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: 38 Search : density
charge density
  چگالی ِ بار   
cagâli-ye bâr

Fr.: densité de charge   

The → electric charge per unit volume in space, or per unit area on a surface, or per unit length of a line. They are respectively called volume- (ρ), surface- (σ), or line (λ) charge density.

charge; → density.

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.

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

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 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 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.

density wave theory
  نگره‌ی ِ موج ِ چگالی   
negare-ye mowj-e cagâli

Fr.: théorie des ondes de densité   

One possible explanation for → spiral arms, first put forward by B. Lindblad in about 1925 and developed later by C.C. Lin and F. H. Shu. According to this theory, spiral arms are not material structures, but regions of somewhat enhanced density, created by → density waves. Density waves are perturbations amplified by the self-gravity of the → galactic disk. The perturbation results from natural non-asymmetry in the disk and enhanced by environmental processes, such as galaxy encounters. Density waves rotate around the → galactic center and periodically compress the disk material upon their passage. If the spiral arms were rigid structures rotating like a pinwheel, the → differential rotation of the galaxy would wind up the arms completely in a relatively short time (with respect to the age of the galaxy), → winding problem. Inside the region defined by the → corotation radius, density waves rotate more slowly than the galaxy's stars and gas; outside that region they rotate faster.
As the density waves rotate, they are overtaken by the individual stars and nebulae/molecular clouds that are rotating around the galaxy at a higher rate. The molecular clouds passing through the density wave are subjected to compression because it is a region of higher density. This triggers the formation of clusters of new stars, which continue to move through the density wave.
The short-lived stars die, most likely as supernovae, before they can leave the spiral density wave. But the longer-lived stars that are formed pass through the density wave and eventually emerge on its front side and continue on their way as a slowly dissipating cluster of stars. Density wave theory explains much of the spiral structure that we see, but there are some problems. First, computer simulations with density waves tend to produce very orderly "grand design" spirals with a well-defined, wrapped 2-arm structure. But there are many spiral galaxies that have a more complex structure than this (→ flocculent spiral galaxy). Second, density wave theory assumes the existence of spiral density waves and then explores the consequences.
See also: → stochastic self-propagating star formation.

density; → wave; → theory.

density-bounded H II region
  ناحیه‌ی ِ H IIی ِ چگالی‌کرانمند   
nâhiye-ye H II-ye cagâli karânmand

Fr.: bornée par la densité   

An → H II region which lacks enough matter to absorb all → Lyman continuum photons of the → exciting star(s). In such an H II region a part of the ionizing photons escape into the → interstellar medium. See also → ionization-bounded H II region.

density; → bounded; → region.

electron density
  چگالی ِ الکترونی   
cagâli-ye elektroni (#)

Fr.: densité électronique   

The number of electrons per unit volume in an ionized medium, like an → H II region, as determined from → emission lines.

electron; → density.

energy density
  چگالی ِ کاروژ   
cagâli-ye kâruž

Fr.: densité d'énergie   

The amount of energy in the form of radiation per unit volume, expressed in ergs cm-3. In particular, the energy density of blackbody radiation at temperature T is aT4, where the radiation constant a = 7.56 × 10-15 erg cm-3 (K)-4.

energy; → density.

flux density
  چگالی ِ شار   
cagâli-ye šârr

Fr.: densité de flux   

Flux of radiation that falls on a detector per unit surface area of the detector per unit bandwidth of the radiation per unit time.

flux; → density.

Lagrangian density
  چگالی ِ لاگرانژی   
cagâli-ye Lagranži

Fr.: densité lagrangienne   

A quantity, denoted Ld, describing a continuous system in the → Lagrangian formalism, and defined as the → Lagrangian per unit volume. It is related to the Lagrangian L by:
L = ∫∫∫Ld d3V.
Lagrangian density is often called Lagrangian when there is no ambiguity.

Lagrangian; → density.

magnetic flux density
  چگالی ِ شار ِ مغناتیسی   
cagâli-ye šâr-e meqnâtisi (#)

Fr.: densité du flux magnétique   

A vector quantity measuring the strength and direction of the magnetic field. It is the → magnetic flux per unit area of a magnetic field at right angles to the magnetic force. Magnetic flux density is expressed in → teslas. Also called → magnetic induction.

magnetic; → flux; → density.

mass density
  چگالی ِ جرمی   
cagâli-ye jermi

Fr.: densité massique   

The mass per unit area of the ring material, integrated through the thickness of the ring. Sometimes called → surface density (Ellis et al., 2007, Planetary Ring Systems, Springer).

mass; → density.

maximum density of water
  چگالی ِ بیشینه‌ی ِ آب   
cagâli-ye bišine-ye âb

Fr.: densité maximale de l'eau   

The density of pure water occurring at 3.98 °C, which is 1.0000 g cm-3, or 1000 kg m-3. Water when cooled down contracts normally until the temperature is 3.98 °C, after which it expands. Because the maximum density of water occurs at about 4 °C, water becomes increasingly lighter at 3 °C, 2 °C, 1 °C, and 0 °C (→ freezing point). The density of liquid water at 0 °C is greater than the density of frozen water at the same temperature. Thus water is heavier as a liquid than as a solid, and this is why ice floats on water. When a mass of water cools below 4 °C, the density decreases and allows water to rise to the surface, where freezing occurs. The layer of ice formed on the surface does not sink and it acts as a thermal isolator, thus protecting the biological environment beneath it. This property of water liquid is very unusual; molecules pack more closely than in the crystal structure of ice. The reason is that → hydrogen bonds between liquid water are not stable, they are continuously broken and new bonds are created. In the crystal structure of ice molecules have a fixed pattern creating empty space between molecules.

maximum; → density; → water.

morphology-density relation
  بازانش ِ ریخت-چگالی   
bâzâneš-e rixt-cagâli

Fr.: relation morphologie-densité   

An observationally determined relationship between the → morphological classification of galaxies and the → environments in which they are located. Specifically, the morphology-density relation indicates that early-type galaxies (→ ETG) are preferentially located in high density environments, whereas late-type galaxies (→ LTG) are preferentially found in low density environments. Hence, spiral galaxies are rare in the high densities of clusters and are common in the lower density group environments. Early-type galaxies, on the other hand, are common in clusters and are rarely found in isolation.

morphology; → density; → relation.

neutral density filter
  پالایه‌ی ِ چگالی ِ نتار   
pâlâye-ye cagâli-ye natâr

Fr.: filtre neutre   

A filter having a flat response over the range of wavelengths of interest. Also called neutral filter or gray filter.

neutral; → density; → filter.

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