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plerion plerion Fr.: plérion A → supernova remnant which has a filled center rather than being a shell. The internal region is "filled" by energetic particles streaming from a rotating → pulsar. The → Crab Nebula is the archetypal plerion. Plerion, from Gk. pleres "full," akin to Pers. por "full," → poly-. |
Pogson's relation bâzâneš-e Pogson Fr.: relation de Pogson The equation that expresses the → magnitude
→ difference between
two objects in terms of the → logarithm of the
→ flux → ratio: Named after Norman Robert Pogson (1829-1891), the English astronomer, who introduced the magnitude scale in 1856; → relation. |
Poinsot's motion jonbeš-e Poinsot Fr.: mouvement à la Poinsot The motion of a torque free rotating rigid body in space, in general whose angular velocity vector precesses regularly about the constant angular momentum factor. After Louis Poinsot (1777-1859), French physicist and mathematician. He was the inventor of geometrical mechanics, showing how a system of forces acting on a rigid body could be resolved into a single force and a couple. |
point spread function (PSF) karyâ-ye gostareš-e noqté, ~ ~ pandé Fr.: fonction d'étalement du point The two-dimensional intensity distribution about the image of a point source. |
Poisson distribution vâbâžeš-e Poisson Fr.: distribution de Poisson A → probability function that characterizes → discrete → random events occurring independently of one another within some definite time or space. It may be regarded as an approximation of the → binomial distribution when the number of events becomes large and the probability of success becomes small. The Poisson distribution is expressed by: f(x) = (λ^{x}e^{-λ})/x!, where λ is the mean number of successes in the interval, e is the base of the → natural logarithm, and x is the number of successes we are interested in. Named after Siméon Denis Poisson (1781-1840), French mathematician, who developed the application of Fourier series to physical problems and made major contributions to the theory of probability and to the calculus of variations; → distribution. |
Poisson's equation hamugeš-e Poisson Fr.: équation de Poisson An equation (∇^{2}φ = 4πGρ) which relates the gravitational (or electromagnetic) potential to the mass density (or charge density). → Poisson distribution; → equation. |
polar equation hamugeš-e qotbi Fr.: équation polaire An equation for a curve written in terms of the → polar coordinates. |
polar motion jonbeš-e qotbi Fr.: mouvement du pÃ´le The irregularly varying motion of the Earth's pole of rotation with respect to the Earth's crust. |
polarization qotbeš (#) Fr.: polarisation 1) Optics: A process or state in which the directions of the electric or magnetic fields
of an → electromagnetic radiation
change in a regular pattern. Light can be polarized by a
variety of ways, involving the following processes: reflection, transmission,
double refraction, and scattering. See also
→ unpolarized light;
→ linear polarization;
→ circular polarization;
→ elliptical polarization.
