Fr.: naine L
A type of → brown dwarf with an → effective temperature ranging from about 2200 K to about 1300 K, corresponding to luminosities about 4 x 10-4 to 3 x 10-5 times that of the Sun. L dwarfs are intermediate in temperature between M and → T dwarfs. Their spectra in the optical show weak titanium oxide (TiO) and vanadium oxide (VO) absorption lines and strong metallic hydrides CrH (8611 and 9969 Å) and FeH (8692 and 9896 Å). Also are present strong neutral atomic lines of alkali metals Na I (8183, 8195 Å), K I (7665, 7699 Å), Rb I (7800, 7948 Å), Cs I (8521, 8943 Å), and sometimes Li I (6708 Å). The prototype of the L-dwarf class is → GD 165B. The spectral classification was first defined by Kirkpatrick et al. 1999, ApJ 519, 802 and Martin et al. 1999, AJ 118, 2466.
For the reasoning behind the choice of the letter L, see Kirkpatrick et al. 1993, ApJ 406, 701; → dwarf.
Moderately warm; tepid.
M.E. lukewarme "tepid," from luke "tepid," of unknown origin, + → warm.
Velarm "lukewarm, tepid," of unknown origin.
Fr.: naine M
A star whose spectrum is dominated by the absorption bands of → titanium oxide (TiO) and → vanadium oxide (VO) and has many neutral metal lines. The → effective temperature of M dwarfs ranges from about 3850 to 2600 K. They are low mass stars with masses ranging from 0.6 times that of the Sun at spectral type M0 to less than 0.1 → solar masses. M dwarfs are very abundant, they account for about 70-80% of stars in the → Galactic disk. The nearest star to the Sun, → Proxima Centauri, is an M dwarf.
Fr.: essaim de météoroïdes
A relatively dense collection of meteoroids at certain spots along some → meteoroid streams.
OB subdwarf (sdOB)
Fr.: sous-naine OB
kutule-ye sorx (#)
Fr.: naine rouge
A small, cool, very faint, main sequence star whose surface temperature is under about 3500 K. Red dwarfs generally have masses of less than one-third that of the Sun. In the neighbourhood of the Sun the majority of stars are red dwarfs.
Sagittarius Dwarf Elliptical Galaxy
kahkešân-e kutule-ye beyzigun-e nimasb
Fr.: galaxie naine elliptique du Sagittaire
A satellite galaxy of the Milky Way discovered only in 1994 since most of it is obscured by the Galactic disc. At only 50,000 light years distant from our Galaxy's core, it is travelling in a polar orbit around the Galaxy. Our Galaxy is slowly devouring it, as evidenced by a filament which stretches around the Milky Way's core like a gossamer loop. It is only about 10,000 light-years in diameter, in comparison to the Milky Way's diameter of 100,000 light years. It is populated by old yellowish stars has four known globular clusters: M54, Arp 2, Terzan 7, and Terzan 8. It should not be confused with the → Sagittarius Dwarf Irregular Galaxy.
Sagittarius Dwarf Irregular Galaxy
kahkešân-e kutule-ye bisâmân-e Nimasb
Fr.: galaxie naine irrégulière du Sagittaire
A dwarf irregular galaxy, discovered in 1977, that is a member of the Local Group of galaxies. It has a diameter of 1,500 light-years and lies about 3.5 million light-years away. SagDIG contains as much as about 108 solar masses of H I gas and is one of the most metal-poor galaxies. It should not be confused with the → Sagittarius Dwarf Elliptical Galaxy.
Fr.: barrière de Schwarzschild
An upper theoretical limit to the → eccentricity of orbits near a → supermassive black hole (SBH). It results from the impact of → relativistic precession on the stellar orbits. This phenomenon acts in such a way as to "repel" inspiralling bodies from the eccentric orbits that would otherwise lead to capture as → extreme mass ratio inspiral (EMRI)s. In other words, the presence of the Schwarzschild barrier reduces the frequency of EMRI events, in contrast to that predicted from → resonant relaxation. Resonant relaxation relies on the orbits having commensurate radial and azimuthal frequencies, so they remain in fixed planes over multiple orbits. In the strong-field potential of a massive object, orbits are no longer Keplerian but undergo significant perihelion precession. Resonant relaxation is only efficient in the regime where precession is negligible. The Schwarzschild barrier refers to the boundary between orbits with and without significant precession. Inside this point resonant relaxation is strongly quenched, potentially reducing inspiral rates.
Schwarzschild black hole
Fr.: trou noir de Schwarzschild
Karl Schwarzschild (1873-1916), German mathematical physicist, who carried out the first relativistic study of black holes. → black hole.
Fr.: métrique de Schwarzschild
Fr.: rayon de Schwarzschild
The critical radius at which a massive body becomes a → black hole, i.e., at which light is unable to escape to infinity: Rs = 2GM / c2, where G is the → gravitational constant, M is the mass, and c the → speed of light. The fomula can be approximated to Rs≅ 3 x (M/Msun), in km. Therefore, the Schwarzschild radius for Sun is about 3 km and for Earth about 1 cm.
