An Etymological Dictionary of Astronomy and Astrophysics
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فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 199 Search : sit
column density
  چگالی ِ ستون   
cagâli-ye sotun

Fr.: densité de colonne   

Density of the interstellar matter lying between an object and the Earth in a cylinder with a unity base.

column; → density.

composite
  همنهاده   
hamnehâdé (#)

Fr.: composite   

Made up of distinct components. The components may retain part of their identities. → compound, → combination.

From O.Fr. composite, from L. compositus, p.p. of componere "to put together," → compose.

Hamnehâdé, p.p. of hamnehâdancompose.

composite number
  عدد ِ همنهاده   
adad-e hamnehâdé

Fr.: nombre composite   

A whole number which is the product of whole numbers other than itself and 1. The opposite of prime number. → compound number.

composite; → number.

composite population
  پرینش ِ همنهاده   
porineš-e hamnehâdé

Fr.: population composite   

A stellar population consisting of more than one → simple population. Also called complex population.

composite; → population.

composite spectrum
  بیناب ِ همنهاده   
binâb-e hamnehâdé

Fr.: spectre composite   

A stellar spectrum displaying lines characterising two stars of different types.

composite; → spectrum.

composition
  همنهش   
hamneheš (#)

Fr.: composition   

1) General: The act or manner of composing; the result or product of composing.
2) Chemistry: The proportion of the elements of which a substance is made up.
3) Mechanics: The determination of a force whose effect is the same as that of two or more given forces acting simultaneously.

Hamnehesh, from ham-, → com-, + neheš verbal noun from nehâdan "to put, place," → compose.

compound proposition
  گزاره‌ی ِ همنات   
gozâre-ye hamnât

Fr.: proposition composée   

A statement formed from simple statements by the use of words such as "and," "or," "not," "implies," or their corresponding symbols.

compound; → proposition.

conditional proposition
  گزاره‌ی ِ بوتاری   
gozâre-ye butâri

Fr.: proposition conditionelle   

A compound → proposition in which one → clause asserts something as true provided that the other clause is true. A conditional statement consists of two parts, a hypothesis in the "if" clause and a conclusion in the "then"clause. For instance, "If it rains, then they cancel school." It rains is the hypothesis. "They cancel school" is the conclusion. The clause following if is traditionally called the → antecedent, whereas the clause following then is called the → consequent.

conditional; → proposition.

critical density
  چگالی ِ پرژنی   
cagâli-ye paržani

Fr.: densité critique   

1) Cosmology: The average density of matter in the Universe that would be needed to eventually halt the → cosmic expansion. In a spatially → flat Universe, the critical density is expressed by ρc = (3c2/8πG)Ht2, where c is the → speed of light, G is the → gravitational constant, and Ht the → Hubble parameter. The critical density is currently 9.3 × 10-30g cm-3, about 6 hydrogen atoms per cubic meter (for H0 = 70 km s-1 Mpc-1).
2) In → gravitational lensing, the minimum density that would be needed by an intervening object to bend light rays. It is expressed by: Σ = (c2/4πG)(dos/doldls), where c is the speed of light, G is the gravitational constant, dos, dol, and dls represent angular diameter distances between the observer and the source, the observer and the lens, and the lens and the source respectively. It has units of mass per unit solid angle.
3) Radiative processes: The density at which the collisional → de-excitation rate equals the → radiative transition rate. The critical density for level j is given by: nc = Σi < j Aji = Σi ≠ j qji, where Aji is the → Einstein coefficient of → spontaneous emission and qji is the rate for collisional de-excitation of → energy level j, summed over all possible processes. This expression often simplifies to the ratio of two numbers, since in many cases there is a single important path for emission and a dominant collisional de-excitation process. In the low density limit the → emissivity is proportional to the product Ne (electron density) x Ni (ion density), whereas for densities exceeding the critical density, the emissivity is proportional to Ni. Thus, line emission in a nebula occurs most efficiently near the critical density.

critical; → density.

current density
  چگالی ِ جریان   
cagâli-ye jarayân

Fr.: densité de courant   

The electric current per unit of cross-sectional area perpendicular to the direction of current flow. It is a vector quantity and represented by symbol J. Electric current density is usually expressed in amperes per square meter.

current; → density

data acquisition
  الفنجش ِ داده‌ها   
alfanješ-e dâdehâ

Fr.: acquisition de données   

The process of controlling telescope operations during observation and obtaining data.

data; acquisition, from L. acquisitionem, from acquirere "get in addition," from → ad- "extra" + quærere "to search for, obtain."

