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.
neutral density filter
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.
cagâli-ye haste-yi (#)
Fr.: densité nucléaire
The density of an atomic nucleus (about 1014 g/cm3).
Fr.: densité nmérique
Number of a particular type of object found in each unit volume.
Fr.: densité optique
The transmittance of a point on a photographic negative equal to the log to the base 10 of the reciprocal of the transmittance through the negative at that point.
period-mean density relation
bâzâneš-e dowré-cagâli-ye miyângin
Fr.: relation période-densité moyenne
A relation that gives a rough estimate of the oscillation period of a → pulsating star as a function of its mean density. This relation is obtained by considering how long it would take a sound wave to travel across the diameter of a model star: Π ≅ (3π/2γGρ)1/2, where ρ is the mean density, γ the ratio of → specific heats (Cp/Cv), and G the → gravitational constant. This relation shows that the pulsation period of a star is inversely proportional to the square root of its mean density. And this is the reason why the pulsation periods decrease along the → instability strip from the luminous, very tenuous → supergiants to the faint, very dense → white dwarfs.
Fr.: densité de Planck
The density corresponding to a → Planck mass in a cubic region of edge length given by the → Planck length: ρP = c5/(ħG2) ≅ 5.16 x 1093 g cm-3, where c is the → speed of light, ħ is the → reduced Planck's constant, and G is the → gravitational constant.
Fr.: densité potentielle
Of a fluid parcel at pressure P, the density that it would acquire if adiabatically brought to a reference pressure.
power spectral density
cagâli-ye binâbi-ye tavân
Fr.: densité spectrale de puissance
Same as → spectral density.
probability density function
karyâ-ye cagâli-ye šavânâyi
Fr.: fonction de densité de probabilité
A mathematical function whose integral over any interval gives the probability that a continuous → random variable has values in this interval. Also known as → density function, frequency function, → probability function.
radiation density constant
pâypa-ye cagâli-ye tâbeš
Fr.: constante de rayonnement
The constant related to the total energy radiated by a → blackbody and defined as: a = 4σ/c, where σ is the → Stefan-Boltzmann constant and c the → speed of light. Its value is a = 7.5657 x 10-15 erg cm-3 K-4. Same as → radiation constant.
radio flux density
cagâli-ye šârr-e râdioyi
Fr.: densité de flux radio
Fr.: densité relative
The ratio of the density of a substance to the density of a given reference material. For a solid or liquid, it is the density (at 20°C) relative to the maximum density of water (at 4°C). For a gas is its density divided by the density of hydrogen (or sometimes dry air) at the same temperature and pressure. Also called → specific density.
Fr.: densité scalaire
Fr.: densité spécifique
Same as → relative density.
Fr.: densité spectrale
For a specified → bandwidth of radiation consisting of a continuous → frequency spectrum, the total → power in the bandwidth divided by the bandwidth. Spectral density describes how the power (or variance) of a time series is distributed with frequency. Also called power spectral density.
Fr.: densité de surface
The amount of a quantity distributed over a surface area divided by the area, such as a surface-charge density.
Fr.: densité de tenseur
A generalization of the tensor concept that like a tensor transforms, except for the appearance of an extra factor, which is the → Jacobian matrix of the transformation of the coordinates, raised to some power, in transformation law. The exponent, which is a positive or negative integer, is called the weight of the tensor density. → weight of a tensor density. Ordinary tensors are tensor densities of weight 0. Tensor density is also called → relative tensor.
Fr.: densité de vecteur
weight of a tensor density
vazn-e cagâli-ye tânsor
Fr.: poids d'une densité de tenseur
A constant the value of which is characteristic for any given → tensor density.