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Fr.: astéroïde S
A moderately bright type of asteroids (albedo 0.10 to 0.22) consisting mainly of iron- and magnesium-silicates such as olivine and pyroxene. They are dominant in the inner main belt within 2.2 AU, common in the central belt within about 3 AU, but become rare further out. The largest is 15 Eunomia (about 330 km in its largest dimension).
Fr.: amas S
A → star cluster situated within an arcsecond, or 0.04 pc, from the → Galactic Center, in the vicinity of the → supermassive black hole Sgr A*. The cluster members are about 40 → main sequence → B-type stars with relatively high orbital → eccentricities (0.3 ≤ e&le 0.95). The most famous member of the S cluster is S2 because of its brightness and its fast orbital motion near Sgr A*. Same as Same as the Sgr A* cluster and S stars. See also other → Galactic center clusters (Figer et al. 2002, ApJ 581, 258; and 1999, ApJ 525, 750).
S Doradus star
setâre-ye S Zarrin-mâhi
Fr.: étoiles S Doradus
A type of massive, → blue supergiant, → variable star, also known as a → Hubble-Sandage variable or a → Luminous Blue Variable (LBV). S Doradus stars are the most luminous stars in the Galaxy and are easily identified in other nearby galaxies. They are named after the prototype, S Doradus, in the → Large Magellanic Cloud.
Fr.: étoile de type S
A → red giant of → spectral type S whose spectrum is dominated by → molecular bands arising from → zirconium → oxide (ZrO). S stars also have strong → cyanogen bands and contain spectral lines of → lithium and → technetium. Almost all S stars are → long-period variables.
S, letter of alphabet; → star.
Fr.: processus s
A → nucleosynthesis process by which → chemical elements heavier than → copper are formed through a slow flux of → neutrons absorbed by atomic nuclei (→ neutron-capture element). The → capture of neutrons occurs on time scales that are long enough to enable unstable nuclei to decay via the emission of a → beta particle before absorbing another neutron. Prominent s-process elements include → barium, → zirconium, and → yttrium. See also: → r-process.
sayârak-e gune-ye S
Fr.: astéroïde de type S
A type of → asteroid containing → pyroxene and → olivine silicates, probably mixed with metallic iron, similar to → stony meteorites. S-type asteroids show high albedo of 0.10-0.22. They include about 17% of known asteroids and occupy the inner → asteroid belt.
setâre-ye gune-ye S
Fr.: étoile de type S
Same as → S star.
Fr.: onde S
→ shear wave.
Fr.: effet de Sachs-Wolfe
The effect of → gravitational potentials on the → anisotropy of the → cosmic microwave background radiation, in which photons from the → CMB are gravitationally → redshifted, causing the CMB spectrum to appear uneven. This effect is the predominant source of fluctuations in the CMB for angular scales above about 10 degrees. It involves two parts: the effect of the potential at the → surface of last scattering, which is the ordinary Sachs-Wolfe effect. And the integrated Sachs-Wolfe (ISW) effec, which is caused by the time variation of gravitational potentials as the photons travel through them. A photon traveling through a decaying → potential well (wall) gains (loses) energy. Without → dark energy the photon is → blueshifted and then → redshifted, so that both effects compensate each other. On the other hand, in an → accelerating Universe driven by dark energy the photon gets more blueshifted. See also → Rees-Sciama effect.
Rainer Kurt Sachs (1932- ) & Arthur Michael Wolfe (1939- ), 1967, ApJ 147, 73; → effect.
Fr.: plateau de Sachs-Wolfe
Sadalmelik (α Aquarii)
A supergiant star of type G2 Ib situated in the constellation → Aquarius. At a distance of 750 light-years, it has a luminosity 3000 times that of the Sun, and a diameter about 60 times the solar diameter. Variant designations: Sadalmelek; Sadlamulk; El Melik; Saad el Melik.
