An Etymological Dictionary of Astronomy and Astrophysics
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The condition in which a physical system is capable of assuming either of two stable states.

→ bi-; → stable.

An abrupt discontinuity in the → stellar wind properties of → hot stars near → effective temperatures about 21,000 K and 10,000 K, corresponding to O9.5-B3 supergiants (Castor et al. 1975, ApJ 195, 157; Lamers et al., 1995, ApJ 455, 269). At these temperatures the → terminal velocity of the wind drops steeply by about a factor two and the → mass loss rate increases steeply by about a factor three to five, when going from high to low temperatures. Bistability jump is related to the degree of ionization in the wind. With a little drop in the temperature, the dominant driving element (Fe) will recombine to lower ionization stages which produces a lower terminal velocity and a relatively high density in the wind. → wind momentum. Additional bistability jumps may occur at higher temperatures where CNO may provide the dominant line driving, especially for low metallicity stars (Vink et al. 2001, A&A 369, 574). However, a recent study using a larger sample finds that there is a gradual decline in the wind terminal velocities of early B supergiants and not a "jump" (Crowther et al. 2006, A&A 446, 279).

→ bistability; → jump.

The mechanism that accounts for the → bistability jump.

→ bistability; → mechanism.

The acceleration of gravity at the → event horizon of a → black hole. For a → Schwarzschild back hole it is given by κ = GM/R_Sch² = c⁴/(4GM).

→ black; → hole; → surface; → gravity.

The total rate of energy output of an object integrated over all wavelengths.

→ bolometric; → luminosity.

A characteristic luminosity around which the → luminosity function of a sample of galaxies changes to a steeper slope or exponentially declines.

→ break; → luminosity.

The velocity of a → rotating star at which the → centrifugal force equals the → gravitational force. Also known as → critical velocity. The simplest expression of the break-up velocity for an OB star, ignoring the → Eddington luminosity, is given by the relation: v = (GM / R)^1/2, where M and R are the mass and radius of the star respectively, and G the → gravitational constant. A more realistic expression takes into account not only the → radiation pressure, but also the non-uniformity of the brightness over the stellar surface, as indicated by → von Zeipel theorem. With these conditions, the break-up velocity has a more complicated formula, corresponding to the velocity reached when somewhere on the star the → total gravity becomes zero.

→ break + up; M.E.; O.E. up, uppe, → hyper-; → velocity.

The ability to receive or contain.
Electricity: → capacitance.

From M.Fr. capacité, from L. capacitatem, from capax "able to hold much," from capere "to take, grasp."

Gonjâyeš "capacity, holding, containing," from gonjdan "to be contained; to hold exactly; to be filled;" Mid.Pers. winj- "to be contained;" Proto-Iranian *uiac-/*uic-; cf. Skt. vyac- "to contain, encompass," vyás- "extent, content, extension;" L. uincire "to bind."

Same as → capillary action.

→ capillary; → -ity.

Math.: The → cardinal number indicating the → number of → elements in a → set. For example, the set A = {a, b, c, d} contains 4 elements, and therefore it has a cardinality of 4 (denoted |A| = 4).

→ cardinal; → -ity.

The relationship between causes and effects

Causality, from → causal + -ity.

Bonârmandi, from bonâr→ cause + -mand suffix denoting relation, affinity + -i noun forming suffix.

The principle that cause must always precede effect.

→ causality; → principle.

The quality of being physically caustic.

→ caustic; → -ity.

1) An apparently hollow formation in the structure of an astronomical object, for example a sizable hole on the surface of a → molecular cloud created by → ultraviolet photons of a → massive star.
2) In a semiconductor laser, two reflective parallel edges forming a resonator that amplifies the light through stimulated emission.

From M.Fr. cavité, from L.L. cavitas "hollowness," from L. cavus "hollow."

Kâvâk, related to verb kâvidan (kâftan) "to dig; to examine, investigate," cf. L. cavus "hollow" (E. derivatives: cavity, concave, cave, excavate), Gk. koilos "hollow," Armenian sor, PIE *kowos "hollow."

A fixed point in a body through which the resultant force of gravitational attraction acts. Same as → center of mass, → center of inertia, → centroid.

→ center; → gravity.

Gerânigâh, from gerâni→ gravity + -gâh "place."

The condition of being → chaotic.

→ chaotic; → -ity.

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.

The laws of physics should be the same if a particle is interchanged with its → antiparticle (→ charge conjugation), or swapped for its mirror image (→ parity symmetry). It is known that charge-parity (CP) symmetry holds for interactions involving → electromagnetism, → gravitation, and → strong interactions, but CP violation is known to occur during → weak interactions involved in → radio decay. Same as → CP-symmetry.

→ charge; → parity; → symmetry.

The extent to which a chemical species, such as an atom or molecule, tends to combine with another to form a chemical compound.

→ chemical; → affinity.

The geometric property of a rigid object that is → chiral.

→ chiral; → -ity.