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Optics: A unit of luminance equal to one candle per square cm.
From Gk. stilbe "lamp."
To cause physical activity in something; e.g. → stimulated emission.
Verb from → stimulus.
Fr.: émission stimulée
The process by which an electron, which is already in an excited state (an upper energy level, in contrast to its lowest possible level or "ground state"), can "stimulate" a transition to a lower level, producing a second photon of the same energy. The quantum energy of the incoming photon should be equal to the energy difference between its present level and the lower level. This process forms the basis of both the → laser and → maser. Same as → induced emission.
stimulated star formation
diseš-e gavâlide-ye setâré
Fr.: formation stimulée d'étoiles
A process in which a star is not formed spontaneously but is provoked by the action of external forces, such as pressure and shock on a molecular cloud by close-by → massive stars, → supernova explosions, etc. See also → sequential star formation.
Something that incites or rouses to action; an incentive.
From L. stimulus "goad, spur;" cognate with Pers. tiz→ sharp.
Gavâz "goad, a stick with a pointed end, for driving cattle," Mid.Pers. *gawâz, lowned in Arm. gawazan "goad;" Av. gauuāza- "whip, stick for driving cattle," from gao- "cattle, cow" (→ Bootes) + āza-, from az- "to drive," azaiti "drives;" cf. Skt. aj- "to drive," ájati "drives;" Gk. agein "to lead, guide, drive;" L. agere "to do, set in motion," → act.
Fr.: approximation de Stirling
Named after James Stirling (1692-1770), a Scottish mathematician; → approximation.
Involving or containing a random variable or variables. A stochastic variable is neither completely determined nor completely random. A system containing one or more stochastic variables is probabilistically determined.
From Gk. stokhastikos "able to guess, conjecturing," from stokhazesthai "to aim at, guess," from stokhos "a guess, target," literally "pointed stake."
Kâturgin, from kâtur, kâturé, → random + -gin, adj. suffix, contraction of âgin "filled."
Fr.: excitation stochastique
The mechanism arising from turbulent convection in the → convective zone of stars, which is responsible for the driving of stellar → pulsation modes. In stars cooler than typically ~ 7 500 K (→ F-type stars and cooler), the stochastic excitation occurs in the convection envelope. In massive stars, it may develop either in the → convective core or in the convective layer beneath the → photosphere. Recent studies suggest that in → Be stars stochastic excitation takes place in the convective core. The stochastic waves can transport → angular momentum from the core to the surface. Fast rotation, as in Be stars, amplifies the stochastic excitation.
Fr.: processus stochastique
Any process involving a sequence of random variables. The future evolution of a stochastic process is therefore described by probability distributions.
stochastic self-propagating star formation
diseš-e setâregân bâ xod-tuceš-e kâturgin
Fr.: formation d'étoiles par auto-propagation stochastique
A mechanism that could be responsible for global → spiral structure in galaxies either by itself or in conjunction with spiral → density waves. In this mechanism, star formation is caused by → supernova-induced → shocks which compress the → interstellar medium. The → massive stars thus formed may, when they explode, induce further → star formation. If conditions are right, the process becomes self-propagating, resulting in agglomerations of young stars and hot gas which are stretched into spiral shaped features by → differential rotation. Merging of small agglomerations into larger ones may then produce large-scale spiral structure over the entire galaxy. The SSPSF model, first suggested by Mueller & Arnett (1976) was developed by Gerola & Seiden (1978). While the → density wave theory postulates that spiral structure is due to a global property of the galaxy, the SSPSF model examines the alternative viewpoint, namely that spiral structure may be induced by more local processes. The two mechanisms are not necessarily mutually exclusive, but they involve very different approaches to the modeling of galaxy evolution. The SSPSF gives a better fit than the density wave theory to the patchy spiral arms found in many spiral galaxies. However, it cannot explain → galactic bars.
1) The statement or discussion of the first principles of any science or art (1913 Webster).
1) Of or pertaining to → stoichiometry.
1) The branch of chemistry that studies chemical processes within the context of the laws of
and conservation of matter and energy.
From Gk. stoikheion "element, component, principle," Stoikheia
"elements" (the title of Euclid's great collection of Gk. mathematics);
loaned in Ar. and Pers. (9-th century A.D.) as ustuqus (
Stoyxiyosanji, from stoyxiyo loan from Gk., as above, + -sanji, → -metry.
After Sir George Gabriel Stokes (1819-1903), a British mathematician and physicist, who made important contributions to fluid dynamics, optics, and mathematical physics; → Stokes law; → Stokes parameter.
Stokes friction factor
karvand-e mâleš-e Stokes
Fr.: facteur de friction de Stokes
For the translational motion of a spherical body moving in a → viscous fluid, the proportionality factor between the uniform flow velocity far from the sphere and the drag force, provided no-slip boundary condition and small → Reynolds numbers: f = 6πηR, where η is the Reynolds number and R radius of the sphere.
qânun-e Stokes (#)
Fr.: loi de Stokes
1) Fluid mechanics: At low velocities, the frictional force on a
spherical body moving through a fluid at constant velocity is equal to
6πRηv, where R is the radius of the sphere,
η the fluid → viscosity, and v the velocity.
Fr.: paramètres de Stokes
Four parameters which are needed to fully describe the
→ polarization state of
→ electromagnetic radiation.
They involve the maximum and minimum intensity, the ellipticity,
and the direction of polarization.
The four Stokes parameters are traditionally defined as follows:
The hard nonmetallic mineral or group of consolidated minerals either in mass or in a fragment of pebble or larger size. See also → rock.
O.E. stan; cf. O.N. steinn, Dan. steen, O.H.G., Ger. Stein; from PIE *stai- "stone," also "to thicken, stiffen" (cf. Skt. styayate "curdles, becomes hard;" Av. stay- "heap;" Gk. stear "fat, tallow," stia, stion "pebble").
Sang "stone, rock;" Mid.Pers. sang; O.Pers. aθanga-; Av. asenga- "stone;" PIE *aken-.
asr-e sang (#)
Fr.: âge du fer
A prehistoric period during which the main material used to make tools and weapons was stone. The Stone Age is usually divided into three separate periods (Paleolithic Period, Mesolithic Period, and Neolithic Period) based on the degree of sophistication in the fashioning and use of tools. The Paleolithic time period is by far the longest, beginning some two million years ago and ending around 10,000 BC to coincide with the end of the last ice age (Pleistocene epoch).
Fr.: météorite pierreuse
A meteorite composed largely of rock-forming (→ silicate) → minerals. Stony meteorites are the most abundant kind, about 95%, of all meteorites. They are divided into two groups: → chondrites and → achondrites.