Fr.: singularité de Schwarzschild
Fr.: solution de Schwarzschild
Briefly following Einstein's publication of → General Relativity, Karl Schwarzschild discovered this solution in 1916 (Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften zu Berlin, Phys.-Math. Klasse, 189); → Schwarzschild black hole.
Fr.: critère de Schwarzschild
The condition in stellar interior under which → convection occurs. It is expressed as: |dT/dr|ad < |dT/dr|rad, where the indices ad and rad stand for adiabatic and radiative respectively. This condition can also be expressed as: ∇ad<∇rad, where ∇ = d lnT / d lnP = P dT / T dP with T and P denoting temperature and pressure respectively. More explicitly, in order for convection to occur the adiabatic temperature gradient should be smaller than the actual temperature gradient of the surrounding gas, which is given by the radiative temperature gradient if convection does not occur. Suppose a hotter → convective cell or gas bubble rises accidentally by a small distance in height. It gets into a layer with a lower gas pressure and therefore expands. Without any heat exchange with the surrounding medium it expands and cools adiabatically. If during this rise and → adiabatic expansion the change in temperature is smaller than in the medium the gas bubble remains hotter than the medium. The expansion of the gas bubble, adjusting to the pressure of the medium, happens very fast, with the speed of sound. It is therefore assumed that the pressure in the gas bubble and in the surroundings is the same and therefore the higher temperature gas bubble will have a lower density than the surrounding gas. The → buoyancy force will therefore accelerate it upward. This always occurs if the adiabatic change of temperature during expansion is smaller than the change of temperature with gas pressure in the surroundings. It is assumed that the mean molecular weight is the same in the rising bubble and the medium. See also → Ledoux's criterion; → mixing length.
Named after Karl Schwarzschild (1873-1916), German mathematical physicist (1906 Göttinger Nachrichten No 1, 41); → criterion.
Sculptor Dwarf Elliptical Galaxy
kahkešân-e kutule-ye beyzigun-e Peykartarâš
Fr.: galaxie naine elliptique du Sculpteur
A → dwarf elliptical galaxy that is a satellite of our → Milky Way. It lies about 285,000 → light-years away in the constellation → Sculptor, and has an → absolute magnitude of -11.28 and a mass of about 3 million → solar masses. The Sculptor Dwarf is a → metal-deficient galaxy containing only 4 percent of the oxygen and carbon elements in our own Galaxy.
Fr.: connaissance de soi
A general term used to describe a collection of computer programs, procedures, and documentation that perform some tasks on a computer system. → hardware.
→ soft + ware, from M.E., from O.E. waru, from P.Gmc. *waro (cf. Swed. vara, Dan. vare, M.Du. were, Du. waar, Ger. Ware "goods").
Narm, → soft + afzâr "instrument, means, tool," from Mid.Pers. afzâr, abzâr, awzâr "instrument, means," Proto-Iranian *abi-cāra- or *upa-cāra-, from cāra-, cf. Av. cārā- "instrument, device, means" (Mid.Pers. câr, cârag "means, remedy;" loaned into Arm. aucar, aucan "instrument, remedy;" Mod.Pers. câré "remedy, cure, help"), from kar- "to do, make, build;" kərənaoiti "he makes" (Pers. kardan, kard- "to do, to make"); cf. Skt. kr- "to do, to make," krnoti "he makes, he does," karoti "he makes, he does," karma "act, deed;" PIE base kwer- "to do, to make").
Fr.: architecture de logiciel
The overall structure of a software system consisting of mutually dependent components that create a logical whole.
Space Situational Awareness (SSA)
âgâhi az siteš-e fazâyi
Fr.: surveillance de l'environnement spatial
A program aimed at monitoring the near-Earth environment for recognizing and preventing space hazards by means of radar and optical observations from either space or the ground. The objective of the → European Space Agency initiative is to support the European independent utilization of, and access to, space for research or services, through the provision of timely and quality data, information, services and knowledge regarding the space environment, the threats and the sustainable exploitation of the outer space surrounding our planet Earth. The SSA Program was authorized at the November 2008 Ministerial Council and formally launched on 1 January 2009. The mandate was extended at the 2012 and 2016 Ministerial Councils, and the program is funded through to 2020. The program comprises three segments: 1) Space Surveillance and Tracking (SST), which is the monitoring and tracking of every object orbiting the Earth, such as satellites, space stations and debris. The objective is the prediction and warning of collisions and re-entry events. 2) → Space Weather (SWE), which aims at detection and forecasting of space weather and its effects through monitoring of the Sun, solar wind, magnetosphere, radiation belts, ionosphere and disturbances in the geomagnetic field. 3) → Near-Earth Objects (NEOs), which provides warning services against potential asteroid impact risks, including discovery, identification, orbit prediction and civil alert capabilities.