From Ar. Sa'd al-Malik (
Sadr (γ Cygni)
From Ar. as-sadr (
The Arrow. A very small → constellation, in fact the third smallest constellation in the sky, lying south of → Vulpecula, and north of → Aquila. The constellation contains the prototype → WZ Sagittae star and M71 (NGC 6838), formerly thought to be an → open cluster but now considered to be a → globular cluster of low condensation. Its brightest star α Sge is a yellow bright → giant of → apparent magnitude +4.37 and → spectral type G1 II about 475 → light-years from Earth. Abbreviation: Sge; Genitive: Sagittae.
From L. sagitta "arrow."
Peykân, → arrow.
The Archer. A large constellation belonging to the → Zodiac, situated between → Scorpius and → Capricorn. It is located in the southern hemisphere at approximately 19h right ascension, 25° south declination. The constellation, part of which lies in the → Milky Way, contains the → Trifid Nebula, → Lagoon nebula, star clusters, and globular clusters. The center of the Galaxy lies in the direction of Sagittarius. Abbreviation: Sgr; Genitive: Sagittarii.
From L. sagittarius "archer," literally "pertaining to arrows," from → sagitta "arrow" + -arius "-ary." In Gk. mythology, Sagittarius is identified as a centaur, half human, half horse. In some legends, the Centaur Chiron was the son of Philyra and Saturn, who was said to have changed himself into a horse to escape his jealous wife, Rhea. Chiron was eventually immortalized in the constellation of → Centaurus, or in some version, Sagittarius.
Nimasb, from Mid.Pers. nêmasp "centaur, Sagittarius," from nêm, nêmag "mid-, half" (Mod.Pers. nim); Av. naēma- "half;" cf. Skt. néma- "half" + asp "horse" (Mod.Pers. asb); O.Pers. asa- "horse;" Av. aspa- "horse," aspā- "mare," aspaiia- "pertaining to the horse;" cf. Skt. áśva- "horse, steed;" Gk. hippos; L. equus; O.Ir. ech; Goth. aihwa-; O.E. eoh "horse;" PIE base *ekwo- "horse."
Sagittarius A (Sgr A)
Fr.: Sagittarius A
A strong radio source at the center of our Galaxy. It is a complex object with three components: Sgr A West is a thermal radio source made of several dust and gas clouds, which orbit → Sgr A* and fall onto it at velocities as high as 1000 km per second. Sgr A East is a → non-thermal source, about 25 → light-years across, that appears to be a → supernova remnant. Sgr A* is the most plausible candidate for the location of a Galactic → supermassive black hole with a mass of about 4 million → solar masses.
Fr.: bras du Sagittaire
Fr.: Sagittarius B2
A massive (3 × 106 → solar masses), dense (up to 108 particles per cm3) → H II region and → molecular cloud complex located near the → Galactic center (about 390 → light-years from it) and about 26,000 light-years from Earth. This complex is one of the largest in the → Milky Way, spanning a region about 150 light-years across. The mean → hydrogen → density within the cloud is 3,000 atoms per cm3, which is about 20-40 times denser than a typical molecular cloud. It is the richest molecular source in the Galaxy in which many different types of → interstellar molecule have been identified, including glycine, the simplest amino acid, and the sugar molecule glycoaldehyde.
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.: effet Sagnac
The → phase difference between two light waves moving in opposite directions along a closed circular loop when the loop is rotating. More specifically, consider a beam of light split into two beams which are then allowed to propagate in two opposite directions along the rim of a rotating disk. When they are recombined, a phase difference occurs between them. The position of the → interference fringes is dependent on the → angular velocity of the setup. This → relativistic effect illustrates the impossibility of synchronizing clocks situated in a rotating → reference frame, as described by Einstein in 1905. The Sagnac effect is used, for example, in optical gyroscopes installed in airplanes or in devices used for measuring the Earth rotation. The Sagnac effect is very important for the correct working of the → Global Positioning System.
Named after Georges Sagnac (1869-1928), French physicist, who discovered the phenomenon in 1913; → effect.