An Etymological Dictionary of Astronomy and Astrophysics
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1) Of or pertaining to → critics or → criticism.
2) Of, relating to, or being a state or level at which a significant change takes place (Dictionary.com).

→ critic; → -al.

Angle of incidence of light proceeding from a denser medium toward a thinner, at which grazing refraction occurs (angle of refraction 90°).

→ critical; → angle.

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: M_crit = 1.18 (a⁴/G^3/2)P_ext^-1/2, where a = (kT/m)^1/2 is the isothermal → sound speed inside the sphere, G is the → gravitational constant, and P_ext the pressure of the external medium (see, e.g., F. H. Shu, 1977, ApJ 214, 488).

→ critical; → Bonnor-Ebert mass.

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 ρ_c = (3c²/8πG)H_t², where c is the → speed of light, G is the → gravitational constant, and H_t 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)(d_os/d_old_ls), where c is the speed of light, G is the gravitational constant, d_os, d_ol, and d_ls 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: n_c = Σ_{i < j} A_ji = Σ_{i ≠ j} q_ji, where A_ji is the → Einstein coefficient of → spontaneous emission and q_ji 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 N_e (electron density) x N_i (ion density), whereas for densities exceeding the critical density, the emissivity is proportional to N_i. Thus, line emission in a nebula occurs most efficiently near the critical density.

→ critical; → density.

Of a fissile material (²³⁵U or ²³⁹Pu), the minimum mass needed for a sustained nuclear chain reaction, as in an atomic bomb.

→ critical; → mass.

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.

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.

1) Velocity of → fluid through a pipe at which the motion changes from → laminar to → turbulent flow.
2) Same as → break-up velocity.

→ critical; → velocity.

Of or pertaining to a state, value, or quantity that is less than critical, especially the condition of a → subcritical reactor.

→ sub-; → critical.

An amount of → fissile material that by its mass or geometry is incapable of sustaining a → fission → chain reaction.

→ subcritical; → mass.

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.
2) Nuclear physics: Of or relating to an arrangement of → fissile material where more neutrons are being produced than are wasted and escape.

→ super-; → critical.

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.