An Etymological Dictionary of Astronomy and Astrophysics
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The fastest → main sequence  → hypervelocity star (HVS) with a → heliocentric  → radial velocity of 1017 ±2.7 km s^-1. The star S5-HVS1 is an → A-type with a → luminosity of ~ 2.35 Msun located at a distance of ~ 9 kpc from the Sun. The current 3D velocity of the star in the Galactic frame is 1755 ± 50 km s^-1. When integrated backwards in time, the orbit of the star points unambiguously to the → Galactic Center, implying that S5-HVS1 was kicked away from → Sgr A* with a velocity of ~ 1800 km s^-1 and travelled for 4.8 Myr to its current location. This is so far the only HVS confidently associated with the Galactic Center. The ejection trajectory and transit time of S5-HVS1 coincide with the orbital plane and age of the annular disk of young stars at the Galactic Centre, and thus may be linked to its formation. With the S5-HVS1 ejection velocity being almost twice the velocity of other hypervelocity stars previously associated with the Galactic Center, the question arises whether they have been generated by the same mechanism or whether the ejection velocity distribution has been constant over time (Koposov, S.E., et al., 2019, arXiv:1907.11725).

S5, or S⁵, short for Southern Stellar Stream Spectroscopic Survey; HVS, → hypervelocity star (HVS).

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.

An almost horizontal region in the → CMB angular power spectrum belonging to a → multipole index 10 ≤ l ≤ 100. This feature is due to the → Sachs-Wolfe effect.

→ Sachs-Wolfe effect; → plateau.

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 (سعد الملک) "the lucky star of the ruler," for unknown reasons.

The star that lies at the center of → Cygnus's → Northern Cross. This F8 → supergiant is situated some 1,500 → light-years away and has an → apparent visual magnitude of 2.20.

From Ar. as-sadr (الصدر) "breast" (of the Cygnus).

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."

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.

→ Sagittarius.

One of the → spiral arms of the Milky Way Galaxy. It lies between the Sun and the the → Scutum-Crux arm. Also known as the Sagittarius-Carina Arm.

→ Sagittarius; → arm.

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.

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; → elliptical; → galaxy.

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.

→ Sagittarius; → dwarf; → irregular; → galaxy.

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.

An equation that gives the number of atoms of a given species in various stages of → ionization that exist in a gas in → thermal equilibrium as a function of the temperature, density, and ionization energies of the atoms.

Named after the Indian astrophysicist Megh Nad Saha (1894-1956), who first derived the equation in 1920; → equation.

A blue/violet light better seen at night on a pointed object, such as the mast of a ship or the wing of an airplane, during a → thunderstorm. The mast appears to be on fire but does not burn. It occurs when the ground below the storm is electrically charged, and there is high voltage in the air between the cloud and the ground. The high voltage causes the electrons and protons of the air molecules to be pulled away from each other, transforming the air into a glowing ionized gas. St. Elmo's fire is sometimes mistaken for → ball lightning. It was identified as an electrical phenomenon first by by Benjamin Franklin in 1749. Also called → corposant.

Saint Elmo the Italian rendering of St. Erasmus of Formiae (died 303) the patron saint of Mediterranean sailors; → fire.

Âtaš, → fire, sepant "saint, holy," → heiligenschein.

A → supergiant star of visual magnitude 2.06 and → spectral type B0.5 Ia marking the right knee of Orion. It is about 700 light-years away.

Saiph "sword," from Ar. as-saiph al-jabbâr (سیف الجبار) "the Sword of the Giant."

The three conditions that are necessary for the generation of a → baryon asymmetry in the → early Universe. These conditions are:
1) The → baryon number should not be → conserved.
2) The → charge conjugation and → CP symmetry should be → violated, and
3) Departure from → thermal equilibrium.

Named after Andrei Sakharov (1921-1989), who in 1967 described these three minimum conditions (A. D. Sakharov, 1967, Zh. Eksp. Teor. Fiz. Pis'ma 5, 32; 1967, JETP Lett. 91B, 24); → condition.

A → post-asymptotic giant branch star that in 1995 underwent sudden re-brightening due to a → helium shell flash, or → very late thermal pulse (VLTP), before embarking on a → white dwarf cooling track. Such an outburst is very rare, and in this case it is the first seen in modern times. Stellar outbursts observed in 1670 (nova CK Vul) and 1918 (nova V605 Aql) may have been caused by the same phenomenon. Since 1995, Sakurai's Object has undergone observable changes on time-scales of weeks to months. Several phases of dust production followed the outburst, with a deep optical minimum beginning in early 1999, such that any changes in the central star have since been inferred from radio and infrared observations. Subsequent observations and modeling have revealed much about the dust shell formation and the outer regions of the ejecta. This object is also the central star of an extended very faint planetary nebula (→ CSPN), confirming that the latest large mass ejection during the planetary nebula phase occurred several thousands years ago (see, e.g. H. L. Worters et al. 2009, MNRAS 393, 108 and references therein).

Named after Yukio Sakurai, a Japanese amateur astronomer, who serendipitously discovered it on February 20, 1996, when searching for comets; → object.

The first mathematical description of the → initial mass function (IMF) of newly formed stars of solar to → intermediate-masses. It is proportional to M ^-2.35, where M is the stellar mass. → Salpeter slope.

Named after the Austrian-Australian-American astrophysicist Edwin Ernest Salpeter (1924-2008); → function.

An equation describing how the nuclei of helium fuse together, in the interior of giant stars, to form carbon nuclei. → triple-alpha process.

Named after the Austrian-Australian-American astrophysicist Edwin Ernest Salpeter (1924-2008); → process.