The study of the polarization of light from astronomical sources can yield
unique information in particular related to the properties of magnetic fields. Verbal noun of → polarize. |
polarization angle zâviye-ye qotbeš (#) Fr.: angle de polarisation Same as → polarizing angle and → Brewster angle. → polarization; → angle. |
polarization charge bâr-e qotbeš Fr.: Same as → bound charge. → polarization; → charge. |
polarization degree daraje-ye qotbeš (#) Fr.: degrÃ© de polarisation → polarization; → degree. |
polarization fraction barxe-ye qotbeš Fr.: fraction de polarisation The ratio expressed by P = (I_{par} - I_{per}) / (I_{par} + I_{per}), where I_{par} and I_{per} are the light intensities with the electric field vector respectively parallel and perpendicular to the incident beam. → polarization; → fraction. |
pollution âludegi (#) Fr.: pollution The introduction of harmful substances or light into the natural environment
as a consequence of human activities. → light pollution. M.E., from O.Fr., from L.L. pollutionem "defilement," from L. polluere "to soil, defile," from pol-, variant of por- "forth, forward, before" + -luere "smear," related to lutum "mud," and to lues "filth; plague, pestilence;;" cf. Gk. luma "filth, dirt, disgrace;" O.Ir. loth "mud, dirt;" Lith. lutynas "pool, puddle;" Pers. âludan, as below. &ACIRC;ludegi, from âludan, âlây-"to pollute, soil, stain;" Mid.Pers. âlutan; from prefixed Proto-Iranian *ā-rūta-, from rav- "to stain, soil;" Mid.Pers. Manichean Parthian rwd "rascal;" PIE base *leu- "dirty; to soil;" cf. L. lues, as above. |
polynomial equation hamugeš-e bolnâmin Fr.: équation polynomiale An equation of the form a_{0} + a_{1}x + a_{2}x^{2} + ... + a_{n}x^{n}, where a_{0} ... a_{n} are → real numbers and a_{n}≠ 0. Same as → algebraic equation. → polynomial; → equation. |
popularization mardom-pasandâneš, mardomâneš Fr.: popularisation The act of popularizing. → popularize; → -tion. |
population porineš Fr.: population Statistics: Any finite or infinite set of individuals, items, or data subject to a statistical study. → disk population; → halo population; → population inversion; → Bose-Einstein distribution. Verbal noun of populate, from M.L. populatus, p.p. of populare "to inhabit," from L. populus "people." In the IE languages the concepts of "full, many, multitude" and "people, group, herd, flock" are related. In Pers. several variants of por "full, much, many" denote "group, population," as in Lori, Qâyeni bor "group, tribe, herd," Torbat-Heydariyeyi, Qomi borr "heap, bundle, group," Qomi borreh "group, assemblage of people," Pashtu parrak "flock, herd," Urdu para "flock, herd," Lârestâni baila "group, tribe," Tabari balik "herd, flock;" other examples from literary Pers. bâré "herd, flock," parré "a rank or file of soldiers, a circular disposition of troops." Therefore, porineš "population," verbal noun of porinidan "to populate," infinitive of porin "populous," from por "mutitude, many, full" + -in attribution suffix. Por, from Mid.Pers. purr "full;" O.Pers. paru- "much, many;" Av. par- "to fill," parav-, pauru-, pouru- "full, much, many;" PIE base *pelu- "full," from *pel- "to be full;" cf. Skt. puru- "much, abundant;" Gk. polus "many," plethos "great number, multitude;" O.E. full. |
Population I star setÃ¢re-ye-e porineš-e I Fr.: étoiles de population I A member of a class of relatively young stars, containing a large fraction of → metals, found mainly in the disk of the Galaxy. → population; I, Roman number 1; → star. |
Population II star setÃ¢re-ye porineš-e II Fr.: étoiles de population II A member of a population of relatively old stars, containing a small fraction of → metals, found mainly in the → halo of the Galaxy and in → globular clusters. → population; II, Roman number 2; → star. |
Population III star setâre-ye porineš-e III Fr.: étoile de population III A member of the first generation of stars, formed out of pristine gas, enriched by → primordial nucleosynthesis alone. The material from which these stars formed consisted mostly of hydrogen and helium. Because neutral hydrogen clouds were free of dust, their cooling mechanism was drastically ineffective. As a result, these star forming clouds had a much higher temperature than in the present epoch, and their → Jeans mass was much higher. Therefore, these first generation of stars were principally massive, with a typical mass scale of order of about 100 M_{sun}. Population III stars started forming about 300 million years after the → Big Bang at → redshifts between 50 and 6, when the Universe had between 1 and 5% of its present age. These stars were probably responsible for the → reionization of the Universe. Given their high mass, they lived only a few million years ending with either a → pair-instability supernova phase or a direct collapse to a → black hole. Population III stars thus initiated the chemical enrichment of the Universe and opened the way to more normal modes of star formation, namely → Population II. Some models predict a bimodal → initial mass function for the first stars, allowing also for solar mass stars. See also → extremely metal-poor star. → population; III, Roman number 3; → star. |
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