An Etymological Dictionary of Astronomy and Astrophysics

The only supernova seen to date in the Andromeda galaxy and the first supernova observed beyond our own Galaxy. It was recorded on Aug. 20, 1885, by Ernst Hartwig (1851-1923) at Dorpat Observatory (Tartu) in Estonia and independently by other astronomers. S Andromedae reached magnitude 6 between Aug. 17 and 20, and had faded to magnitude 16 by February 1890. It is now believed that S Andromedae was a Type Ia supernova. Also known as SN 1885A.

See also: S, from the second variable star to be discovered in constellation → Andromeda

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).

See also: S stands for silicaceous, → silicate;
→ asteroid.

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).

See also: S, because of proximity to → Sgr A*; → cluster.

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.

See also: → Dorado; → star.

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.

See also: S, letter of alphabet; → star.

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.

See also: s stands for → slow; → process.

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.

See also: S for → stone; → type; → asteroid.

Same as → S star.

See also: S, letter of alphabet; → type; → star.

→ shear wave.

See also: S, referring to → shear; → wave.

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).

See also: 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.

See also: 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.

See also: → 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.

See also: 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.

See also: 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.

Etymology (EN): From L. sagitta “arrow.”

Etymology (PE): 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.

Etymology (EN): 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.

Etymology (PE): 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.

See also: → 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.

See also: → 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.

See also: → 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.

See also: → 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.

See also: → 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.

See also: 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.

See also: 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.

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

Etymology (PE): 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.

See also: 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.

See also: 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).

See also: 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.

See also: 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.

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

The value of the exponent in the → initial mass function as
derived by Salpeter (1955) for solar to → intermediate mass stars in the Solar neighborhood: &Gamma = 1.35 or &alpha = 2.35,
or x = -1.35. Also known as Sapleter index.

See also: → Salpeter function; → slope.

1. A crystalline compound, sodium chloride, NaCl, occurring as a mineral, used for food seasoning and preservation.
   
2. Chem.: A solid compound formed when the hydrogen of an acid has been replaced by a metal.

Etymology (EN): O.E. sealt; cf. O.N., O.Fris., Goth. salt, Du. zout, Ger. Salz from PIE *sal- “salt;” cf.
Gk. hals (genitive halos) “salt, sea;” L. sal; O.Ir. salann; Welsh halen;
O.C.S. sali “salt.”

Etymology (PE): Namak “salt;” Mid.Pers. namak “salt.”

Oceanography: One of several alternating columns of rising and descending water resulting from a → mixing process that occurs when warm salty water overlies a colder and relatively fresher layer of water. If the overlying salty water loses enough heat, it sinks down into the colder, fresher water, lengthening into a finger of salty water. Becuse the finger loses heat faster than it loses salt, the salt finger will continue to sink (salty water is denser than fresh water of the same temperature). Hence the salt finger loses more heat and displaces the colder water around it, which rises up and mixes into the warm salty layer above. Salt fingers are an example of → double-diffusive convection and play an important role in oceanic mixing. See also → fingering instability, → fingering convection.

See also: → salt; → finger.

A chemical compound, potassium nitrate, KNO₃. It is a naturally occurring mineral source of nitrogen, and is used in the manufacture of fireworks, fluxes, gunpowder, etc.

Etymology (EN): M.E. sal peter, salpetre, from O.Fr. salpetre, from M.L. sal petrae “salt of rock,” from L. sal, → salt

• petra “rock, stone.”

Etymology (PE): Šuré, related to šur “salty;” Mid.Pers. šôr “salty,” šorag “salt land;” cf. Skt. ksurá- “razor, sharp knife;” Gk. ksuron “razor;” PIE base *kseu- “to rub, whet.”

Statistics: A portion of the units of a population. The units are selected based on a randomized process with a known probability of selection. The sample is used to make inferences about the population by examining or measuring the units in the sample. → specimen = nemuné (نمونه).

Etymology (EN): M.E., from O.Fr. essample, from L. exemplum “a sample,” literally “that which is taken out,” from eximere “to take out, remove.”

Etymology (PE): Nemunân, from nemun, from nemudan “to show;” Mid.Pers. nimūdan, nimây- “to show,” from O.Pers./Av. ni- “down; into,” → ni- (PIE), + māy- “to measure;” cf. Skt. mati “measures,” matra- “measure;”
Gk. metron “measure;” L. metrum; PIE base *me- “to measure.”

Statistics: Each possible outcome in a → sample space.

See also: → sample; → point.

The number of sampling units which are to be included in the sample. In the case of a multi-stage sample this number refers to the number of units at the final stage in the sampling.

See also: → sample; → size.

Statistics: A set which consists of all possible outcomes of a random experiment.

See also: → sample; → space.

The act, process, or technique of selecting a number of cases from all the cases in a particular population.

Etymology (EN): → sample + → -ing.

Etymology (PE): Nemunân-giri, literally “taking sample,” from nemunân→ sample + giri verbal noun of gereftan “to take, seize, hold;” Mid.Pers. griftan, gir- “to take, hold, restrain;” O.Pers./Av. grab- “to take, seize,”
cf. 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;” PIE *ghrebh- “to seize.”

That part of the difference between the expected value of the sample estimator and the true value of the characteristic which results from the sampling procedure, the estimating procedure, or their combination.

See also: → sampling; → bias.

That part of the difference between a population value and an estimate thereof, derived from a random sample, which is due to the fact that only a sample of values is observed; as distinct from errors due to imperfect selection, bias in response or estimation, errors of observation and recording, etc.

See also: → sampling; → error.

Same as → Nyquist-Shannon sampling theorem.

See also: → sampling; → theorem.

One of the units into which an aggregate is divided for the purpose of sampling, each unit being regarded as individual and indivisible when the selection is made.

See also: → sampling; → unit.

Hard granular powder, consisting of fine grains of rock or minerals, usually quartz fragments, found on beaches, in deserts, and in soil.

Etymology (EN): O.E. sand; cf. O.N. sandr, O.Fris. sond, M.Du. sant, Ger. Sand; PIE base *samatha- (cf. Gk. psammos “sand,” L. sabulum).

Etymology (PE): Mâsé “sand,” of unknown origin.

Variously colored → sedimentary rock composed mainly of sand-like quartz grains cemented by calcite, clay, or iron oxide.
The sand accumulated originally underwater in shallow seas or lakes, or on the ground along shorelines or in desert regions.

See also: → sand; → stone.

A strong wind carrying sand through the air.

See also: → sand; → storm.

A deep → objective prism survey of the → Large Magellanic Cloud carried out with the Curtis Schmidt telescope on Cerro Tololo in Chile. A total of 1272 stars, generally brighter than → photographic magnitude ~ 14, are listed in the catalog as proven or probable LMC members. The stars are identified on the charts in the LMC Atlas of Hodge & Wright (1967).

See also: By Nicholas Sanduleak (1933-1990), American astronomer, published in 1970 as Contribution No. 89 of the Cerro Tololo Inter-American Observatory; → catalog.

One of the 5 or 6 → epagomenal days added to the 12 months of 30 days each in the → French Republican Calendar. Sansculottides began on September 17 or 18 and approximately ended on the → autumnal equinox, on September 22 or 23 of the → Gregorian calendar. These days were kept as festivals of Virtue, Genius, Labor, Opinion, and Rewards. There was a sixth Sanculottide, called Revolution, in → leap years.

See also: From Fr. sans-culotte, literally “without knee breeches,”
a revolutionary of the lower class in the French revolution. The appellation was originally a term of contempt applied by the aristocrats but later was adopted as a popular name by the French revolutionaries. It refers to the fashionable culottes (silk knee breeches) of the aristocrats as distinguished from the working class sans-culottes, who traditionally wore pantalons (long trousers).

A general whole-sky catalog compiled by the Smithsonian Astrophysical Observatory which results from the combination of several earlier catalogs.
The compilation gives positions and proper motions for 258,997 stars, of which 8,712 are double and 499 variable, with an average distribution of 6 stars per square degree. The star positions have an average standard deviation of 0’’.2 at their original epochs (0’’.5 at epoch 1963.5). The equinox is 1950.0 and the system that of the FK4.

Etymology (EN): SAO acrynome of the Smithsonian Astrophysical Observatory;
→ star; → catalog.

The period of 223 → synodic month, equaling 6585.32 days or 18 years, 11.33 days, after which the Sun, Earth, and Moon return to approximately the same relative geometry. When two eclipses are separated by a period of one Saros, they occur at the same node with the Moon at nearly the same distance from Earth and at the same time of year. Thus, the Saros is a useful tool for organizing eclipses into families or series. Each series typically lasts 12 or 13 centuries and contains 70 or more eclipses (F. Espenak, NASA).

See also: Gk. saros, from Akkadian shār, Sumerian shar “multitude, large number.”

The ancient astronomers knew the Saros cycle, but they did not use the term Saros. In the Almagest, Ptolemy refers to the Saros as the “periodic time” (periodikos chronos) and gives it the following properties: 223 → synodic months = 239 → anomalistic months = 242 → draconistic months = 6,585 ¹/₃ days = 241 revolutions in longitude plus 10 ²/₃ degrees. Edmund Halley seems to have been the first to apply this term to an eclipse cycle, in 1691.

1. A body that revolves around a planet; a moon. → Galilean satellite; → regular satellite; → irregular satellite.
   

2. Something that depends on, accompanies, or serves something else. → satellite galaxy; → satellite line.

Etymology (EN): From M.Fr. satellite, from L. satellitem “attendant.”

Etymology (PE): 1) Mâhvâré, from mâh, → moon, + -vâré, -vâr similarity suffix.
2) Bandevâr, from bandé “bound, fastened; (devoted) servant, domestic;” Mid.Pers. bandag,
from bastan, band-, vastan “to bind, shut” (O.Pers./Av. band- “to bind, fetter,” banda- “band, tie” (cf.
Skt. bandh- “to bind, tie, fasten;” PIE *bhendh- “to bind;” Ger. binden; E. bind).

A galaxy that orbits a larger one due to gravitational attraction. The Milky Way has at least ten satellite galaxies: the Large Magellanic Cloud, the Small Magellanic Cloud, Ursa Minor Dwarf, Draco Dwarf, Sculptor Dwarf, Sextans Dwarf, Carina Dwarf, Fornax Dwarf, Ursa Major I, and → Sagittarius Dwarf Elliptical Galaxy.

