limit and derive it (1942, ApJ 96, 161). |
Scheat (β Peg) Asb-šâné Fr.: Scheat The second-brightest star in the constellation → Pegasus. It is a giant star of spectral type M2.5 II-III whose magnitude varies between 2.3 and 2.7. Scheat, from Ar. as-sâq "leg," erroneously taken from
the Ar. name of δ Aquarii as-sâq al-sâkib al-ma'
( Asb-šâné, literally "the Horse's Shoulder," from asb→ horse + šâné "shoulder" (Lori šona, Kurd. šân, Gilaki cân, con), maybe related to Skt. skandhá- "shoulder, trunk of tree, bulk" (Pali khandha-, Ashkun kándä, Bashkarih kân, Tôrwâldi kan "shoulder"), from skand- "to jump, leap, spring out," skandati "he jumps;" cf. L. scandere "to climb." |
Schechter function karyâ-ye Schechter Fr.: fonction de Schechter A mathematical expression that describes the → luminosity function of galaxies. The function correctly reflects the facts that the luminosity function decreases with increasing luminosity and that the decrease is particularly marked at high luminosities. It is expressed as: φ(L) = φ^{*}(L/L^{*})^{α} exp (-L/L^{*}), which has two parts and three parameters: φ^{*} is an empirically determined amplitude, α is an empirically derived exponent, and L^{*} is a characteristic luminosity which separates the low and high luminosity parts. For small luminosities (L much smaller than L^{*}) the Schechter function approaches a power law, while at high luminosities (L much larger than L^{*}) the frequency of galaxies drops exponentially. φ^{*}, L^{*}, and the faint-end slope α depend on the observed wavelength range, on the → redshift, and on the environment where the galaxies are observed. Named after the American astronomer Paul Schechter (1948-), who proposed the function in 1976 (ApJ 203, 297); → function. |
Schmidt law qânun-e Schmidt Fr.: loi de Schmidt A power-law relation between → star formation rate (SFR) and a corresponding measure of gas density. For external galaxies it is usually expressed in terms of the observable surface density of gas (Σ_{gas}): SFR ∝ Σ_{gas}^{n}. The exponent n is determined to be 1.4 ± 0.15 (Kennicutt 1998, ApJ 498, 541). The validity of the Schmidt law has been tested in dozens of empirical studies. The Schmidt law provides a tight parametrization of the global star formation law, extending over several orders of magnitude in SFR and gas density. Named after Maarten Schmidt (1929-), a dutch-born American astronomer, who also discovered the first → quasar (3C 273) in 1963. |
Schmidt telescope teleskop-e Schmidt, durbin-e ~ (#) Fr.: télescope de Schmidt A telescope with a spherical concave primary mirror in which the aberration produced by the spherical mirror is compensated for by a thin correcting lens placed at the opening of the telescope tube. Its very wide-field performance makes it suitable for surveys. Named after Bernhard Woldemar Schmidt (1879-1935), a German optician of Estonian origin, who invented the telescope in 1930; → telescope. |
Schmidt-Cassegrain telescope teleskop-e Schmidt-Cassegrain, durbin-e ~ (#) Fr.: télescope Schmidt-Cassegrain A mixture of the → Cassegrain telescope with a very short → focal length and of a Schmidt design (due to the presence of the → corrective plate), used mainly in → amateur astronomy. The main advantage of this telescope is its compact design. However, Schmidt-Cassegrain telescopes produce fainter images with less contrast than other telescope designs with similar → aperture sizes. This is due to the comparatively large → secondary mirror required to reflect the light back the → eyepiece. |
Schmidt-Kennicutt relation bâzâneš-e Schmidt-Kennicutt Fr.: relation Schmidt-Kennicutt Same as the → Schmidt law. Named after the American astrophysicists Maarten Schmidt (1929-), the pioneer of research in this field, and Robert C. Kennicutt, Jr. (1951-), who developed the study; → relation. |
scholar dânešpažuh, dânešvar (#) Fr.: 1) lettré, érudit; 2) boursier 1) A learned or erudite person, especially one who has profound knowledge of a
particular subject. → scientist.
