The condition in which the wavefronts arriving from different parts of a region of sky undergo almost identical phase perturbations. See also → isoplanatic patch.
Fr.: instabilité de Jeans
An instability that occurs in a → self-gravitating → interstellar cloud which is in → hydrostatic equilibrium. Density fluctuations caused by a perturbation may condense the material leading to the domination of gravitational force and the cloud collapse. The advent of instability involves a threshold called the → Jeans length or the → Jeans mass.
nâpâydâri-ye Kelvin-Helmholtz (#)
Fr.: instabilité de Kelvin-Helmholtz
An → instability raised when there is sufficient velocity difference across the interface between two uniformly moving → incompressible fluid layers, or when velocity → shear is present within a continuous fluid.
Keplerian angular velocity
tondâ-ye zâviye-yi-ye Kepleri
Fr.: vitesse angulaire keplérienne
The angular velocity of a point in a circular orbit around a central mass. It is given by: ΩK = (GM/r3)1/2, where G is the → gravitational constant, M is the mass of the gravitating object, and r is the radius of the orbit of the point around the object.
Keplerian orbital velocity
tondâ-ye madâr-e Kepleri
Fr.: vitesse d'orbite képlérienne
The velocity of an object orbiting another object according to → Kepler's laws.
Fr.: viscosité cinématique
The ratio of the → dynamic viscosity (η) to the density (ρ) of a fluid: ν = η/ρ. The unit of kinematic viscosity in the → SI system is m2s-1. In the → cgs system, cm2s-1, equal to 10-4 m2s-1, is called the → stokes (st).
Fr.: hélicité cinétique
In fluid mechanics, a quantity that describes helical flow. It is defined by the integrated scalar product of the velocity field and the → vorticity: KK = ∫ dVu . (∇ x u). In the absence of magnetic field, this quantity is conserved by the → Euler equation. See also → magnetic helicity.
Kramers' opacity law
qânun-e kederi-ye Kramers (#)
Fr.: loi de l'opacité de Kramers
Same as → Kramers' law.
Named after Henrik Kramers (1894-1952); → law.
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:
law of identity
Fr.: principe d'identité
Same as → principle of identity.
Fr.: intensité de raie
A measure of the total effect of an absorption or emission line. The line intensity is equal to the integration of the absorption coefficient over the entire shape of the absorption line.
line profile variability (LPV)
vartandegi-ye farâpâl-e xatt
Fr.: variabilité du profil de raie
nâpâydâri-ye xatti (#)
Fr.: instabilité linéaire
An instability that can be described (to first-order accuracy) by linear (or tangent linear) equations.
Fr.: vitesse linéaire
The rate of change of the position of an object that is traveling along a straight path. In other words, the velocity of an object when its moving direction is not changing. For a given → angular velocity (ω), the linear velocity v of the particle is directly proportional to the distance of the particle from the center of the circular path: v = ω ×r.
The property, condition, or state of being linear.
Fr.: environnement faible en métaux
A medium in which chemical elements have abundances smaller than the solar values.
The → total → brightness
of a star or other astronomical object.
It is expressed in watts and represents the total amount of
→ energy that the object radiates each
→ second over all
wavelength regions of the → electromagnetic spectrum.
Because this quantity is independent of distance, it is an
→ intrinsic brightness.
Verbal noun of → luminous.
rade-ye tâbandegi (#)
Fr.: classe de luminosité
A classification of stellar spectra according to luminosity for a given → spectral type. The luminosity class is an indication of a star's → surface gravity. It is shown by a Roman numeral as follows: I (→ supergiants), II (bright → giants), III (normal giants), IV (→ subgiants), and V (→ dwarf stars, or → main-sequence stars). Luminosity classes VI (→ subdwarfs) and VII (→ white dwarfs) are rarely used. Subclasses a, b, and c are especially used for supergiants, while the most luminous → hypergiants are assigned luminosity class Ia-0.
Fr.: distance de luminosité
1) Distance derived by comparison of → observed and
→ intrinsic luminosities.
If an object has a known luminosity L, and the observed flux is
S, the luminosity distance is defined by
DL = (L/4πS)1/2.
Fr.: fonction de luminosité
Number → distribution of → stars or galaxies (→ galaxy) with respect to their → absolute magnitudes. The luminosity function shows the → number of stars of a given intrinsic luminosity (or the number of galaxies per integrated magnitude band) in a given → volume of space.