Alcyone (η Tauri)
Alkuone, Nayyer-e Sorayyâ, Raxšân-e Parvin
In Gk. mythology, a daughter of Aelous who, with her husband, Ceyx, was transformed into a kingfisher.
Nayyer-e Sorayyâ "the bight of the Pleiades," from Ar. nayyir
"luminous" + Thorayyâ "the Pleiades".
Meteo.: A weather phenomenon associated with atmospheric high pressure. In the Northern Hemisphere an anticyclone rotates in the clockwise direction. The rotation is caused by the movement of colder higher pressure air that is moving away from the poles toward the equator being affected by the rotation of the Earth.
Fr.: zone aurorale
A roughly circular band around either geomagnetic pole above which there is a maximum of auroral activity. It lies about 10 to 15° of geomagnetic latitude from the geomagnetic poles.
Fr.: composante décalée vers le bleu
A constituent of a composite astronomical object which has a motion directed towards the observer, as revealed by its spectrum.
Fr.: zone de Brillouin
Crystallography: One of the several regions which, in reciprocal space, represent the solution of the wave equations for the propagation of → phonons or electrons in solids. The first Brillouin zone is the Wigner-Seitz cell of the reciprocal lattice. It is a polyhedron obtained by connecting a lattice point to its first neighbors and drawing the planes perpendicular to these connecting lines and passing through their midpoints. The second Brillouin zone is obtained by a similar construction but the second-nearest neighbours.
After Léon Brillouin (1889-1969) French physicist; → zone.
Central Molecular Zone (CMZ)
zonâr-e molekuli-ye markazi
Fr.: zone moléculaire centrale
A vast, turbulent region encircling the → Milky Way's nucleus that contains a large fraction of the → Galaxy's dense → molecular clouds and → star formation regions. Spanning -1 to +1.5 degrees of → Sgr A*, the CMZ is about 400 pc × 100 pc in size and contains at least 107→ solar masses of → giant molecular clouds, approximately 10% of the Galaxy's molecular gas. The gas in the CMZ is at higher temperature than typical giant molecular clouds and has high velocity dispersion reflecting the → turbulent nature of the gas in the area. Despite these extensive molecular reserves, the → star formation rate within the CMZ is actually lower than expected based on the analysis of nearby → star-forming regions in the quiescent → Galactic disk. A common assumption is that this is a result of the extreme conditions within the CMZ, where the density, pressure, temperature, → velocity dispersion and → radiation field are all significantly greater than elsewhere in the Milky Way (Clark et al. 2018, The Messenger 173, 22 and references therein).
circumstellar habitable zone
zonâr-e zistpazir-e pirâsetâreyi
Fr.: zone habitable circumstellaire
A zone around a star within which a planet can have temperatures that permit liquid water, depending on the luminosity of the star and the distance of the planet from it.
A constituent part; an individual part of an organized whole.
L. componentem, pr.p. of componere "to put together," → compose.
A solid bounded by a plane and the surface generated by a straight line which always touches a simple closed curve on the plane and passes through a fixed point not on the plane.
L. conus "a wedge, peak, cone," from Gk. konos "cone, spinning top, pine cone," from PIE base *ko(n)- "to sharpen."
Maxrut from Ar.
Fr.: zone de convection
Same as → convective zone.
Fr.: zone convective
A zone inside a star in which → convection takes place; the central → core in → massive stars and the zone underneath → photosphere in low mass stars like the Sun. Recent works predict the presence of a thin convection zone in sufficiently massive main sequence stars which originates from the iron opacity and partial helium ionization. See also → iron convection zone.
Any circulatory wind system in the atmosphere in which the motion is anticlockwise in the northern hemisphere (that is in the same sense as that of Earth) and clockwise in the southern hemisphere, around a center of low pressure.
From Gk. kyklon "moving in a circle, whirling around," pr.p. of kykloun "move in a circle, whirl," from kyklos "circle," cognate with Pers. carx→ cycle.
Dione (Saturn IV)
The fourth largest moon of Saturn and the second densest after Titan. Its diameter is 1,120 km and its orbit 377,400 km from Saturn. It is composed primarily of water ice but must have a considerable fraction of denser material like silicate rock.
Discovered in 1684 by Jean-Dominique Cassini, Italian born French astronomer (1625-1712). In Gk. mythology Dione was the mother of Aphrodite (Venus) by Zeus (Jupiter).
discrete absorption component (DAC)
hamne-ye daršami-ye gosasté
Fr.: composante d'absorption discrète
The rapid, systematic changes in the absorption parts of the → P Cygni profiles of the ultraviolet → resonance lines (Si IV, C IV, and N V) observed in a majority of massive → hot stars. DACs are typically seen to accelerate to the → blue wing of the profile over a few days, becoming narrower as they approach the → terminal velocity.
Math.: A symbol or number placed above and after another symbol or number (called the base) to denote the power to which the latter is to be raised. Examples: n in the expresseion an; 3 in the expression 23.
From L. exponentem, pr.p. of exponere "put forth, explain," from → ex- "forth" + ponere "to put, to place."
xam-e nemâyi (#)
Fr.: courbe exponentielle
A curve that represents an → exponential function.
Fr.: équation exponentielle
An equation in which unknowns appear as exponents. Examples: 23x + 1 = 32.
Fr.: fonction exponentielle
A function in the form of y = bx defined for every → real number x, with positive base b > 1.
In an exponential manner.