M.E. scoler(e); O.E. scolere "student," from M.L. scholaris, from L.L. scholaris "of a school," from L. schola, from Gk. skhole "school, lecture, discussion; leisure, spare time." Dânešpažuh, from dâneš→ science + pažuh agent noun of pažuhidan "to search," → research. Dânešvar, from dâneš, as befor, + -var possession suffix. |
Schottky barrier varqe-ye Schottky Fr.: barrière de Schottky A junction between a metal and a semiconductor, which exhibits rectifying characteristics. A Schottky barrier has a very fast switching action and low forward voltage drop of about 0.3 volts, compared with 0.6 volts in silicon diodes, which use adjacent p-type and n-type semiconductors. Named after Walter Hans Schottky (1886-1976), German physicist, who described the phenomenon; → barrier. |
Schottky defect âk-e Schottky Fr.: défaut de Schottky An unoccupied position in a crystal lattice which forms when oppositely charged ions leave their lattice sites, creating vacancies. Named after Walter Hans Schottky (1886-1976), German physicist; → defect. |
Schottky diode diod-e Schottky (#) Fr.: diode Schottky A → semiconductor diode containing a → Schottky barrier. Such a diode has a low forward voltage drop and very fast switching characteristics. Also called Schottky barrier diode and hot electron diode. → Schottky barrier; → diode. |
Schottky noise nufe-ye Schottky Fr.: bruit de Schottky Excess voltage generated by random fluctuations in the emission of electrons from a hot cathode, causing a hissing or sputtering sound (shot noise) in an audio amplifier and causing snow on a television screen. Same as → shot effect, → shot noise. Named after Walter Hans Schottky (1886-1976), German physicist; → noise. |
Schrödinger equation hamugeš-e Schrödinger Fr.: équation de Schrödinger A fundamental equation of physics in → quantum mechanics the solution of which gives the → wave function, that is a mathematical expression that contains all the information known about a particle. This → partial differential equation describes also how the wave function of a physical system evolves over time. Named after Erwin Schrödinger (1887-1961), the Austrian theoretical physicist, Nobel Prize 1933, who first developed the version of quantum mechanics known as → wave mechanics; → equation. |
Schrödinger's cat gorbe-ye Schrödinger (#) Fr.: chat de Schrödinger A → thought experiment intended to illustrate the → superposition principle in → quantum mechanics. A cat is put in a steel box which is separated from the outside world. The box also contains a vial of lethal acid, a tiny amount of a radioactive substance, a → Geiger counter, and a hammer. If an atom decays and the Geiger counter detects an → alpha particle, the hammer breaks the vial which kills the cat. According to Schrödinger, as long as the box stays closed the cat's fate is tied to the → wave function of the atom, which is itself in a superposition of decayed and un-decayed states. Thus the cat must itself be in a superposition of dead and alive states before the observer opens the box, "observes" the cat, and "collapses" its wave function. However, Schrödinger's argument fails because it rests on the assumption that macroscopic objects can remain unobserved in a superposition state. When an atom decays, its wave function becomes entangled with the enormously complex wave function of the macroscopic Geiger counter. The atom is therefore "observed" by the Geiger counter. Since a Geiger counter cannot, for all practical purposes, be isolated from the rest of the world, the rest of the world observes the atom, and the cat is either dead or alive. → collapse of the wave function. Named after Erwin Schrödinger (1887-1961), → Schrödinger equation, who proposed the thought experiment in 1935 in order to illustrate the inconsistency of the Copenhagen interpretation of quantum mechanics; cat, from M.E. cat, catte; O.E. catt, catte (cf. O.Fris, M.D. katte, O.H.G. kazza, Ir. cat, Welsh cath), probably from L.L. cattus, catta "cat." Gorbé, from Mid.Pers. gurbag "cat;" → Schrodinger equation, |