See also: → satellite; → galaxy.

Radio astro.: Of an OH source, which emits at 1665 and 1667 MHz as the main frequencies, one of the lines arising from transitions at 1612 and 1730 MHz.

See also: → satellite; → line.

1. Chem.: To add as much of a liquid, solid, or gas to a solution as it can absorb at a given temperature.
   
2. To fill something with so many things that no more can be added.

Etymology (EN): From L. saturatus, p.p. of saturare “to fill full, sate, drench,” from satur “sated, full,” from PIE base *sā- “to satisfy.”

Etymology (PE): Anjâlidan “to saturate, to fill” (Dehxodâ, Steingass), ultimately from Proto-Iranian *ham-gar-, from *ham- “together,” denoting “much, many,”
→ syn-, + *gar- “to soak, moisten;” cf. Sogdian wγyr- “to soak, steep,” from *aua-gar-, from which derives Pers. âqâridan, âqeštan “to steep, soak; mix.”

1. Chem.: The qualifier of a solution that has as much solute as possible.
   

2. (Of colors) Of maximum chroma or purity.

See also: Past participle of → saturate (v..

Air that contains the maximum amount of → water vapor that is possible at the given → temperature and → pressure, i.e. air in which the → relative humidity is 100%.

See also: → saturated; → air.

A liquid whose temperature and pressure are such that any decrease in pressure without change in temperature causes it to boil.

Etymology (EN): → saturate; → liquid.

A solution which can exist in equilibrium with excess of solute. The saturation concentration is a function of the temperature.

Etymology (EN): → saturate; → solution.

A vapor at the pressure and temperature at which it can exist in dynamical equilibrium with its liquid. Any compression of its volume at constant temperature causes it to condense to liquid at a rate sufficient to maintain a constant pressure. The term “saturated” is a misnomer, since it does not have the same meaning as a → saturated solution in chemistry. There is no question of one substance being dissolved in another.

Etymology (EN): → saturate; → vapor.

Physics: Degree of magnetization of a substance which cannot be exceeded however strong the applied magnetizing field.
Detectors: The condition of a detector or a pixel when it is submitted to a signal so strong that it cannot handle it properly; the result is a non-linear, useless response.

See also: Verbal noun of → saturate.

The maximum current that can be obtained in a specific circuit under specified conditions.

See also: → saturation; → current.

The maximum intrinsic magnetic induction possible in a material.

See also: → saturation; → induction.

In radar, a signal whose amplitude is greater than the dynamic range of the receiving system.

See also: → saturation; → signal.

The sixth → planet from the Sun and the second largest with an equatorial diameter of 120,536 km orbiting at an average distance of 1,429,400,000 km (9.54 → astronomical units) from Sun. With an → eccentricity of 0.05555,
its distance from the Sun ranges from 1.35 billion km (9.024 AU) at its → perihelion to 1.509 billion km (10.086 AU) at its → aphelion.

Its average orbital speed being 9.69 km/s, it takes Saturn 29.457 Earth years (or 10,759 Earth days) to complete a single revolution around the Sun. However, Saturn also takes just over 10 and a half hours (10 hours 33 minutes) to rotate once on its axis. This means that a single year on Saturn lasts about 24,491 Saturnian solar days.

Saturn has a mass of 5.6836 × 10²⁶ kg (95.159 → Earth masses) and a mean density of 0.687 g cm^-3.

Like Jupiter, Saturn is about 75% → hydrogen and 25% → helium with traces of → water, → methane, and → ammonia, similar to the composition of the primordial Solar Nebula from which the solar system was formed.

The temperature on Saturn is ~ -185 °C.

Like Jupiter, Saturn has a solid core of iron-nickel and rock (silicon and oxygen compounds). The core has an estimated mass of 9-22 Earth Masses and a diameter of about 25,000 km (about 2 Earth diameter). The core is enveloped by a liquid → metallic hydrogen layer and a → molecular hydrogen layer. Saturn’s interior is hot (12,000 K at the core). The planet radiates more energy into space than it receives from the Sun. Most of the extra energy is generated by the → Kelvin-Helmholtz mechanism as in Jupiter. Saturn has 62 known satellites. → Saturn’s ring. On 1 July 2004 NASA/ESA’s → Cassini-Huygens became the first to orbit Saturn, beginning a 13 year mission that revealed many secrets and surprises about Saturn and its system of rings and moons.

Etymology (EN): O.E. Sætern “Italic god,” also “most remote planet” (then known), from L. Saturnus, Italic god of agriculture, possibly from Etruscan.

Etymology (PE): Keyvân Mid.Pers. Kêwân, borrowed from Aramean kâwân, from Assyrian kaiamânu.

A planetary nebula in the Aquarius constellation discovered by William Herschel in 1782.
It has a size of about 0.3 x 0.2 light-years and lies about 1400 light-years away. Also known as NGC 7009.

See also: → Saturn, such named by Lord Rosse in the 1840s, because the object has a vague resemblance to the planet Saturn in low-resolution telescopes;
→ nebula.

A system of rings around Saturn made up of countless small particles, ranging in size from micrometers to meters, that orbit the planet. The ring particles are made almost entirely of → water ice, with some contamination from → dust and other chemicals. The ring system is divided into six major components: D, C, B, A, F, and G rings, listed from inside to outside. But in reality, these major divisions are subdivided into thousands of individual → ringlets. The large gap between the A and B rings is called the Cassini division. Saturn’s rings are extraordinarily thin: though they are 250,000 km or more in diameter, they are less than one kilometer thick. → A ring, → B ring, → C ring, → D ring, → F ring, → G ring.

See also: → Saturn; → ring.

1. To rescue from danger or possible harm, injury, or loss.
   

   2. To keep safe, intact, or unhurt; safeguard; preserve.
      

   3. Computers: To copy (a file) from RAM onto a disk or other storage medium (Dictionary.com).

Etymology (EN): M.E. sa(u)ven, from O.Fr. sauver “keep (safe), protect, redeem,” from L.L. salvare “make safe, secure,” from L. salvus “safe;” ultimately from PIE root *sol- “whole,” → general.

Etymology (PE): Bužidan, variants
buxtan, boxtan “to save, liberate;” boxt “saved, redeemed;” Mid.Pers. bôz- “to free, to release;” Bactrian βoγ “to save;” Av. bûj- “to save, redeem;” cf. Baluci bôtk / bôj “to open”, butk / busk “to be released (from jail), be fired (a gun), be emptied;” Pers.

buzidan/buz- “to pluck off hair, wool;”

cf. Gk. phugo, L. fugio “I flee”, Goth. us-baugjan “to wipe off” (Cheung 2007).

Same as → single-lined binary.

See also: SB, for → spectroscopic binary; 1, for → single-lined; → binary.

Same as → double-lined binary.

See also: SB, for → spectroscopic binary; 2, for → double-lined; → binary.

The ability of something, especially a computer system, to adapt to increased demands.

See also: → scalable; → -ity.

The quality of a system that can be expanded or reduced in scale. Scalability allows computer equipment and software programs to be upgraded easily, rather than needing to be replaced.

See also: → scale; → -able.

Any quantity which is sufficiently defined only with its magnitude, when given in appropriate units. Compare → vector.
See also:
→ electric scalar potential, → scalar field, → scalar potential, → scalar processor, → scalar product, → scalar wave, → scalar-tensor theory, → tensor-vector-scalar (TeVeS) theory.

See also: Of or pertaining to → scale.

A → tensor density of → order 0.

See also: → scalar; → density.

A → field whose value at every point of space is independent of → direction and → position. Examples include → temperature distribution throughout space and → pressure distribution in a → fluid. Similarly, a → potential field, such as the Newtonian → gravitational field or the electric potential in → electrostatics are scalar fields. In quantum field theory, a scalar field is associated with → spin zero particles, such as → mesons or → bosons. Therefore, the → Higgs boson is associated with a scalar field. The → derivative of a scalar field results in a → vector field is called the → gradient. In contrast to a vector field, a scalar field is → invariant under the → rotation of the → coordinate system. The → inflation in the → early Universe is supposed to be driven by a scalar field, called the → inflaton field.

See also: → scalar; → field.

The energy density fluctuations in the → photon-baryon plasma
that bring about hotter and colder regions. This perturbation creates velocity distributions that are out of phase with the acoustic density mode. The fluid velocity from hot to cold regions causes blueshift of the photons, resulting in → quadrupole anisotropy.

See also: → scalar; → perturbation.

Computers: A type of central processing unit in which only one operation on data is executed at a time. By contrast, in a vector processor, a single instruction operates simultaneously on multiple data items.

See also: → scalar; → processor.

A multiplication of two vectors giving a scalar. The scaler product of V₁ and V₂ is defined by:
V₁.V₂ = V₁.V₂ cos α, where V₁ and V₂ are the magnitudes of the vectors and α is the angle between them. Same as dot product. See also → vector product.

See also: → scalar; → product.

In theories of gravitation, a kind of → gravitational wave, transversal and/or longitudinal, characterized by → spin zero.

See also: → scalar; → wave.

An alternative to the standard → general relativity of gravity that contains not only the → tensor field (or → metric), but also a → scalar field. In this formalism, the → gravitational constant is considered to vary over time. As a consequence, the measured strength of the gravitational interaction is a function of time. Same as → Jordan-Brans-Dicke theory.

See also: → scalar; → tensor; → theory.

1a) A succession or progression of steps or degrees.

1b) A standard of measurement or estimation; point of reference by which to gauge or rate.

2. To reduce or increase according to a common proportion (often followed by down or up).

Etymology (EN): M.E., from L. scalae “ladder, stairs.”

Etymology (PE): Marpel, literally “measuring stick, measuring step,” on the model of Ger. Maßstab
from Mass “measure” + Stab “stick, bar, pole, baton.” The first element from
Mod./Mid.Pers. mar “measure, count,” from Av.
mar- “to count, remember;” Skt.
smr, smarati “to remember, he remembers;” L. memor, memoria; Gk.
mermera “care,” martyr “witness.” The second element pel “stick, a bit of wood;” pel can also be interpreted as the contraction of pellé “staircase, ladder.”

In computer science, to reduce the processing power of the same node/system by reducing its resources (CPU, RAM, etc.). This type of → vertical scaling is opposite to → scale up. See also → scale in, → scale out.

