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.: 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. 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.
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.
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.
Galactic habitable zone
zonâr-e zistpazir-e kahkešân
Fr.: zone habitable galactique
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.
habitable zone (HZ)
Fr.: zone habitable
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 creation 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, from O.Fr. habitation, from L. habitare "to live, dwell," frequentative of habere "to have, to hold, possess," from PIE base *ghrebh- "to seize, take, hold, have, give, receive" (cf. Mod.Pers. gereftan "to take, seize;" Mid.Pers. griftan; O.Pers./Av. grab- "to take, seize;" Skt. grah-, grabh- "to seize, take," graha "seizing, holding, perceiving;" M.L.G. grabben "to grab," from P.Gmc. *grab, E. grab "to take or grasp suddenly"); → zone.
Hayashi forbidden zone
zonâr-e baſkam-e Hayashi
Fr.: zone interdite de Hayashi
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.
iron convection zone (FeCZ)
zonâr-e hambaz-e âhan
Fr.: zone convective du fer
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).
A form of oxygen, O3, in which the molecule is made of three atoms instead of the usual two.
From Ger. Ozon, coined in 1840 by Ger. chemist Christian Friedrich Schönbein (1799-1868) from Gk. ozon, neute pr.p. of ozein "to smell." So called for its peculiar odor.
surâx-e ozon (#)
Fr.: trou d'ozone
Not really a "hole," but a region of exceptionally depleted ozone in the stratosphere over the Antarctic that happens at the beginning of Southern Hemisphere spring (August-October). It was first noticed in the 1970s by a research group from the British Antarctic Survey.
lâye-ye ozon (#)
Fr.: couche d'ozone
An atmospheric layer that contains a high proportion of oxygen that exists as ozone. It acts as a filtering mechanism against incoming ultraviolet radiation. It is located between the troposphere and the stratosphere, around 15 to 20 kilometers above the Earth's surface.
separ-e ozon (#)
Fr.: bouclier d'ozone
The ozone layer within the stratosphere that filters out potentially lethal intensities of ultraviolet radiation from the Sun.
→ ozone; shield, from O.E. scield, scild, related to sciell "seashell, eggshell," from P.Gmc. *skeldus (cf. Du. schild, Ger. Schild, Goth. skildus); PIE base *(s)kel- "to cut."
Separ "shield," from Mid.Pers. spar "shield;" cf. Skt. phalaka- "board, lath, leaf, shield," phálati "(he) splits;" Gk. aspalon "skin, hide," spolas "flayed skin," sphalassein "to cleave, to disrupt;" O.H.G. spaltan "to split;" Goth. spilda "board;" PIE base *(s)p(h)el- "to split, to break off;" → ozone.
partial ionization zone
zonâr-e yoneš-e pâri
Fr.: zone d'ionisation partielle
One of several zones of the stellar interior where increased → opacity can provide the → kappa mechanism to drive → pulsations. See also → Kramers' law. In these zones where the gases are partially ionized, part of the energy released during a layer's compression can be used for further ionization, rather than raising the temperature of the gas. Partial ionization zones modulate the flow of energy through the layers of the star and are the direct cause of → stellar pulsation. The partial ionization zones were first identified by the Russian astronomer Sergei A. Zhevakin (1916-2001) in the 1950s. In most stars there are two main ionization zones. The hydrogen partial ionization zone where both the ionization of neutral hydrogen (H ↔ H+ + e-) and the first ionization of helium (He ↔ He+ + e-) occurs in layers with a characteristic temperature of 1.5 x 104 K. The second, deeper zone is called the He+ partial ionization zone, and involves the second ionization of helium (He+↔ He++ + e-), which occurs deeper at a characteristic temperature of 4 x 104 K. The location of these ionization zones within the star determines its pulsational properties. In fact if the → effective temperature of the star is ≥ 7500 K, the pulsation is not active, because the ionization zones will be located very near to the surface. In this region the density is quite low and there is not enough mass available to drive the oscillations. This explains the blue (hot) edge of the instability strip on the → H-R diagram. Otherwise if a star's surface temperature is too low, ≤ 5500 K, the onset of efficient convection in its outer layers may dampen the oscillations. The red (cool) edge of the instability strip is believed to be the result of the damping effect of convection. He+ ionization is the driving agent in → Cepheids. See also → gamma mechanism.
Fr.: zone radiative
The region of a star in which the energy generated by → nuclear fusion in the core is transferred outward by → electromagnetic radiation and not by → convection. Such zones occur in the interior of low-mass stars, like the Sun, and the envelopes of → massive star. The radiative zone of the Sun starts at the edge of the core of the Sun, about 0.2 solar radii, and extends up to about 0.7 radii, just below the → convective zone.
zonâr-e zamân, zamân-zonâr
Fr.: fuseau horaire
Any of the 24 zones on the Earth surface delimited by → meridians at approximately 15° intervals. In each time zone a common standard time is used, and the time is one hour earlier than the zone immediately to the east.
Fr.: Zone torride
The part of the Earth's surface between the tropics of Cancer and Capricorn
Fr.: zone de Zhevakin
Named after the Russian astronomer Sergei A. Zhevakin (1916-2001), who identified these zone in the 1950s; → mechanism.
The portion of the surface of a sphere included between two parallel planes.
From L. zona "geographical belt, celestial zone," from Gk. zone "a belt," related to zonnynai "to gird," from PIE base *yes- "to gird, girdle;" cognate with Pers. parhun "circle," as below.
Zonâr most probably loan from from Gk., as above; it is related to Pers. parhun "circle," ultimately from Proto-Iranian *pari-iâhana- "girdle, belt," from pari-, variant pirâ-, → circum-, + iâhana- "to girdle," cf. Av. yâh- "to girdle." The Pers. pirâhan "shirt" is a variant of parhun.