Schröter's effect oskar-e Schröter Fr.: effet de Schröter A phenomenon in which the observed and predicted phases of Venus do not coincide. At eastern elongation, when the planet is visible in the evening sky, dichotomy (half-phase) usually comes a day or two earlier than predicted, while at western elongation dichotomy occurs a day or two later. Named after Johan Schröter (1745-1816), German astronomer, who first described the effect in 1793; → effect. |
Schrodinger equation hamugeš-e Schrödinger Fr.: équation de Schrödinger A fundamental equation of physics in → quantum mechanics the solution of which gives the → wave function, that is a mathematical expression that contains all the information known about a particle. This → partial differential equation describes also how the wave function of a physical system evolves over time. Named after Erwin Schrödinger (1887-1961), the Austrian theoretical physicist, Nobel Prize 1933, who first developed the version of quantum mechanics known as → wave mechanics; → equation. |
Schrodinger's cat gorbe-ye Schrödinger (#) Fr.: chat de Schrödinger A → thought experiment intended to illustrate the → superposition principle in → quantum mechanics. A cat is put in a steel box which is separated from the outside world. The box also contains a vial of lethal acid, a tiny amount of a radioactive substance, a → Geiger counter, and a hammer. If an atom decays and the Geiger counter detects an → alpha particle, the hammer breaks the vial which kills the cat. According to Schrödinger, as long as the box stays closed the cat's fate is tied to the → wave function of the atom, which is itself in a superposition of decayed and un-decayed states. Thus the cat must itself be in a superposition of dead and alive states before the observer opens the box, "observes" the cat, and "collapses" its wave function. However, Schrödinger's argument fails because it rests on the assumption that macroscopic objects can remain unobserved in a superposition state. When an atom decays, its wave function becomes entangled with the enormously complex wave function of the macroscopic Geiger counter. The atom is therefore "observed" by the Geiger counter. Since a Geiger counter cannot, for all practical purposes, be isolated from the rest of the world, the rest of the world observes the atom, and the cat is either dead or alive. → collapse of the wave function. Named after Erwin Schrödinger (1887-1961), → Schrodinger equation, who proposed the thought experiment in 1935 in order to illustrate the inconsistency of the Copenhagen interpretation of quantum mechanics; cat, from M.E. cat, catte; O.E. catt, catte (cf. O.Fris, M.D. katte, O.H.G. kazza, Ir. cat, Welsh cath), probably from L.L. cattus, catta "cat." Gorbé, from Mid.Pers. gurbag "cat;" → Schrodinger equation, |
Schroter's effect oskar-e Schröter Fr.: effet de Schröter A phenomenon in which the observed and predicted phases of Venus do not coincide. At eastern elongation, when the planet is visible in the evening sky, dichotomy (half-phase) usually comes a day or two earlier than predicted, while at western elongation dichotomy occurs a day or two later. Named after Johan Schröter (1745-1816), German astronomer, who first described the effect in 1793; → effect. |
Schwarzschild barrier varqe-ye Schwarzschild Fr.: barrière de Schwarzschild An upper theoretical limit to the → eccentricity of orbits near a → supermassive black hole (SBH). It results from the impact of → relativistic precession on the stellar orbits. This phenomenon acts in such a way as to "repel" inspiralling bodies from the eccentric orbits that would otherwise lead to capture as → extreme mass ratio inspiral (EMRI)s. In other words, the presence of the Schwarzschild barrier reduces the frequency of EMRI events, in contrast to that predicted from → resonant relaxation. Resonant relaxation relies on the orbits having commensurate radial and azimuthal frequencies, so they remain in fixed planes over multiple orbits. In the strong-field potential of a massive object, orbits are no longer Keplerian but undergo significant perihelion precession. Resonant relaxation is only efficient in the regime where precession is negligible. The Schwarzschild barrier refers to the boundary between orbits with and without significant precession. Inside this point resonant relaxation is strongly quenched, potentially reducing inspiral rates. |
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