See also: → scale; → down.

Math.: A number which scales, or multiplies, some quantity. In the equation y = Cx, C is the scale factor for x. C is also the coefficient of x, and may be called the constant of proportionality of y to x.
Geometry: The ratio of any two corresponding lengths in two similar geometric figures. The ratio of areas of two similar figures is the square of the scale factor.

See also: → scale; → factor.

The height within which some parameter, such as pressure or density, decreases by a factor of e. For example, an atmospheric scale height of 100 km means that the value at 100 km is 1/e the value at the surface.

See also: → scale; → height.

In computer science, to reduce the number of nodes (servers), as opposed to → scale out. Scale-in is a type of → horizontal scaling. See also → scale up, → scale down.

See also: → scale; → in.

In computer science, to upgrade a system by increasing the number of nodes.
For example, instead of going from a CPU of X and memory of Y to a CPU with 4X and 4Y memory, use 4 machines with CPU of X and memory of Y. This is a type of → horizontal scaling. See also → scale in, → scale up, → scale down.

See also: → scale; → out.

In computer science, to increase the processing power of the same node/system by increasing its resources (CPU, RAM, etc.).
This is a type of → vertical scaling
opposite to → scale down. For example, instead of a machine with a CPU running at speed of X and having Y gigabytes of memory, use a machine with a CPU running at speed of 4X and a memory of 4Y gigabytes. See also → scale in, → scale out.

See also: → scale; → up.

A triangle no two sides of which are equal.

Etymology (EN): From L.L. scalenus, from Gk. skalenos “uneven, unequal, rough,” from skallein “chop, hoe,” related to
skolios “crooked,” from PIE base *(s)qel- “crooked, curved, bent;” → triangle.

Etymology (PE): Sebar, → triangle; nâjur-pahlu “dissimilar sides,” from nâjur “dissimilar, ill-matched” + pahlu “side, flank” (Mid.Pers. pahlug “side, rib,” Av. pərəsu- “rib,” Ossetic fars “side, flank,” cf. Skt. párśu- “rib,” Lith. piršys (pl.) “horse breast”).

An electronic circuit devised to give a single pulse after a prescribed number of input pulses have been received.

See also: Agent noun from → scale.

1. Altering original variable values (according to a specific function or an algorithm) into a range that meet particular criteria.
   

2. The ability of a system, or process, to handle a growing load in regards to capacity. It has two types: → vertical scaling and → horizontal scaling.

See also: → scale; → -ing.

The part of the head where the hair grows from.

Etymology (EN): M.E., perhaps from O.Norse skālpr “sheath,” related to O.Norse skalli “a bald head,” from PIE root *(s)ker- “to cut,” → bark.

Etymology (PE): Keler, from Zâhedân Baloci keler “scalp,” cognate with kâlun, → bark.

1. (v.) To sweep a surface with a beam of light or electrons in order to reproduce or transmit a picture.
   In radar, to sweep an airspace or region with a succession of directed beams from a radar aerial system.
   
2. (n.) An act or instance of scanning. The image or display so obtained.

Etymology (EN): M.E. scannen, from L.L. scandere “to read or mark so as to show metrical structure,”
originally, in classical L., “to climb” (the connecting notion is of the rising and falling rhythm of poetry), from PIE *skand- “to spring, leap” (cf. Skt. skandati “he jumps;” Gk. skandalon “a snare, trap, stumbling block;” O.Ir. scendim “I jump”).

Etymology (PE): Rajrub, literally “sweeping along rows,” from raj “row, line”

• rub “to sweep.” The first component raj, variants raž, rak, râk, rezg (Lori), ris, risé, radé, rasté, râsté, related to râst “right, true; just, upright, straight,” → row, → right.

The second component rub stem of rubidan, ruftan “to sweep,” related to robudan “to rub, carry off;” Mid.Pers. rôb- “to rub, sweep, attract;”
Av. urūpaiieinti “to cause racking pain(?);” cf. Skt. rop- “to suffer from abdominal pain,” rurupas “to cause violent pain,” ropaná- “causing racking pain,” rópi- “racking pain;” L. rumpere “to break;” O.E. reofan “to break, tear.”

Any device for exposing an image on film, a sensitized plate, etc., by tracing light along a series of many closely spaced parallel lines.

See also: Agent noun of → scan.

The process of analyzing or synthetizing successively the light values of the elements making up a picture area, according to a pre-determined method.

See also: Verbal noun of → scan.

A → sundial consisting of an inverted half sphere and a central vertical → gnomon used by ancient Greeks. See also → Eratosthenes experiment.

Etymology (EN): Gk. skaphe “boat, skiff; a bowl.”

A flat triangular bone a pair of which form the back part of the shoulder. Commonly known as → shoulder blade.

Etymology (EN): L. scapula “shoulder.”

Etymology (PE): Šâné, Mid.Pers. šânag “shoulder-blade.”
Ketf, loan from Ar. kataf “shoulder.”

Geology: A line of cliffs produced by faulting, erosion, or landslides. → cliff.

Etymology (EN): From It. scarpa.

Etymology (PE): Tondé “a steep slope of a mountain,” from tond “swift, rapid, brisk; fierce, severe” (Mid.Pers. tund “sharp, violent;” Sogdian tund “violent;” cf. Skt. tod- “to thrust, give a push,” tudáti “he thrusts;” L. tundere “to thrust, to hit” (Fr. percer, E. pierce, ultimately from L. pertusus, from p.p. of pertundere “to thrust or bore through;”
PIE base *(s)teud- “to thrust, to beat”); cf. dialectal Anzali tin, Laki den, Tâleši teš “steep rock.”

1. To cause → electromagnetic waves or a → beam of → particles to be irregularly → deflected, → dispersed, or → reflected, or be turned aside in the process of → scattering.
   

2. The act of → scattering; something that is → scattered.

Etymology (EN): M.E. scateren, schateren “to disperse, break up, destroy;” cf. M.Du. schaderen “to scatter.”

Etymology (PE): Parâkandan “to scatter, to disperse;” Mid.Pers. parakandan “to scatter” (cf. apakandan “to throw”), from Proto-Iranian *pari-kan-, from *pari, *par- “around” (cf. Pers. pirâ-, variant par- “around, about,” from Mid.Pers. pêrâ; O.Pers. pariy “around, about,” Av. pairi “around, over,” per- “to pass over, beyond;”
cf. Skt. pari;
PIE base *per- “through, across, beyond;” cf. Gk. peri “around, about,
beyond;” L. per “through”) + *kan- “to throw, place, put” (cf. Pers. afgandan “to throw; to lay, place;” kandan “to dig; to extract;” Mid.Pers. kan-, kandan “to dig;”
O.Pers. kan- “to dig,” akaniya- “it was dug;” Av. kan- “to dig,” uskən- “to dig out;” cf. Skt. khan- “to dig,” khanati “he digs,” kha- “cavity, hollow, cave, aperture”).

1. Occurring or distributed over widely spaced and irregular intervals in time or space.
   

2. The quality of a particle that has undergone → scattering.

See also: Past participle of → scatter.

A → particle that causes → scattering of another particle through interaction with it.

See also: → scatter; → -er.

The process in which the direction of motion of → particles or → waves is changed randomly because of their → interactions (→ collisions) with other particles of the → medium transversed.

Two parameters govern scattering: 1) the wavelength (λ) of the incident radiation, and 2) the size of the scattering particle (r), usually expressed as the nondimensional size parameter, x = 2πr / λ. The size parameter defines three types of scattering:

1. x much less than 1 (or r much smaller than λ), → Rayleigh scattering;
   

2. x ~ 1 (or rλ), → Mie scattering; and
   

3. x much larger than 1 (or r much larger than λ), → geometric scattering.
   

See also: → atmospheric scattering, → backscattering, → Brillouin scattering, → coherent scattering, → Compton scattering, → elastic scattering, → forward scattering, → last scattering, → last scattering surface, → multiple scattering, → noncoherent scattering, → quasi-single-scattering approximation, → Raman scattering, → scattering angle, → scattering coefficient, → scattering of stars, → selective scattering, → single scattering, → spin-flip scattering, → surface of last scattering, → Thomson scattering.

Related terms: → diffraction; → diffusion; → dispersion; → distribution.

See also: Verbal noun of → scatter.

The angle between the → incident radiation on a → particle (such as a water droplet in a rainbow) and the scattered radiation (such as the light ray leaving the droplet). Scattering angle is a function of → impact parameter. In other words, The angle along which the change of direction has taken place, irrespective whether radiation is scattered by particles or reflected (refracted) by a surface.

See also: → scattering; → angle.

The fraction of light scattered per unit distance in a medium.

See also: → scattering; → coefficient.

The progressive increase of random motions of → disk stars with increasing stellar → ages. While some initial random
motion seems likely in the disturbed conditions of disks
when the oldest stars formed, the observation is generally attributed to scattering processes. Both massive gas → clumps and → spiral waves are considered as scattering agents (J. A. Sellwood & J. J. Binney, 2002, astro-ph/0203510 and references therein).

See also: → scattering; → star.

The second-brightest star in the constellation → Pegasus. It is a giant star of spectral type M2.5 II-III whose magnitude varies between 2.3 and 2.7.

Etymology (EN): Scheat, from Ar. as-sâq “leg,” erroneously taken from the Ar. name of δ Aquarii as-sâq al-sâkib al-ma’ (الساق الساکب‌الماء) “the leg of the water-bearer.”

Etymology (PE): Asb-šâné, literally “the Horse’s Shoulder,” from asb→ horse + šâné “shoulder” (Lori šona, Kurd. šân, Gilaki cân, con), maybe related to Skt. skandhá- “shoulder, trunk of tree, bulk” (Pali khandha-, Ashkun kándä, Bashkarih kân, Tôrwâldi kan “shoulder”), from skand- “to jump, leap, spring out,” skandati “he jumps;” cf. L. scandere “to climb.”

