An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

1) Chemistry: A family of → organic molecules composed of carbon and hydrogen atoms (→ hydrocarbons) in which → carbon atoms appear in multiple loops (polycyclic) with strong chemical → bonds that exist between them (aromatic). PAHs are formed during the incomplete burning of coal, oil and gas, garbage, or other organic substances like tobacco or charbroiled meat. As a pollutant, they are of concern because some compounds (benzo(a)pyrene) have been identified as tending to cause cancer.
2) Astro.: Strong → infrared emission features at 3.3, 6.2, 7.7, 8.6, and 11.2 microns, which are attributed to PAH molecules in the form of → dust grains with radii smaller than 10 Å. PAHs are common toward various sources characterized by a strong → ultraviolet radiation field (such as → active galaxies, → reflexion nebulae, → planetary nebulae, → H II regions). In particular, they trace the presence of → massive stars. The PAH features are thought to result from the fluorescent emission process of PAH molecules and ions excited by ultraviolet radiation of nearby stars. PAHs are likely produced by → photoevaporation of → very small grains.

→ poly-; → cyclic; → aromatic; → hydrocarbon.

A one-dimensional closed figure consisting of a series of points, each of which is called a → vertex, and the line segments, called → sides, joining the vertices. Polygons of three sides are called → triangles, and of four sides → quadrilaterals.

→ poly- + → -gon.

1) Geometry: A three-dimensional figure formed by several → polygons.
2) A set P ⊆ ℜⁿ is a polyhedron if there is a system of finitely many unequalities Ax ≤ b such that P = {x ∈ ℜⁿ | Ax ≤ b}.

→ poly-; → hedron.

An equation of the form a₀ + a₁x + a₂x² + ... + a_nxⁿ, where a₀ ... a_n are → real numbers and a_n≠ 0. Same as → algebraic equation.

→ polynomial; → equation.

The act of popularizing.

→ popularize; → -tion.

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.

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.

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.

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.

In physics, specifically statistical mechanics, the state of an atomic or molecular system in which the number of members in an excited state is larger than those in lower energy states. → optical pumping; → inverted population.

→ population; → inversion.

1) A part of any whole, either separated from or integrated with it.
2) To divide into or distribute in portions or shares.

M.E. porcion, from O.Fr. porcion "part, portion, share," from partion- "share, part, piece," related to pars, → part.

Pâreš, from pâr "part, portion, piece" (variants pâré, parré "portion, segment (of an orange)," pargâlé, "piece, portion; patch;" (dialects Kermâni pariké "portion, half;" Tabari perik "minute quantity, particle;" Lârestâni pakva "patch;" Borujerdi parru "patch"); Mid.Pers. pârag "piece, part, portion; gift, offering, bribe;" Av. pāra- "debt," from par- "to remunerate, equalize; to condemn;" PIE *per- "to sell, hand over, distribute; to assigne;" Gk. peprotai "it has been granted;" L. pars, as above; Skt. purti- "reward;" Hitt. pars-, parsiya- "to break, crumble") + -eš suffix.

1a) Condition with reference to place; location; situation.
1b) A place occupied or to be occupied. → mean position.
2a) To put in a particular or appropriate position; place.
2b) To determine the position of; locate.

M.E. posicioun, from O.Fr. posicion, from L. positionem "act or fact of placing, position, affirmation," from positus, p.p. stem of ponere "to put, place."

Neheš, verbal noun from nehâdan "to place, put; to set;" Mid.Pers. nihâtan, from ne-, ni- "down; into," → ni- (PIE), + dâ- "to put; to establish; to give," dadâiti "he gives;" cf. Skt. dadâti "he gives;" Gk. didomi "I give;" L. do "I give;" PIE base *do- "to give"). Neheš-dâdan, from neheš "position," + dâdan "to give, yield, put," → datum.

The convention for measuring angles on the sky in astronomy (Abbreviated as PA). It is the direction of an imaginary arrow in the sky, measured from north through east: 0° = north, 90° = east, 180° = south, and so on to 359° and back to 0°. Applied to a binary system it is the direction of a secondary body or feature from a primary, measured in the system. .

→ position; → angle.

In single dish astronomy, an observing mode in which the telescope is moved between the object position and a user defined reference position. The aim is to eliminate unwanted signals in the baseline. → beam switching; → frequency switching.

→ position; → switching.

Relating to or determined by position.

→ position; → -al.

The branch of astronomy that is used to determine the location of objects on the celestial sphere, as seen at a particular date, time, and location on the Earth. Same as → spherical astronomy.

→ positional; → astronomy.

A system of representing → numbers in which the → position of a → digit in a string of digits affects its value. The decimal system is a positional notation for expressing numbers. Same as → place-value notation and → positional number system.

→ positional; → notation.

A → number system in which the value of each digit is determined by which place it appears in the full number. The lowest place value is the rightmost position, and each successive position to the left has a higher place value. In the → number system conversion, the rightmost position represents the "ones" column, the next position represents the "tens" column, the next position represents "hundreds", etc. The values of each position correspond to powers of the → base of the number system. For example, in the usual decimal number system, which uses base 10, the place values correspond to powers of 10. Same as → place-value notation and → positional notation. See also → number system conversion.

→ positional; → number; → system.

The act or process of putting in a particular position or determining the psition of.

Verbal noun of → position; → -ing.

Same as → direct correlation.

→ positive; → correlation.