1) Of or pertaining to → critics or
Fr.: angle critique
Angle of incidence of light proceeding from a denser medium toward a thinner, at which grazing refraction occurs (angle of refraction 90°).
critical Bonnor-Ebert mass
jerm-e paržani-ye Bonnor-Ebert
Fr.: masse critique de Bonnor-Ebert
The upper value of mass that a → Bonnor-Ebert sphere must have in order that → hydrodynamic equilibrium be maintained. This → critical mass is given by: Mcrit = 1.18 (a4/G3/2)Pext-1/2, where a = (kT/m)1/2 is the isothermal → sound speed inside the sphere, G is the → gravitational constant, and Pext the pressure of the external medium (see, e.g., F. H. Shu, 1977, ApJ 214, 488).
→ critical; → Bonnor-Ebert mass.
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).
Fr.: masse critique
Of a fissile material (235U or 239Pu), the minimum mass needed for a sustained nuclear chain reaction, as in an atomic bomb.
Fr.: métallicité critique
The → metallicity of a → star-forming → molecular cloud when → cooling → rates by → metals dominate the → gravitational → heating during → protostellar collapse. The minimum → Jeans mass achieved by gravitational → fragmentation depends on the presence/absence of → coolants in the cloud. Since cooling rate in metal lines is more efficient than in primordial molecular lines (H2 and HD), metals favor fragmentation in gas and formation of → low-mass stars.
→ critical; → metallicity.
Fr.: opalescence critique
The strong → scattering that occurs in a fluid near the → critical point. Large density fluctuations bring about inhomogeneities of all sizes. This results in the scattering of light at all wavelengths giving a milky appearance to the fluid.
→ critical; → opalescence.
Fr.: vitesse critique
1) Velocity of → fluid through a pipe at which the motion
changes from → laminar to
→ turbulent flow.
Of or pertaining to a state, value, or quantity that is less than critical, especially the condition of a → subcritical reactor.
Fr.: masse sous-critique
An amount of → fissile material that by its mass or geometry is incapable of sustaining a → fission → chain reaction.
→ subcritical; → mass.
Fr.: réacteur sous-critique
A → nuclear reactor in which the rate of production of → fission neutrons is lower than the rate of production in the previous generation, and therefore the number of fissions decreases over time.
→ subcritical; → reactor.
1) Thermodynamics: Describing a condition in which a substance has
a temperature or pressure above its critical value of temperature or pressure.
Fr.: fluide supercritique
A fluid that is at a temperature and pressure above its thermodynamic critical point. In these conditions the substance acquires unique characteristics of density and mobility. Supercritical fluids exist deep inside some planets; for example, there is supercritical water deep inside the Earth.
→ supercritical; → fluid.