A mathematical expression that describes the → luminosity function of galaxies. The function correctly reflects the facts that the luminosity function decreases with increasing luminosity and that the decrease is particularly marked at high luminosities. It is expressed as:

φ(L) = φ(L/L)^α exp (-L/L), which has two parts and three parameters: φ is an empirically determined amplitude, α is an empirically derived exponent, and L is a characteristic luminosity which separates the low and high luminosity parts.
For small luminosities (L much smaller than L) the Schechter function approaches a power law, while at high luminosities (L much larger than L) the frequency of galaxies drops exponentially. φ, L^*, and the faint-end slope α depend on the observed wavelength range, on the → redshift, and on the environment where the galaxies are observed.

See also: Named after the American astronomer Paul Schechter (1948-), who proposed the function in 1976 (ApJ 203, 297); → function.

A power-law relation between → star formation rate (SFR) and a corresponding measure of gas density. For external galaxies it is usually expressed in terms of the observable surface density of gas (Σ_gas):

SFR ∝ Σ_gasⁿ. The exponent n is determined to be 1.4 ± 0.15 (Kennicutt 1998, ApJ 498, 541). The validity of the Schmidt law has been tested in dozens of empirical studies.

The Schmidt law provides a tight parametrization of the global star formation law, extending over several orders of magnitude in SFR and gas density.

See also: Named after Maarten Schmidt (1929-), a dutch-born American astronomer, who also discovered the first → quasar (3C 273) in 1963.

A telescope with a spherical concave primary mirror in which the aberration produced by the spherical mirror is compensated for by a thin correcting lens placed at the opening of the telescope tube. Its very wide-field performance makes it suitable for surveys.

See also: Named after Bernhard Woldemar Schmidt (1879-1935), a German optician of
Estonian origin, who invented the telescope in 1930; → telescope.

A mixture of the → Cassegrain telescope with a very short → focal length and of a Schmidt design (due to the presence of the → corrective plate), used mainly in → amateur astronomy. The main advantage of this telescope is its compact design. However, Schmidt-Cassegrain telescopes produce fainter images with less contrast than other telescope designs with similar → aperture sizes. This is due to the comparatively large → secondary mirror required to reflect the light back the → eyepiece.

See also: → Schmidt telescope; → Cassegrain telescope.

Same as the → Schmidt law.

See also: Named after the American astrophysicists Maarten Schmidt (1929-), the pioneer of research in this field, and Robert C. Kennicutt, Jr. (1951-), who developed the study; → relation.

1. A learned or erudite person, especially one who has profound knowledge of a particular subject. → scientist.
   
2. A student who has been awarded a scholarship.

Etymology (EN): M.E. scoler(e); O.E. scolere “student,” from M.L. scholaris, from L.L. scholaris “of a school,” from L. schola, from Gk. skhole “school, lecture, discussion; leisure, spare time.”

Etymology (PE): Dâne&#353pažuh, from dâneš→ science + pažuh agent noun of pažuhidan “to search,” → research. Dânešvar, from dâneš, as befor, + -var possession suffix.

During the → main sequence stage, a star burns the hydrogen in its core and transforms it into helium. When the helium mass amounts to about 10% of the initial stellar mass, the star can no longer maintain the → hydrostatic equilibrium in its core; the star increases its volume and leaves the main sequence in order to become a → red giant.

See also: Named after the Brazilian astrophysicist Mario Schönberg (1914-1990) and Subramahmanyan Chandrasekhar, → Chandrasekhar limit, who were the first to point out this limit and derive it (1942, ApJ 96, 161).

1. An institution where instruction is given, especially to persons under college age.
   

   2. An institution for instruction in a particular skill or field.
      

   3. A college or university (Dictionary.com).

Etymology (EN): M.E. scole, O.E. scôl, from L. schola, from Gk. scholé “spare time, leisure,” from skhein “to get.”

Etymology (PE): Dabestân, from Mid.Pers. dibistân “school,” literally “place of writing” or “the place where documents are kept,” from dib, dip “→ document,” + -istân suffix of place, → summer.

A junction between a metal and a semiconductor, which exhibits rectifying characteristics. A Schottky barrier has a very fast switching action and low forward voltage drop of about 0.3 volts, compared with 0.6 volts in silicon diodes, which use adjacent p-type and n-type semiconductors.

See also: Named after Walter Hans Schottky (1886-1976), German physicist, who described the phenomenon; → barrier.

An unoccupied position in a crystal lattice which forms when oppositely charged ions leave their lattice sites, creating vacancies.

See also: Named after Walter Hans Schottky (1886-1976), German physicist; → defect.

A → semiconductor diode containing a → Schottky barrier. Such a diode has a low forward voltage drop and very fast switching characteristics. Also called Schottky barrier diode and hot electron diode.

See also: → Schottky barrier; → diode.

Excess voltage generated by random fluctuations in the emission of electrons from a hot cathode, causing a hissing or sputtering sound (shot noise) in an audio amplifier and causing snow on a television screen. Same as → shot effect, → shot noise.

See also: Named after Walter Hans Schottky (1886-1976), German physicist;
→ noise.

A fundamental equation of physics in → quantum mechanics the solution of which gives the → wave function, that is a mathematical expression that contains all the information known about a particle. This → partial differential equation describes also how the wave function of a physical system evolves over time.

See also: Named after Erwin Schrödinger (1887-1961), the Austrian theoretical physicist, Nobel Prize 1933, who first developed the version of quantum mechanics known as → wave mechanics; → equation.

A fundamental equation of physics in → quantum mechanics the solution of which gives the → wave function, that is a mathematical expression that contains all the information known about a particle. This → partial differential equation describes also how the wave function of a physical system evolves over time.

See also: Named after Erwin Schrödinger (1887-1961), the Austrian theoretical physicist, Nobel Prize 1933, who first developed the version of quantum mechanics known as → wave mechanics; → equation.

A → thought experiment intended to illustrate the → superposition principle in → quantum mechanics.
A cat is put in a steel box which is separated from the outside world. The box also contains a vial of lethal acid, a tiny amount of a radioactive substance, a → Geiger counter, and a hammer. If an atom decays and the Geiger counter detects an
→ alpha particle, the hammer breaks the vial which kills the cat. According to Schrödinger, as long as the box stays closed the cat’s fate is tied to the → wave function of the atom, which is itself in a superposition of decayed and un-decayed states. Thus the cat must itself be in a superposition of dead and alive states before the observer opens the box, “observes” the cat, and “collapses” its wave function. However, Schrödinger’s argument fails because it rests on the assumption that macroscopic objects can remain unobserved in a superposition state. When an atom decays, its wave function
becomes entangled with the enormously complex wave function of the macroscopic Geiger counter. The atom is therefore “observed” by the Geiger counter. Since a Geiger counter cannot, for all practical purposes, be isolated from the rest of the world, the rest of the world observes the atom, and the cat is either dead or alive. → collapse of the wave function.

Etymology (EN): Named after Erwin Schrödinger (1887-1961), → Schrodinger equation, who proposed the thought experiment in 1935 in order to illustrate the inconsistency of the Copenhagen interpretation of quantum mechanics; cat, from M.E. cat, catte; O.E. catt, catte (cf. O.Fris, M.D. katte, O.H.G. kazza, Ir. cat, Welsh cath), probably from L.L. cattus, catta “cat.”

Etymology (PE): Gorbé, from Mid.Pers. gurbag “cat;” → Schrodinger equation,

A → thought experiment intended to illustrate the → superposition principle in → quantum mechanics.
A cat is put in a steel box which is separated from the outside world. The box also contains a vial of lethal acid, a tiny amount of a radioactive substance, a → Geiger counter, and a hammer. If an atom decays and the Geiger counter detects an
→ alpha particle, the hammer breaks the vial which kills the cat. According to Schrödinger, as long as the box stays closed the cat’s fate is tied to the → wave function of the atom, which is itself in a superposition of decayed and un-decayed states. Thus the cat must itself be in a superposition of dead and alive states before the observer opens the box, “observes” the cat, and “collapses” its wave function. However, Schrödinger’s argument fails because it rests on the assumption that macroscopic objects can remain unobserved in a superposition state. When an atom decays, its wave function
becomes entangled with the enormously complex wave function of the macroscopic Geiger counter. The atom is therefore “observed” by the Geiger counter. Since a Geiger counter cannot, for all practical purposes, be isolated from the rest of the world, the rest of the world observes the atom, and the cat is either dead or alive. → collapse of the wave function.

Etymology (EN): Named after Erwin Schrödinger (1887-1961), → Schrödinger equation, who proposed the thought experiment in 1935 in order to illustrate the inconsistency of the Copenhagen interpretation of quantum mechanics; cat, from M.E. cat, catte; O.E. catt, catte (cf. O.Fris, M.D. katte, O.H.G. kazza, Ir. cat, Welsh cath), probably from L.L. cattus, catta “cat.”

Etymology (PE): Gorbé, from Mid.Pers. gurbag “cat;” → Schrodinger equation,

A phenomenon in which the observed and predicted phases of Venus do not coincide. At eastern elongation, when the planet is visible in the evening sky, dichotomy (half-phase) usually comes a day or two earlier than predicted, while at western elongation dichotomy occurs a day or two later.

See also: Named after Johan Schröter (1745-1816), German astronomer, who first described the effect in 1793; → effect.

A phenomenon in which the observed and predicted phases of Venus do not coincide. At eastern elongation, when the planet is visible in the evening sky, dichotomy (half-phase) usually comes a day or two earlier than predicted, while at western elongation dichotomy occurs a day or two later.

See also: Named after Johan Schröter (1745-1816), German astronomer, who first described the effect in 1793; → effect.

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.

See also: → Schwarzschild black hole; → barrier.

A → black hole with zero → angular momentum (non-rotating) and zero electric charge derived from Karl Schwarzschild 1916 exact solution to Einstein’s vacuum → field equations.

See also: Karl Schwarzschild (1873-1916), German mathematical physicist, who carried out the first relativistic study of black holes. → black hole.

In → general relativity, the → metric that describes the → space-time outside a static mass with spherically symmetric distribution.

See also: → Schwarzschild black hole; → metric.

The critical radius at which a massive body becomes a → black hole, i.e., at which light is unable to escape to infinity:
R_s = 2GM / c², where G is the → gravitational constant, M is the mass, and c the → speed of light. The fomula can be approximated to R_s≅ 3 x (M/Msun), in km. Therefore, the Schwarzschild radius for Sun is about 3 km and for Earth about 1 cm.

See also: → Schwarzschild black hole; → radius.

A region of infinite → space-time curvature postulated to lie within a → black hole.