Alfanješ, verbal noun of alfanjidan (variant alfaqdan) "to acquire, get," Bactrian αλφανζ "to acquire," Sogdian δβ'yz "to acquire, gain, get" (Cheung 2007); → data.

de Sitter Universe
  گیتی ِ دوسیتر   
giti-ye de Sitter

Fr.: Univers de de Sitter   

A solution to → Einstein's field equations of → general relativity which contains no ordinary matter (ΩM = 0) or radiation (ΩR = 0), is → Euclidean (k = 0), but has a → cosmological constantΛ > 0). The Universe expands exponentially forever. This solution was the first model expanding of → expanding Universe. See also → empty Universe, → Milne Universe.

After the Dutch mathematician and physicist Willem de Sitter (1872-1934) who worked out the model in 1917; → universe.

decomposition
  واهمنهش   
vâhamneheš

Fr.: décomposition   

The process or the state of breaking down a physical entity or an organic material.

From → de- + → composition.

density
  چگالی   
cagâli (#)

Fr.: densité   

The amount of any quantity per unit volume. The mass density is the mass per unit volume. The energy density is the energy per unit volume; particle density is the number of particles per unit volume.
See also:
charge density, → column density, → critical density, → current density, → density fluctuation, → density parameter, → density profile, → density wave, → density-bounded H II region, → density-wave theory, → electron density, → energy density, → flux density, → magnetic flux density, → maximum density of water, → neutral density filter, → nuclear density, → number density, → optical density, → period-mean density relation, → Planck density, → potential density, → power spectral density, → probability density function, → radio flux density, → relative density, → specific density, → spectral density, → surface density.

Noun form of → dense.

density cusp
  تیزه‌ی ِ چگالی   
tize-ye cagâli

Fr.: cuspide de densité   

A localized increase in number of → stellar black holes near a → supermassive black hole predicted by models of galactic → stellar dynamics (Bahcall, Wolf, 1976, ApJ, 209, 214). Same as → stellar cusp.

density; → cusp.

density fluctuation
  افتاخیز ِ چگالی   
oftâxizhâ-ye cagâli

Fr.: fluctuations de densité   

In the early Universe, localized enhancements in the density of either matter alone or matter and radiation. According to models, very small initial fluctuations (less than 1 percent) can lead to subsequent formation of galaxies.

density; → fluctuation.

density of an element
  چگالی ِ بن‌پار   
cagâli-ye bonpâr

Fr.: densité d'élément   

The number of units of mass of the → chemical element that are present in a certain volume of a medium. The density of an element depends on temperature and pressure. The element Osmium has the highest known density: 22.61 g/cc; in comparison gold is 19.32 g/cc and lead 11.35 g/cc.

density; → element.

density parameter
  پارامون ِ چگالی   
pârâmun-e cagâli

Fr.: paramètre de densité   

One of the four terms that describe an arranged version of the → Friedmann equations. They are all time dependent.
1) For matter: Ωm = 8πGρm/(3H2), where G is the → gravitational constant, ρm is the mean matter density, and H the → Hubble parameter. The matter density parameter is also expressed as Ωm = ρmcrit, where ρcrit is the → critical density.
2) For radiation: Ωr = 8πGρr/(3H2), where ρr is the radiation equivalent of matter density. This parameter is also expressed as Ωr = ρrcrit.
3) For the → cosmological constant: ΩΛ = Λc2/(3H2). Similarly, ΩΛ = ρΛcrit, where &rhoΛ = Λc2/(8πG) is sometimes referred to as the density of → dark energy.
4) For the → curvature of space-timeΩk = -kc2/(R2H2), where k is the → curvature constant and R the → cosmic scale factor.
Note that: Ωm + Ωr + ΩΛ + Ωk = 1, and Ωtotal = Ωm + Ωr + ΩΛ = 1 - Ωk.

density; → parameter.

density profile
  فراپال ِ چگالی   
farâpâl-e cagâli

Fr.: profile de densité   

1) A → profile representing the → density of a quantity.
2) A → profile representing the distribution of stars as a function of their number in a region.

density; → profile.

density wave
  موج ِ چگالی   
mowj-e cagâli (#)

Fr.: onde de densité   

A wave phenomenon in which the density fluctuations of a physical quantity propagates in a compressible medium. For example, the → spiral arms of a → galaxy are believed to be due to a density wave which results from the natural instability of the → galactic disk caused by its own gravitational force. A common example of a density wave concerns traffic flow. A slow-moving vehicle on a narrow two-lane road causes a high density of cars to pile up behind it. As it moves down the highway the "traffic density wave" moves slowly too. But the density wave of cars does not keep the same cars in it. Instead, the first cars leave the density wave when they pass the slow vehicle and continue on at a more normal speed and new ones are added as they approach the density wave from behind. Moreover, the speed with which the density wave moves is lower than the average speed of the traffic and that the density wave can persist well after its original cause is gone. See → density wave theory.

density; → wave.

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