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

The set of all points in a → space-time diagram that are reached by signals travelling from a specified point at the speed of light.

→ future; → light; → cone.

A region of the Galaxy whose boundaries are set by its calm and safe environment and access to the chemical materials necessary for building terrestrial planets similar to the Earth. → circumstellar habitable zone; → habitable zone.

→ galactic, → habitable; → zone.

The exponent appearing in the power law that describes the → effective temperature of a → rotating star as a function of the → effective gravity, as deduced from the → von Zeipel theorem or law. Generalizing this law, the effective temperature is usually expressed as T_eff∝ g_eff^β, where β is the gravity darkening exponent with a value of 0.25. It has, however, been shown that the relation between the effective temperature and gravity is not exactly a power law. Moreover, the value of β = 0.25 is appropriate only in the limit of slow rotators and is smaller for fast rotating stars (Espinosa Lara & Rieutord, 2011, A&A 533, A43).

→ gravity; → darkening; → exponent.

A zone around a → star where the → temperature would be in the range 0-100 °C to sustain → liquid water on the surface of rocky planets (or sufficiently large moons). Water is thought to be a necessary component to the → formation and evolution of Earth-type life. This zone depends on the parent star's luminosity and distance; it will be farther from hotter stars. A more accurate definition of HZ needs to include other factors, such as orbital → eccentricity, heat sources other than stellar irradiation, and atmospheric properties. Same as → circumstellar habitable zone; → ecosphere.

→ habitable; → zone.

The region to the right the → Hayashi track, representing objects that cannot be in → hydrostatic equilibrium. Energy transport in these objects would take place with a → superadiabatic temperature gradient.

→ Hayashi track; → forbidden; → zone.

A → convective zone close to the surface of → hot stars caused by a peak in the → opacity due to iron recombination. A physical connection may exist between → microturbulence in hot star atmospheres and a subsurface FeCZ. The strength of the FeCZ is predicted to increase with → metallicity and → luminosity, but decrease with → effective temperature. The FeCZ in hot stars might also produce localized surface magnetic fields. The consequence of the FeCZ might be strongest in → Wolf-Rayet stars. These stars are so hot that the → iron opacity peak, and therefore FeCZ, can be directly at the stellar surface or, better said, at the → sonic point of the wind flow. This may relate to the very strong → clumping found observationally in Wolf-Rayet winds, and may be required for an understanding of the very high → mass loss rates of Wolf-Rayet stars (See Cantiello et al. 2009, A&A 499, 279).

→ iron; → convection; → zone.

A curve on a → Hertzsprung-Russell diagram connecting all stars having the same age.

Isochrone, back formation from isochronal, from Gk. → iso- + khronos "time."

Izozamân, from → iso- + zamân→ time.

A line drawn through all points on a weather map having the same amount of → cloud cover.

From Gk. → iso- + nephos "cloud," cognate with Pers. nam "humidity, moisture;" Av. napta- "moist," nabās-câ- "cloud," nabah- "sky;" Skt. nábhas- "moisture, cloud, mist;" L. nebula "mist," nimbus "rainstorm, rain cloud;" O.H.G. nebul; Ger. Nebel "fog;" O.E. nifol "dark;" from PIE *nebh- "cloud, vapor, fog, moist, sky."

Izoabr, from izo-, → iso-, + abr → cloud.

One of several nuclides having the same number of neutrons in their nuclei but differing in the number of protons.

Isotone, from → iso- + tone, from Gk tonos "strain, tone, mode," literally, "a stretching," akin to teinein "to stretch," cognate with Pers. tanidan "to spin, twist, weave" (Mid.Pers. tanitan; Av. tan- "to stretch, extend;" cf. Skt. tan- to spin, stretch;" tanoti "stretches," tantram "loom;" Gk. teinein "to stretch, pull tight;" L. tendere "to stretch; PIE base *ten- "to stretch;" Pers. târ "string," tân "thread," tur "fishing net, net, snare," and tâl "thread" (Borujerdi dialect) belong to this family; variants tanta "cobweb," tanadu, tafen, kartané, kârtané, kâtené, Pashtu tanistah "cobweb;" cf. Skt. tantu- "cobweb, thread, string").

The function δⁱ_k of two variables i and j defined by δⁱ_k = 1 if i = j, and δⁱ_k = 0 if i ≠ j.

Leopold Kronecker (1823-1891), a German mathematician; delta, Gk. letter of alphabet.

The set of all directions in which a light signal can travel toward an event (past light cone) or from an event (future light cone).

→ light; → cone.

A → sedimentary rock composed principally of calcium carbonate. Limestone is usually formed from shells of once-living organisms or other organic processes, but may also form by inorganic precipitation.

→ lime; → stone.

The cone that confines the pressure disturbance created by a → supersonic object moving in a → compressible medium.

→ Mach number; → cone.

Describing something that is partially or totally occupied.

→ non; → empty.

A set containing at least → one → element.

→ nonempty; → set.

Math.: The fact that no → solution exists for a given → problem. In mathematics, nonexistence usually is a matter of impossibility.

→ non-; → existence.

1) The smallest whole number, symbol 1; unity. The → multiplicative identity for real and complex numbers.
2) Being or amounting to a single unit or individual or entire thing, item, or object rather than two or more; a single.

O.E. an, from P.Gmc. *ainaz (cf. O.N. einn, Dan. een, O.Fris. an, Du. een, Ger. ein, Goth. ains), PIE base *oinos, cognate with Pers. yek, as below.

Yek "one, alone," from Mid.Pers. êwak, êv, yak, êk; cf. Baluci hivak "one, single" (Proto-Iranian *aiua-ka-); O.Pers. aiva- "one, alone;" Av. aēuua- "one, alone;" Skt. éka- "one, alone, single;" Gk. oios "alone, lonely;" L. unus "one;" E. one.

A hypothetical flow in which all the flow parameters may be expressed as functions of time and one space coordinate only. This single space coordinate is usually the distance measured along the center-line of some conduit in which the fluid is flowing (B. Massey, Mechanics of Fluids, Taylor & Francis, 2006).

→ one; → dimensional; → flow.

A → magnetohydrodynamics treatment in which the → plasma consists only of one particle species and moves with the bulk speed. The thermal motion of the particles is neglected and thus there is no motion of particles relative to each other.

→ one; → fluid; → magnetohydrodynamics.

A relationship between two distinct sets of elements such that every member of the first set can be paired with a unique element in the second set; and every member of the second set can be paired with a unique element in the first set. Two sets so related are said to be isomorphic.

→ one; → correspondence.