See also: → Schwarzschild black hole; → singularity.

The first exact solution of → Einstein’s field equations that describes the → space-time geometry outside a spherical distribution of mass.

See also: 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.

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.

See also: Named after Karl Schwarzschild (1873-1916), German mathematical physicist (1906 Göttinger Nachrichten No 1, 41); → criterion.

1. The study of the physical and natural phenomena, especially by using systematic observation and experiment.
   

2. A systematically organized body of knowledge about a particular subject.
   See also: → knowledge,
   → cognition.

Etymology (EN): M.E., from O.Fr. science, from L. scientia “knowledge,” from sciens (genitive scientis), pr.p. of scire “to know,” probably originally “to separate one thing from another, to distinguish,” related to scindere “to cut, divide;” PIE base *skei- “to cut, split;” cf. Pers. gosastan “to tear, cut, break,” from Mid.Pers. wisistan “to break, split,” Av. saed-, sid- “to split, break,” asista- “unsplit, unharmed;” Skt. chid- “to split, break, cut off;” Gk. skhizein “to split;”
Goth. skaidan; O.E. sceadan “to divide, separate.”

Etymology (PE): Dâneš, verbal noun of dân-, dânestan “to know” (Mid.Pers. dânistan “to know”), variant šenâxtan, šenâs- “to recognize, to know” (Mid.Pers. šnâxtan, šnâs- “to know, recognize”);
O.Pers./Av. xšnā- “to know, learn, come to know, recognize;” cf. Skt. jñā- “to recognize, know,” jānāti “he knows;” Gk. gignoskein “to know, think, judge,” cognate with L. gnoscere, noscere “to come to know” (Fr. connaître; Sp. conocer); P.Gmc. *knoeanan; O.E. cnawan, E. know; Rus. znat “to know;” PIE base *gno- “to know.”

A form of fiction that draws imaginatively on scientific knowledge and speculation in its plot, setting, theme, etc. (Dictionary.com).

See also: → science; → fiction.

Of or pertaining to science or the sciences.
Systematic or accurate in the manner of an exact science.

Etymology (EN): From M.Fr. scientifique, from M.L. scientificus “pertaining to science,” from L. scientia “knowledge,” → science,

• -ficus “making,” from facere “to make.” → -ic

Etymology (PE): Dâneši, dânešik, from dâneš, → science

• -i, -ik, → ic.

An agreement by competent observers of a series of observations of the same phenomena. From time to time scientific facts are revised by additional data
(G. Smooth, Lawrence Berkeley Lab website).

See also: → scientific; → fact.

The process by which scientists, collectively and over time, endeavor to construct an accurate (that is, reliable, consistent, and non-arbitrary) representation of the world.
The scientific method has four steps:

1. Observation and description of a phenomenon or group of phenomena.
   

2. Formulation of an hypothesis to explain the phenomena. In physics, the hypothesis often takes the form of a causal mechanism or a mathematical relation.
   

3. Use of the hypothesis to predict the existence of other phenomena, or to predict quantitatively the results of new observations.
   

4. Performance of experimental tests of the predictions by several independent experimenters and properly performed experiments.
   

If the experiments bear out the hypothesis it may come to be regarded as a theory or law of nature. If the experiments do not bear out the hypothesis, it must be rejected or modified. What is key in the description of the scientific method just given is the predictive power (the ability to get more out of the theory than you put in) of the hypothesis or theory, as tested by experiment. It is often said in science that theories can never be proved, only disproved. There is always the possibility that a new observation or a new experiment will conflict with a long-standing theory (Frank L. H. Wolfs, University of Rochester).

See also: → scientific; → method.

A compact format for writing very large or very small numbers. Numbers are made up of three parts: the coefficient, the base and the exponent.
For example 3.58 x 10⁴ is the scientific notation for 35,800.

See also: → scientific; → notation.

The quality of the practices and theories that aim at establishing reproducible regularities
in phenomena by using experimental method and providing a clearly formulated description.

See also: → scientific + → -ity.

An expert in science, especially one of the physical or natural sciences. → scholar.

Etymology (EN): From → science + -ist an agent noun suffix.

Etymology (PE): Dânešmand, from dâneš, → science, + -mand suffix of possession.

1. Rapid variation in the brightness, wavelength, and mean position of stars caused by turbulence in the Earth’s atmosphere.
   

2. In radio astronomy, rapid fluctuations in the detected intensity of radiation from compact cosmic radio sources due to disturbances in ionized gas through which the radiation has passed. → interstellar scintillation.

Etymology (EN): From L. scintillationem (nominative scintillatio),
from scintillatus p.p. of scintillare “to send out sparks, to flash,” from
scintilla “particle of fire, spark.”

Etymology (PE): Susu, from su “light,” related to suz “burning,” present stem of suxtan; Mid.Pers. sôxtan, sôzidan “to burn,” Av. base saoc- “to burn, inflame” sūcā- “brilliance,” upa.suxta- “inflamed;” cf. Skt. śoc- “to light, glow, burn,” śocati “burns,”
śoka- “light, flame;” PIE base *(s)keuk- “to shine.”

A device for detecting and measuring ionizing radiation by means of flashes produced when the radiation particles strike a sensitive layer of phosphor.

See also: → scintillation; → counter.

Relating to a constraint or system that does not contain time explicitly. For example, a pendulum with an inextensible string of length l₀ is described by the equation: x² + y² = l₀² is both → holonomic and scleronomous.

Etymology (EN): From Gk. sclero-, from skleros “hard” + -nomous, → -nomy.

Etymology (PE): Saxtdâtik, from saxt, → hard,

• dâtik, → -nomy.

The Scorpion. A large and impressive constellation in the → Zodiac, which lies between → Libra to the west and → Sagittarius to the east. Scorpius is located in the southern hemisphere near the center of the Milky Way at approximately 17h right ascension, -40° declination. The bright, red star → Antares marks the heart of the scorpion. The constellation contains deep sky objects such as the open clusters M6 and M7, and the globular clusters M4 and M80. Also in the southern end of the constellation there is the open star cluster NGC 6231. Abbreviation: Sco; genitive: Scorpii.

Etymology (EN): M.E., from O.Fr. scorpion, from L. scorpionem (nominative scorpio), from Gk. skorpios “a scorpion,” from PIE base *(s)ker- “to cut,” → shear. According to Gk. mythology, the constellation represents a giant scorpion sent forth by the earth-goddess Gaia to kill the giant Orion when he threatened to slay all the beasts of the earth. Orion and the Scorpion were afterward placed amongst the stars as a pair of constellations. The two opponents are never seen in the sky at the same time, for one constellation sets as the other rises. The scorpion’s claws were originally formed by Libra.

Etymology (PE): Každom “scorpion,” variants kajdom, gaždom literally “crooked tail,”
from Mid.Pers. gazdum literally “stinging tail,” from gaz present stem of gazidan (also Mod.Pers.) “to sting, to bite” + dum, dumb (Mod.Pers. dom, domb) “tail;”
Av. duma- “tail.”

The first and the brightest X-ray source in the sky, after the Sun, discovered in 1962. Scorpius X-1 is a low-mass → X-ray binary consisting of a compact object like a → neutron star or a → black hole, and a low-mass stellar companion. The compact object has a mass of 1.4 → solar masses and the companion 0.42 solar masses. The orbital period is 18.9 hours, and the system lies at a distance of about 9,000 → light-years. The X-rays come from → accretion, where material from the companion overflows its → Roche lobe and spirals down onto the compact object. The luminosity results from the transformation of the falling material’s → gravitational potential energy to heat by → viscosity in the → accretion disk.

See also: Named such by the discoverers (Giacconi et al. 1962), because it was the first extrasolar → X-ray source of the sky detected in the constellation → Scorpius.

The nearest → OB association to the Sun. It contains several hundred stars, mostly → B stars which concentrate in the three subgroups: Upper Scorpius, Upper Centaurus Lupus, and Lower Centaurus Crux. Upper Scorpius is the youngest subgroup, Upper Centaurus Lupus the oldest subgroup of the association.

Isochrone fitting to the Hertzsprung-Russell diagram indicates that the star formation occurred some 5-20 Myr ago.
Based on data from the → Hipparcos catalog, it turns out that the Sco-Cen association lies at a distance of 118-145 → parsecs, with the exact value depending on the subgroup of the association.

The Sco-Cen association is probably a member of the → Gould Belt (Preibisch & Mamajek, 2008, astro-ph/0809.0407).

See also: → Scorpius; → Centaurus; → association.

Vision that occurs when the eye is dark-adapted. In scotopic vision, the level of luminance is so low that the retinal cones are not
stimulated, and there is no color vision. Same as scotopia; → dark adaptation.

Etymology (EN): Scotopic, from L. Gk. skoto- combining form of skotos “darkness” + -opia akin to ope “view, look,” ops “eye, face;” → vision.

Etymology (PE): Did, → vision; târiki noun from târik “dark,” Mid.Pers. târig “dark,” târ “darkness,” Av. taθra- “darkness,” taθrya- “dark,” cf. Skt. támisrâ- “darkness, dark night,” L. tenebrae “darkness,” Hittite taš(u)uant- “blind,” O.H.G. demar “twilight.”

1. A large, usually flat surface onto which an image is projected for viewing.
   

2. The portion of a computer terminal or monitor upon which information is displayed.

Etymology (EN): M.E. screne; O.Fr. escren “a screen against heat,” from M.Du. scherm “screen, cover,” or Frank. *skrank “barrier;” cf. O.H.G. skirm, skerm “protection,” scrank “barrier;” Ger. Schrank “cupboard.”

Etymology (PE): Pardé, from Mid.Pers. pardag “curtain, veil, covering;” loaned in Armenian partak “veil,” and Georgian p’ardag-i “curtain;” cognate with Gk. pelas, pella, L. pellis “skin;” O.E. filmen “thin skin;” PIE root *pel- “to cover.”

A character used for on-screen → display. See also → printer font.

See also: → screen; → font.

The → Coulomb interaction reduced owing to the presence of
other electrons. See → shielding effect.

See also: → screen; → coulomb; → interaction.

Same as → shielding effect.

See also: → screen; → effect.

A piece of metal, consisting of a threaded and usually tapered shank that has a slotted head by which it is turned into something in order to fasten things together.

