Fr.: méthode d'Argelander
A technique to estimate the brightness of a → variable star. It involves comparing the variable with a sequence of neighboring stars of slightly different → magnitudes.
Friedrich Wilhelm Argelander (1799-1875), German astronomer. His most important work was his compilation of the Bonner Durchmusterung; → method.
Fr.: processus de Blandford-Zanjek
A mechanism for the extraction of energy from a rotating → Kerr black hole. It relies on the assumption that the material → accreted by a → black hole would probably be → magnetized and increasingly so as the material gets closer to the → event horizon. Since all black holes of current astrophysical interest are probably accreting from magnetized disks, this has led to suggestions that the Blandford-Znajek process plays a vital role in → active galactic nuclei (AGN) and other accreting black hole systems. The power, P, generated is given by: P = (4π/μ0) B2RS2c, where B is the → magnetic field of the → accretion disk, and RS is the → Schwarzschild radius of the black hole. As an example, for a 108 solar mass black hole with a 1 T magnetic field, the power generated is approximately 2.7 × 1038 W. In perspective, the annual energy consumption of the world is estimated around 5 × 1020 J. The example case presented produces more energy in a single second than the entire globe consumes in a year. While this is a bold claim to make, it is only an example case where not all the energy produced is extractable as useable energy. However, at that point, even a system which is less that < 10-15 % efficient would be sufficient to supply enough energy to power the world for a full year. Of course, the system itself is limited in its lifetime due to the extraction of energy by slowing down the rotation of the black hole. Hence, the system can only exist as long as the black hole has angular momentum, continuing to rotate. At some point, the rotation will cease and the energy source will be unusable (D. Nagasawa, PH240, Stanford University, Fall 2011).
Blandford, R. D., & Znajek, R. L., 1977, MNRAS 179, 433; → process.
Fr.: région montagneuse, hauts plateaux
A mountainous or elevated region; → plateau.
Kuhsâr "mountainous, hilly area," from kuh, → mountain, + -sâr suffix denoting profusion, abundance, variant -zâr, → catastrophe.
âdâk (#), âbxost (#), jaziré (#), tomb (#)
A tract of land completely surrounded by water, and not large enough to be called a → continent (Dictionary.com).
M.E. iland, from O.E. igland "island," from ieg "island;" PIE *akwa- "water," cf. Pers. âb, → water, + → land.
Âdâk, âdak, adak "island" (Dehxodâ), probably from Proto-Ir. *āpdaka-
"placed in water," from *âp-, → water, cf. Pers. âb,
+ *da- "to place, put," cf. Pers. dâdan "to give,"
→ thesis, + suffix *-ka.
The hypothesis first put forward by Immanuel Kant (1724-1804) according to which the objects termed "spiral nebulae" were stellar systems comparable to our own → Milky Way galaxy. At the end of the 18th century, William Herschel (1738-1822) using his giant reflectors discovered thousands of such nebulae. However, in spite of advances in observations it was never possible to prove Kant's idea until the second decade of the twentieth century. The observations using the Mount Wilson 2.50m (100 inch) telescope allowed Edwin Hubble in 1924 to firmly establish that the "spiral nebulae" were unquestionably extragalactic.
The term "island Universe" was first introduced by the German Alexander von Humboldt in 1850; → island; → Universe.
xoški (#), zamin (#)
Any part of the earth's surface not covered by a body of water.
M.E., from O.E. land, lond, "ground, soil, territory;" PIE base *lendh- "land, heath" (cf. O.N., O.Fris. Du., Ger., Goth. land; O.Ir. land; Welsh llan "enclosure, church," Breton lann "heath," source of Fr. lande; O.C.S. ledina "waste land, heath," Czech lada "fallow land").
Xoški, from xošk, → dry, + noun suffix -i; zamin "land, → earth."
nasim-e xoški (#)
Fr.: brise de terre
A coastal breeze blowing from land to sea after sunset, caused by the temperature difference when the sea surface is warmer than the adjacent land. The warmer air above the water continues to rise, and cooler air from over the land replaces it, creating a breeze.
