An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A quantity without an associated → physical dimension. Dimensionless quantities are defined as the ratio of two quantities with the same dimension. The magnitude of such quantities is independent of the system of units used. A dimensionless quantity is not always a ratio; for instance, the number of people in a room is a dimensionless quantity. Examples include the → Alfven Mach number, → Ekman number, → Froude number, → Mach number, → Prandtl number, → Rayleigh number, → Reynolds number, → Richardson number, → Rossby number, → Toomre parameter. See also → large number.

→ dimension; → quantity.

The quality or state of being → disciplinary.

→ disciplinary; → -ity.

A break in sequence or continuity of anything. → Balmer discontinuity
Math.: A point at which a function is not continuous.
Geology: A surface separating rock layers of differing properties or compositions.

M.L. discontinuitas, from discontinuus, from → dis- + continuus, → continuous.

Nâpeyvastegi, noun from nâpeyvasté "discontinuous," from nâ- "non, un-," → a-, + peyvasté, → continuous.

1) General: The process by which an → accretion disk cools, causing it to fragment into self-gravitating → clumps.
2) A model of → planet formation in which a region of a → circumstellar disk becomes dense and cool enough to be unstable to → gravitational collapse, resulting in the formation of a gaseous → protoplanet. Disk instability has been put forward as a mechanism for forming massive → giant planets on short time-scales (1 Ky-1 My).
3) In models of galactic structure, a process that is thought to cause the formation of → pseudo-bulges.
4) → disk instability model (DIM).

→ disk; → instability.

A model describing → dwarf novae and → Soft X-ray Transient (SXT)s. Accordingly, these objects are triggered by an → accretion disk instability due to an abrupt change in opacities (→ opacity) at → temperatures at which hydrogen is partially ionized. All versions of the DIM have this ingredient. They differ in assumptions about → viscosity, and about what happens at the inner and outer disk radii. Basically, during → quiescence, material accumulates in the accretion disk until a critical point is reached. The disk then becomes unstable and is dumped onto the → compact object, releasing a burst of → X-rays. However, the greater duration of SXT bursts (months) and the time interval between bursts (decades) cannot be accounted for by the standard disk instability model used for dwarf novae, and additional factors such as X-ray illumination and irradiation of the accretion disk are required for the model to match the observed properties of SXTs (J-P Lasota and J-M Hameury, 1995).

→ disk; → instability; → model.

The state or quality of being distributive.

Noun of → distributive.

The state or fact of being diverse; difference; unlikeness.

→ diverse; → -ity.

The average velocity of a charged particle in a plasma in response to an applied electric field.

→ drift; → velocity.

The quality or character of being twofold, as the → wave-particle duality.

M.E dualitie, from L.L. dualitas.

Dogânegi, from dogânag + -i.

The property of a metal that allows it to be elongated into wire or threads without fracture. For example, → copper and → silver are highly ductile metals.

→ ductile; → -ity.

The ability to withstand damage or decay.

→ durable; → -ity.

Same as → viscosity and → absolute viscosity.

→ dynamic; → viscosity.

The amount by which the orbit deviates from circularity: e = c/a, where c is the distance from the center to a focus and a the semi-major axis. If e = 0, the orbit is a circle. If e < 1, the orbit is an ellipse, if e > 1 it is a hyperbola, and if e = 1 it is a parabola. The eccentricity is one of the six → orbital elements that define a → Keplerian orbit.

→ eccentric; → -ity.

Same as → Eddington limit.

→ Eddington limit; → luminosity.

In a → rotating star, the sum of the → gravity and the → centrifugal acceleration. The effective gravity is a function of the rotation velocity (Ω) and the → colatitude (θ). At the pole (θ = 0°) and the equator (θ = 90°) the effective gravity is radial. See also → total gravity.

→ effective; → gravity.

The laws of physics are the same in all → inertial reference frames and are invariant under the → Lorentz transformation. The → speed of light is a → physical constant, i.e. it is the same for all observers in uniform motion. Einsteinian relativity is prompted by the → Newton-Maxwell incompatibility. See also: → Galilean relativity, → Newtonian relativity.

→ Einstein; → relativity.

The ability of a body which has been → deformed by an applied → force to return to its original shape when the force is removed. Up to a certain point the material obeys → Hooke's law. See also → ductility, → plasticity.

→ elastic + → -ity.

The strength of an electric field at any point as measured by the force exerted upon a unit positive charge placed at that point.

→ electric; → intensity.

A measure of a material's ability to conduct an electrical current. It is the reciprocal of the → resistivity. Conductivity is expressed by σ = ne²l/(2mv), where n is the number of electrons per cm³ volume of the → conductor, e is the → electron charge, l is the → mean free path, m is the → electron mass, and v is the arithmetic mean velocity of thermal motion of electrons at a given temperature.

→ electrical; → conductivity.

1) The physical phenomena arising from the behavior of → electrons and → protons that is caused by the → attraction of particles with opposite → charges and the → repulsion of particles with the same charge.
2) The → science of electric charges and → currents.
3) A → flow of electrons that is used to generate → light and → power electric devices.

From L. electrum "amber," from Gk. elektron "amber" + -ity a suffix used to form abstract nouns expressing state or condition.

Barq, Pers. term, used also in Ar. and Hebrew (barak "lightening"); variants in Pers.: varq, barx, balk, belak, bala; Lârestâni belak; Tabari, Lahijâni, Semnâni, Sorxeyi, Sangesari belk; Gilaki val; Lori beleyz; Kurd. bilese; Tokharian AB pâlk; Mid/Mod.Pers. bir "lightening," Mid.Pers. brâh "brilliance, splendour," br'z- "to shine, beam," Mod.Pers. barâz "beauty, grace, elegance;" Av. brāz- "to shine, beam; splendour," brazāiti "shines;" cf. Skt. bhrāj- "to shine, beam, sparkle," bhrajate "shines;" Gk. phlegein "to burn;" L. fulgere "to shine," fulmen "lightning," flagrare "to blaze, burn;" O.H.G. beraht "bright;" O.E. beorht "bright;" E. → bright; PIE base *bherəg-; *bhrēg- "to shine; white."