Etymology (EN): M.E. scrwe, screw, from M.Fr. escroue “nut, cylindrical socket,” of uncertain origin.

Etymology (PE): Pic “screw,” present stem of picidan “to twist, entwine, coil.”

A minor and faint → constellation in the southern sky at 0h 30m → right ascension, 33° south → declination. Its brightest star is variable with a mean magnitude of only 4.31. Sculptor contains the south Galactic pole. It also contains the → Sculptor Dwarf, which is a member of the → Local Group. Abbreviation: Scl; Genitive: Sculptoris.

Etymology (EN): Sculptor was introduced by Nicolas Louis de Lacaille (1713-1762).
He originally named it Apparatus Sculptoris “the sculptor’s studio,” but the name was later shortened. From L. sculp(ere) “to carve” + a suffix forming personal agent nouns.

Etymology (PE): Peykartarâš, from peykar “form, figure, body” (from Mid.Pers. pahikar “picture, image;” from O.Pers. patikara- “picture, (sculpted) likeness,” from patiy “against” (Av. paiti; Skt. prati; Gk. poti/proti + kara- “doer, maker,” from kar- “to do, make, build;” Av. kar-; Skt. kr-; cf. Skt. pratikrti- “an image, likeness, model; counterpart”) + tarâš “cutter,” from tarâšidan “to cut, hew; scape; shave;” (Mid.Pers. tâšitan “to cut, cleave; create by putting together different elements;” Av. taš- “to cut off, fashion, shape, create,” taša- “axe” (Mod.Pers. taš tišé “axe”),
tašan- “creator;” cf. Skt. taks- “to fom by cutting, tool, hammer, form,” taksan- “wood-cutter, carpenter;” Gk. tekton “carpenter,”
tekhne “art, skill, craft, method, system;” L. textere “to weave;” PIE *teks- “to fashion”).

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.

See also: → Sculptor; → dwarf; → elliptical; → galaxy.

The nearest group of galaxies to our → Local Group, lying near the south Galactic pole at about 10 million → light-years distance. The Sculptor Group is dominated by five galaxies, four spiral (NGC 247, 253, 300, and 7793) and one irregular (NGC 55). The brightest of the five is NGC 253. The nearest galaxy in this group is NGC 55 which at a distance of 5 million light-years lies on the border of the Local Group.

See also: → Sculptor; → group.

The Shield. A small constellation in the southern Milky Way, at 18h 40m right ascension, 10° south declination. Its brightest star has a visual magnitude of 3.85. Scutum contains several open clusters, as well as a globular cluster and a planetary nebula. The two best known deep sky objects in Scutum
are M11 (NGC 6705), a dense open cluster, and M26, another open cluster also known as NGC 6694. The globular cluster NGC 6712 and the planetary nebula IC 1295 can be found in the eastern part of the constellation. Abbreviation: Sct; Genitive: Scuti.

Etymology (EN): Scutum was created by Johannes Hevelius in 1683, who originally named it L. Scutum Sobiescianum “the shield of Sobieski” to commemorate the victory of the Polish forces led by King John III Sobieski in the Battle of Vienna, and thus refers to Sobieski’s Janina Coat of Arms. Later, the name was shortened to Scutum “shield.”

Etymology (PE): 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.”

A spiral arm of our Galaxy located between the Sagittarius Arm and the Norma Arm, though it is rather less prominent than either of these two better defined spiral arms. It originates relatively close to the Sun’s present position in the Galaxy, and follows a sweeping arc of about 80,000 light years to the opposite side of the Galactic disk.

See also: → Scutum; → Crux; → arm.

An agricultural implement consisting of a long, curving blade fastened at an angle to a handle, for cutting grass, grain, etc., by hand (Dictionary.com).

Etymology (EN): M.E. sythe, sithe, from O.E. sithe, sigdi “sickle;” cf. West Frisian seine “scythe,” Du. zicht “sickle,” Ger. Sense “scythe;” from PIE root *sek- “to cut.”

Etymology (PE): Dahre “scythe,” variant of dâs, → sickle; dialectal variants (Dari Yazd) dare, (Laki) dara “butcher’s cleaver,” (Gilân, Lâsgard, Sorxe) dâra, (Tabari) dahra, dâhra, darra.

1. A large lake or landlocked body of water.
   

2. A large area or great number of something. → Fermi sea.

Etymology (EN): O.E. sæ “sheet of water, sea, lake;” cf. Du. zee, Ger. See, O.N. sær “sea,” Goth saiws “marsh.”

Etymology (PE): Daryâ “sea;” Mid.Pers. daryâp variant zrah; O.Pers. drayah-; Av. zrayah- “sea;” cf. Skt. jráyas- “expanse, space, flat surface.”

The → apparent horizon formed by the sea.

See also: → sea; → horizon.

To explore or examine in order to find something.

Etymology (EN): M.E. serchen, cerchen, from O.Fr. cerchier “to search,” from L. circare “to go about, wander, traverse,” from circus “circle.”

Etymology (PE): Jost-o-ju interfixed jost and juy past and present stem of jostan/juyidan “to seek, strive for;” Proto-Iranian *iud- “to struggle for something, to fight” (Av. yūδ- “to fight, struggle;” Mod.Pers. justan, juy- “to search, seek, ask for”); cf. Mid.Pers. vijuyihitan “to search, seek.”

The scientific attempt to detect → intelligent extraterrestrial → life by surveying the sky to find the existence of → transmissions,
especially → radio waves or → light, from a → civilization on a distant → planet.
The SETI Institute, that carries out the project, is a private non-profit center founded in 1984. There are many methods that SETI scientific teams use to search for extraterrestrial intelligence. Many of these search billions of radio frequencies that reach Earth from all over the → Universe, looking for an intelligent → radio signal. Other SETI teams search by looking for signals in pulses of light emanating from the stars.

See also: → search; → extraterrestrial; → intelligence.

The hard shell of a marine mollusk.

Etymology (EN): → sea; → shell.

Etymology (PE): Sadaf, loan from Ar. Kelâcak from Tabari, variant kelâcin, cf. Gilaki guš kuli. The component kel-, kul might be related to PIE *qarq- “to be hard,” → crab.

One of the four periods of the year astronomically defined by the position of the Sun with respect to the equator. As a result of the obliquity of the ecliptic, the angular distance between the Sun and the equator varies in the course of the year. This circumstance gives rise to seasons. The current lengths of the astronomical seasons, around the year 2000, are about: spring 92.76 days, summer 93.65 days, autumn 89.84 days, and winter 88.99 days. The seasons are unequal because the Earth’s orbit is slightly elliptical and the Sun is not exactly at the center of the orbit. Moreover, the Earth moves faster when it is close to the Sun than when it is farther away, so the seasons that occur when the Earth is close to the Sun pass more quickly.

Etymology (EN): M.E. sesoun, seson, from O.Fr. seison “a sowing, planting,” from L. sationem (nominative satio) “a sowing,” from p.p. stem of serere “to scatter seed over land.”

Etymology (PE): Fasl, from Ar. faSl “cutting, dividing; section.”

1. Geometry: A straight line that intersects a curve in two or more points.
   

2. Trigonometry: For an → acute angle of a → right triangle, the function defined as the ratio of the → hypotenuse to the adjacent side. For any angle, the function defined as the ratio of the → radius vector to the → abscissa. Abbreviation sec.

Etymology (EN): From L. secant-, stem of secans, pr.p. of secare “to cut,”
→ section.

Etymology (PE): 1) Sekanjân, agent noun from sekanjidan “to shave, cut, scape,” cognate with šekastan “to break,” → section.
2) Sekânt, loan from Fr.

A pioneering work in → spectral classification conducted in the 1860s. Secchi divided stars into four main groups based on the visual observation of spectra.

Class I: The white and bluish stars with a continuous spectrum crossed by hydrogen bands, the metallic bands being absent or weak. Examples, → Sirius, → Vega.

Class II: Yellow stars, with spectra in which the hydrogen bands were less prominent and the metallic lines more strong. Examples, Sun, → Capella.

Class III: Red or orange stars, showing bands or flutings. Examples, → Antares, → Betelgeuse.

Class IV: Red stars, showing bands similar to Class III, but with the sharp edge of the flutings toward the other end of the spectrum. Secchi’s scheme was superseded by the photographic → Harvard classification system.

See also: Pietro Angelo Secchi (1818-1878), Italian astronomer and Jesuit priest; → classification.

1. Next after the first in place, time, or value.
   

2. The unit of time in the → International System of Units; symbol s. It is defined by taking the fixed numerical value of the cesium frequency Δν_Cs, the unperturbed → ground state  → hyperfine transition  → frequency of the → cesium-133 atom, to be 9 192 631 770 when expressed in the unit → hertz (Hz), which is equal to s^-1.

Etymology (EN): M.E., from O.Fr. second, from L. secundus “following, next in order,” from root of sequi “to follow;” PIE base *sek^w- “to follow;” cf. Pers. az from; Mid.Pers. hac “from;”
Av. hac-, hax- “to follow,” hacaiti “follows”
(O.Pers. hacā “from;” Av. hacā “from, out of;” Skt. sácā “with”); Skt. sácate “accompanies, follows;” Gk. hepesthai “to follow;” Lith. seku “to follow.”

Etymology (PE): 1) Dovom, dovomin “ordinal number of do,
two” (Mid.Pers. do; Av. dva-; cf.
Skt. dvi-; Gk. duo; L. duo; (Fr. deux; E. two; Ger. zwei).
2) Sâniyé, from Ar. sâniyat (feminine) “second.”

Math: In calculus, limiting an equation to its → second derivative,
for example: e^x≅ 1 + x + x²/2. Also called linear approximation. → first approximation.

See also: → second; → approximation.