Xoški "land," from xošk "dry;" Mid.Pers. xušk "dry;" O.Pers. uška- "mainland;" Av. huška- "dry;" cf. Skt. śuska- "dry, dried out;" Gk. auos "dry, dried up;" O.E. sēar "dried up, withered;" Lith. sausas "dry, barren."
Fr.: amortissement de Landau
The process wherein a → plasma gains energy at the expense of the → Langmuir wave. In the presence of the → Landau resonance, the particles in resonance moving slightly faster than the wave lose energy, while those moving slightly slower will gain energy. Since the Maxwellian distribution is decreasing with velocity, in a Maxwellian plasma, near the Landau resonance, there are more particles at lower velocities than at higher velocities. Also called collisionless damping.
Lev Landau (1908-1968), a prominent Soviet physicist, 1962 Nobel Prize in Physics for his development of a mathematical theory of → superfluidity; → damping.
Fr.: niveau de Landau
The → energy level which can be occupied by a → free electron in a → magnetic field.
→ Landau damping; → level.
Fr.: résonance de Landau
For parallel propagating → electrostatic waves in a → plasma, the → resonance which occurs when the particle velocity equals the parallel phase velocity of the wave.
→ Landau damping; → damping.
Fr.: facteur de Landé
The constant of proportionality relating the separations of lines of successive pairs of adjacent components of the levels of a spectral multiplet to the larger of the two J-values for the respective pairs. The interval between two successive components J and J + 1 is proportional to J + 1.
After Alfred Landé (1888-1976), a German-American physicist, known for his contributions to quantum theory; → facteur.
A → space probe designed to land on a → planet or other solid → celestial body.
Fr.: hauts plateaux lunaires
A light color area on the → Moon, as contrasted with → lunar maria. Also called terra.
Rosseland mean opacity
kederi-ye miyângin-e Rosseland
Fr.: opacité moyenne de Rosseland
The → opacity of a gas of given composition, temperature, and density averaged over the various wavelengths of the radiation being absorbed and scattered. The radiation is assumed to be in → thermal equilibrium with the gas, and hence have a → blackbody spectrum. Since → monochromatic opacity in stellar plasma has a complex frequency dependence, the Rosseland mean opacity facilitates the analysis. Denoted κR, it is defined by: 1/κR = (π/4σT3) ∫(1/kν) (∂B/∂T)νdν, summed from 0 to ∞, where σ is the → Stefan-Boltzmann constant, T temperature, B(T,ν) the → Planck function, and kν monochromatic opacity (See Rogers, F.J., Iglesias, C. A. Radiative atomic Rosseland mean opacity tables, 1992, ApJS 79, 507).
Named after Svein Rosseland (1894-1985), a Norwegian astrophysicist, who obtained the expression in 1924; → mean; → opacity.
Fr.: critère de Solberg-Høiland
A criterion for → convective stability in → massive stars. The Solberg-Høiland stability criterion corresponds to the inclusion of the effect of → rotation (variation of → centrifugal force) in the convective stability criterion. It is a combination of → Ledoux's criterion (or possibly → Schwarzschild's criterion) and → Rayleigh's criterion. Both the dynamical shear and Solberg-Høiland instabilities occur in the case of a very large → angular velocity decrease outwards. Therefore, in a → rotating star the Ledoux or Schwarzschild criteria for convective instability should be replaced by the Solberg-Høiland criterion. More specifically, this criterion accounts for the difference of the centrifugal force for an adiabatically displaced fluid element. It is also known as the axisymmetric baroclinic instability. It arises when the net force (gravity + buoyancy + centrifugal force) applied to a fluid parcel in an adiabatical displacement has components only in the direction of the displacement (A. Maeder, Physics, Formation and Evolution of Rotating Stars, 2009, Springer).
E. Høiland, 1939, On the Interpretation and Application of the
Circulation Theorems of V. Bjerknes. Archiv for mathematik og
naturvidenskab. B. XLII. Nr. 5. Oslo.