An early evolutionary period in the process of star formation which succeeds the → first collapse.
When the mass of the → first core has increased by about a factor 2 and the radius has decreased by a similar factor, the central temperature of the core reaches about 2000 K. At this point the → molecular hydrogen begins to dissociate into atoms. This reduces the → adiabatic index (γ) below the critical value 4/3, with the result that the material at the center of the core becomes unstable and begins to collapse. Most of the gravitational energy generated by this collapse goes into the → dissociation of H₂ molecules, so that the temperature rises only slowly with increasing density. In this second collapse phase, as in the first, the density distribution in the collapsing region becomes more and more sharply peaked at center, and the time scale becomes shorter and shorter with increasing central density. The central collapse of the core continues until the hydrogen molecules are nearly all dissociated and γ again rises above 4/3. The central pressure then rises rapidly and once again becomes sufficient to decelerate and stop the collapse at the center. A small core in the → hydrostatic equilibrium then arises, bounded by a shock front in which the surrounding infalling material is suddenly stopped. The initial mass and radius of the second core are about 3 x 10³⁰ g (1.5 x 10^-3M_sun) and 9 x 10¹⁰ cm (1.3 R_sun) respectively, and the central density and temperature are about 2 x 10^-2 g cm^-3 and 2 x 10⁴ K, respectively. The second core will evolve into a → young stellar object (R. B. Larson, 1969, MNRAS 145, 271).

See also: → second; → collapse.

The beginning of the total phase of a solar eclipse when the leading edge of the Moon touches the eastern edge of the Sun completely obscuring the Sun.

See also: → second; → contact.

A hydrostatic object predicted to result from the → second collapse of a → molecular cloud in an early stage of star formation.

See also: → second; → core.

In → calculus, the → derivative of a → first derivative. It is usually written as f’’(x), d²y/d²x, or y’’.

See also: → second; → derivative.

A method, used in → calculus, for determining whether a given → stationary point of a → function is a → local minimum or → local maximum.

See also: → second; → derivative; → test.

A → dredge-up process that occurs after core helium burning, in which the convective envelope penetrates much more deeply, pushing hydrogen burning shell into close proximity with the helium burning shell (→ first dredge-up). This arrangement is unstable and leads to burning pulses. The reason is that the hydrogen shell burns out until there is enough helium for the helium combustion to occur and all the helium is rapidly burnt. Afterward the hydrogen shell again burns outward and the process repeats.

See also: → second; → dredge-up.

A star whose formation is induced by an older star itself formed previously in the same region. See also → stimulated star formation, → sequential star formation, → triggered star formation.

See also: → second; → generation; → star.

The surface area of a black hole’s horizon can never decrease.

See also: → second; → law; → black hole; → mechanics.

1. Heat cannot be transferred from a colder to a hotter body without some other effect, i.e. without → work being done. Expressed in terms of → entropy: the entropy of an → isolated system tends toward a maximum and its available energy tends toward a minimum.
   

2. In language of → statistical physics, an isolated physical system will tend toward an equilibrium → macrostate with as large a total → entropy as possible, because then the number of → microstates is the largest. See also → Kelvin’s postulate, → Clausius’s postulate.

See also: → second; → law;
→ thermodynamics.

In quantum mechanics, the quantization of the field that replaces potential in Newtonian mechanics, whereby the field variables become operators from which the creation (of particle) operators and destruction operators can be constructed.

See also: → second; → quantization.

An n extension of → first-order logic that quantifies not only → variables that range over → individuals, but also quantifies over → relations.

See also: → second; → order; → predicate; → logic.

1. Derived or derivative; not primary or original.
   

2. Belonging or pertaining to a second order, division, stage, period, rank, grade, etc.
   

3. → secondary body.
   

See also:
→ secondary atmosphere, → secondary calibrator, → secondary cell, → secondary cosmic rays, → secondary crater, → secondary eclipse, → secondary electrons, → secondary emission, → secondary mirror, → secondary star.

Etymology (EN): From → second + -ary a suffix occurring on adjectives (elementary; honorary; stationary) and
nouns denoting objects, especially receptacles or places (library; rosary; glossary).

Etymology (PE): Dovomân, from dovom, → second.

An atmosphere of a planet that forms after primordial gases had been
lost or had failed to accumulate. A secondary atmosphere develops from internal volcanic outgassing, or by accumulation of material from
comet impacts. It is characteristic of terrestrial planets, such as Earth, Mercury, Venus, and Mars. → primordial atmosphere.

See also: → secondary; → atmosphere.

A body that revolves around a more massive body under the → gravitational attraction of the latter
is called the → primary body.

The less massive component in a → binary system.

See also: → secondary; → body.

An indicator of extragalactic distances that relies on → primary calibrators in our Galaxy.
Secondary calibrators of the distance scale depend on statistical measures of the properties of a class of objects, such as the brightness of H II regions, globular clusters, red and blue stars, or the neutral hydrogen 21-cm line width or velocity dispersion (of spiral galaxies), etc. Same as secondary distance indicator.

See also: → secondary; → calibrator.

An electric cell that can be charged by passing a current through it in reverse direction to its discharge. Same as → accumulator. See also → primary cell.

See also: → secondary; → cell.

A burst of secondary charged and neutral particles arising when → primary cosmic rays collide with the atmospheric oxygen or nitrogen nuclei in the upper atmosphere. The
collision produces mostly → pions (π), along with some → kaons (K), → antiprotons, and → antineutrons. Neutral pions very quickly decay, usually into two → gamma rays. Charged pions also decay but after a longer time. Therefore, some of the pions may collide with yet another nucleus of the air before decaying, which would be into a → muon and a → neutrino. The fragments of the incoming nucleus also interact again, also producing new particles.

See also: → secondary; → cosmic; → ray.

A crater formed by the relatively low-velocity impact of fragments ejected from a large primary crater. Secondary craters tend to cluster in a ring around the primary crater.

See also: → secondary; → crater.

Of a transiting → exoplanet, the event and the interval of time during which the planet passes behind its host star. → primary eclipse.

See also: → secondary; → eclipse.

Electrons ejected from the atoms of a material when bombarded with high energy electrons. Secondary electrons are produced when an incident electron excites an electron in the material and loses some of its energy in the process. The excited electron moves toward the surface of the sample undergoing elastic and inelastic collisions until it reaches the surface, where it can escape if it still has sufficient energy. The secondary electron yield depends on many factors, and is generally higher for high atomic number targets, and at higher angles of incidence.

See also: → secondary; → electron.

The emission of → secondary electrons from the surface of a material when an incident particle (often, charged particle such as electron or ion) impacts the material with sufficient energy.

See also: → secondary; → emission.

The second reflecting surface in a → reflecting telescope. It directs the light either out a side opening of the tube (→ Newtonian telescope) or back toward a → focal point behind and through the → primary mirror (→ Cassegrain telescope). The secondary is usually suspended in the beam and therefore obstructs part of the primary.

See also: → secondary; → mirror.

A fainter rainbow appearing about 10° above the → primary rainbow, as viewed by the observer. The secondary rainbow is about twice as wide, and has its colors reversed.

See also: → secondary; → rainbow.

In a → binary system, the star that revolves around the more massive → primary component.

See also: → secondary; → star.

1. Something that is or is kept secret, hidden, or concealed; a → mystery.
   

2. Done, made, or conducted without the knowledge of others (Dictionary.com).

Etymology (EN): From L. secretus “set apart, withdrawn; hidden, concealed,” p.p. of secernere “to set apart, part, divide; exclude,” from se- “without, apart,” properly “on one’s own” + cernere “to separate,” → crisis.

Etymology (PE): Râz, from Mid.Pers. râz “secret, mystery;” cognate with Mod.Pers. rastan/rah- “to escape, be liberated;” O.Pers. (+*aua-) avarad- “to leave, abandon;” cf. Skt. rah- “to be lost, be lonely,” rahas- “loneliness, privacy; a secret, mystery” (Cheung 2007).

The officials or office entrusted with administrative duties, maintaining records, and overseeing or performing secretarial duties, especially for an international organization (Dictionary.com).

Etymology (EN): From Fr. secrétariat, from M.L. secretariatus, from secretarius, → secretary.

Etymology (PE): Dabirxâné, literally “house of secretaries,” from dabir, → secretary, + xâné, → house.

A person, usually an official, who is in charge of the records, correspondence, minutes of meetings, and related affairs of an organization, company, association, etc. (Dictionary.com).

Etymology (EN): M.E. secretarie “one trusted with private or secret matters; confidant,” from M.L. secretarius “confidential officer, confidant, clerk, notary,”
from L. secretum “a secret, a hidden thing.”

Etymology (PE): Dabir, from Mid.Pers. dipîr, contraction of dipîvar (Mid.Pers. dip, dīp “document;” dīb “letter”); from O.Pers., from Proto-Ir. *dipī-uara- “he who preserves the documents;” cf. O.Pers. dipī- “inscription” + *Huar- “to cover;” cf. Av. vār- “to cover, hide, protect.”

The head or chief administrative officer of a secretariat.

See also: → secretary; → general.

A part that is cut off or separated.
A distinct part or subdivision of anything. → cross section; → intersection

Etymology (EN): From M.Fr. section, from L. sectionem “a cutting, division,” from secare “to cut;” PIE base *sek- “cut” (cf. O.C.S. seko, sesti “to cut,” Lith. isekti “to engrave, carve;” O.S. segasna, O.E. sigðe “scythe;” O.E. secg “sword,” seax “knife, short sword”).

Etymology (PE): Sekanj “a scraping, shaving,
cutting,” cognate with Pers. šekast-, šekastan “to break;” Av. skand- “to break,” Skt. khand- “to break,” khanda- “piece;” Pers. dialect Tabari šag “a special razor used to make incisions in the walls of unripe opium poppies in order to extract the milky sap,” may be related to PIE *sek- “cut,” as above.

1a) General: Going on from age to age; continuing through long ages.

1b) Astro.: Gradual or taking place over a long period. → secular acceleration; → secular change.

2a) (adj.) Worldly or material rather than spiritual.
2b) (adj.) Not overtly or specifically relating to religion or to a religious body.
2c) (adj. & n.) Relating to or advocating secularism; a layperson.

Etymology (EN): Secular from O.Fr. seculer, from L.L. sæcularis “of an age, occurring once in an age,” from sæculum “age, span of time, generation, the spirit of the age.”

Etymology (PE): 1) Diryâz “long lasting, from dir “slowly, tardily; late” (Mid.Pers. dêr, variants dagr, drâz “long;” (Mod.Pers. derâz “long,” variant Laki, Kurdi
derež); O.Pers. darga- “long;” Av. darəga-, darəγa- “long,” drājištəm
“longest;” cf. Skt. dirghá- “long (in space and time);” L. longus “long;” Gk. dolikhos “elongated;” O.H.G., Ger. lang; Goth. laggs “long;” PIE base *dlonghos- “long”)

• yâz present stem of yâzidan “to stretch out the arms; grow up” (Parthian Mid.Pers. y’d “to reach a goal, come to, stretch out;” Av. yat- to reach, take one’s place,” yaiiata “places,”
  frā-iiatāt “has reached;” cf. Skt. yat- “to be in place, put in place, line up;” PIE base *iet- “to be in place”).
  

2. Giyâné, giyâni from giyân, variant of Mod.Pers. jahân, keyhân, geyhân “world,” giti “world, material world, time,” Mid.Pers. gêhân “world,” gêtig “the material world; wordly,” Manichean Mid.Pers. gyh “world,” gyh’n “worlds;” Av. gaē&thetaā- “being, world, matter, mankind,” gaya- “life, manner of living”, root gay- “to live” (present tense jiva-), O.Pers. gaiθā- “live-stock,” cognate with Skt. jīv- “to live,” jīva- “alive, living;” Gk. bios “life,” L. vivus “living, alive,” vita “life;” PIE base *g^wei- “to live” (cf. O.E. cwic “alive;” O.C.S. zivo “to live;” Lith. gyvas “living, alive;” O.Ir. bethu “life,” bith “age, life, world;” Welsh byd “world”). The Pers. words zistan “to live,” zendé “alive,” zendegi “life,” and jân “vital spirit, soul; mind” belong to this family.

The smallest component of the aberration of starlight which is caused by the motion of the solar system through space. → annual aberration; → diurnal aberration.

See also: → secular; → aberration.

The apparent gradual increase in the → Moon’s motion in its orbit, as measured relative to → mean solar time. Secular acceleration corresponds to an extremely gradual reduction in the speed of the → Earth’s rotation. The slow-down of the Earth’s spin comes mainly from → tidal frictions from the Moon. Historically, Edmond Halley (1656-1742) was the first to suggest that the Moon’s mean rate of motion relative to the stars was gradually increasing. In 1693, Halley compared eclipses of recent, medieval, and classical Babylonian time, and discovered that the Moon’s mean motion had been gradually increasing. Using Lunar Laser Ranging measurement, based on laser reflectors left by the Apollo astronauts on the Moon’s surface (1969 to 1972), the secular acceleration is derived to be -25".4 ± 0".1 century ² (Xu Huaguan et al., 1996, in Earth, Moon and Planets 73, 101). This corresponds to a linear increase of about 3.5 cm yr^-1 in the mean Earth-Moon distance.

See also: → secular; → acceleration.

A continuous, non-periodic change in one of the attributes of the states of a system. Often, a change in an orbit due to dissipation of energy. See also → canonical change.

See also: → secular; → change.

Instability caused by a slow dissipation of energy.

See also: → secular; → instability.

The angle subtended at a star by a baseline that is the distance the Sun moves in a given interval of time with respect to the local standard of rest (4.09 AU per year).

See also: → secular; → parallax.

A variation of planetary orbital elements which is always in the same direction as time increases.

See also: → secular; → perturbation.

1. The condition in which the equilibrium configuration of a system is stable over long periods of time.
   
2. The condition of a star when it is stable against arbitrary adiabatic perturbations.

See also: → secular; → stability.

In perturbation theory used in celestial mechanics, a steadily increasing disturbance. → periodic term.

See also: → secular; → term.

Same as → secular perturbation.

See also: → secular; → variation.

The view that religious considerations should be excluded from civil affairs or public education.

See also: → secular.

The process of organizing society or aspects of social life around non-religious values or principles.

See also: Verbal noun of secularize “giyânidan” (گیانیدن); → secular

1. Free from or not exposed to danger or harm; safe.
   

2. To get hold or possession of; procure; obtain.

Etymology (EN): From L. securus “free from care, quiet, easy,” also “careless, reckless;” of things, “free from danger, safe,” from *se cura, from se “without, free from,” + cura, → care.

Etymology (PE): Zilé, from Tabari zil, zilé “firm, fixed,” zil hâkerdan “to fix, fasten,” of unknown origin.

1. Freedom from care, anxiety, or doubt; well-founded confidence.
   

2. Something that secures or makes safe; protection; defense.
   

3. A department or organization responsible for protection or safety (Dictionary.com).

See also: → secure; → -ity.

A technique that uses → spectral energy distribution results from models to reproduce observational data.

See also: → spectral energy distribution; → fitting.

Mineral or organic material which has been transported and deposited by an agent of erosion such as water, wind, and ice.

Etymology (EN): From Fr. sédiment, from L. sedimentum “a settling, sinking down,” from stem of sedere “to settle, sit”

Etymology (PE): Nehešt past stem of neheštan “to place, deposit,” from ne- “down, below,” → ni- (PIE), + heštan “to place, put” from Mid.Pers. hištan, hilidan “to let, set, leave, abandon;” Parthian Mid.Pers. hyrz; O.Pers. hard- “to send forth,” ava.hard- “to abandon;”
Av. harəz- “to discharge, send out; to filter,” hərəzaiti “releases, shoots;” cf. Skt. srj- “to let go or fly, throw, cast, emit, put forth;” Pali sajati “to let loose, send forth.”

Of, pertaining to, or of the nature of sediment.

See also: Adj. of → sediment.

A rock composed of materials that were transported to their present position by wind or water. → Sandstone, → shale, and → limestone are sedimentary rocks.

See also: → sedimentary; → rock.

A trans-Neptunian object (numbered 90377) and a likely → dwarf planet, it is the most distant large object yet found orbiting the Sun. It is at present over 90 A.U.s away, 3 times as far as Pluto. Its precise diameter is unknown, probably 1,600-2,200 km (about 12-17% of Earth). Its estimated orbital period is 12,050 years. Formerly known as 2003 VB12

See also: In Inuit mythology, Sedna (Inuktitut Sanna) is a goddess of the marine animals, especially mammals such as seals.

The second phase in the evolution of a → supernova remnant (SNR) occurring after the → free expansion phase. After the passage of the → reverse shock, the interior of the SNR is so hot that the energy losses by radiation are very small (all atoms are → ionized,
no → recombination). The expansion is driven by the → thermal pressure of the hot gas and can therefore be regarded as → adiabatic; the → cooling of the gas is only due to the → expansion. Pressure forces accelerate the swept-up → interstellar medium (ISM) converting → thermal energy (which came from original explosion) into → kinetic energy of the → shell of swept-up mass. As the mass of the ISM swept up by the shell increases, it eventually reaches densities which start to impede the free expansion. → Rayleigh-Taylor instabilities arise once the mass of the swept-up ISM approaches that of the ejected material. This causes the SNR’s ejecta to become mixed with the gas that was just shocked by the initial → shock wave. The Sedov-Taylor phase lasts some 10⁴ years and
is followed by the radiative or → snowplow phase. Also called → adiabatic phase.

See also: After Sedov, L. (1959, Similarity and Dimensional Methods in Mechanics, New York, Academic Press) and Taylor, G. I. (1950, Proc. Roy. Soc. London, A, 201, 159 and 175); → phase.

To perceive with the eyes; look at.

Etymology (EN): M.E. seen, from O.E. seon “to see, look, behold, understand, know,” ultimately from PIE *sek^w- “to see, notice;” cognate with Du. zien “to see,” Ger. sehen “to see,” Danish, Swedish and Norwegian Bokmal se “to see,” L. signum “mark, token.”

Etymology (PE): Didan “to see, regard, catch sight of, contemplate, experience;” Mid.Pers. ditan; O.Pers. dī- “to see;” Av. dā(y)- “to see,” didāti “sees;” cf. Skt. dhī- “to perceive, think, ponder; thought, reflection, meditation,” dādhye; Gk. dedorka “have seen.”

An → electromotive force produced in a closed electric circuit formed by connecting conductors of different metals in series when the two junctions junctions are maintained at different temperatures. The circuit constitutes a → thermocouple.

See also: Named for the German physicist Thomas Seebeck (1770-1831), who discovered the effect; → effect.

A small single crystal of a semiconductor from which is grown the large single crystal for the manufacture of semiconductor devices.

Etymology (EN): O.E. sed, sæd; cf. O.N. sað, O.S. sad, O.Fris. sed, M.Du. saet, O.H.G. sat, Ger. Saat; PIE base *se- “to sow.”

Etymology (PE): Toxm “seed” (Tabari tim “seed; race,” Laki tôm “seed”), from Mid.Pers. tôhm, tôhmak, tôm, tuxm “seed; extraction; descent;” Av. taoxman- “seed;” O.Pers. taum&#299:- “family;” cf. Skt. tókman- “offspring, children, race, child,” tokma- “young shoot, young blade of corn.”

A nucleus from which a variety of → fusion  → chain reactions derive in → stellar nucleosynthesis.

See also: → seed; → nucleus.

A measure of the blurring and degradation of the image of astronomical objects caused by → turbulence in the Earth’s atmosphere, including the telescope environment. Seeing causes the images of stars to break up into → speckle patterns, which change very rapidly with time. See also → Fried parameter; → differential image motion monitor.

Etymology (EN): → see; → -ing.

Etymology (PE): Šekân “wrinkle, plait; curl; rupture, breach,” variant of šekan “fold, curl; ripples on water,” from šekastan “to break, split;” Mid.Pers. škastan “to break;” Av. scind-, scand “to break, cleave;”
Proto-Iranian *skand- “to break, cleave;” PIE sken- “to cut off.”

The angular size of a stellar image for long exposures, as determined by the ratio λ/r₀, where λ is the wavelength and r₀ the typical size of → turbulence patches. → Fried parameter.
The most common seeing measurement is the → full-width at half-maximumof the seeing disk. → Airy disk.

See also: → seeing; → disk.

An optical instrument that follows the variation of → atmospheric turbulence by continuously measuring the → seeing conditions.

See also: → seeing; → monitor.

1. Of a line, that portion bounded by two points.
   
2. Of a circle, that portion of a plane bounded by an arc of the circle and its chord.
   
3. Of a sphere, the solid formed between two parallel planes that cut through a sphere.
   
4. In computer science, a portion of a program, often one that can be loaded and executed independently of other portions.

Etymology (EN): From L. segmentum “a strip or piece cut off,” originally a geometric term, from secare “to cut” + -mentum “-ment.”

Etymology (PE): Borank, from Kermâni borang “a slice (of fruit);” Borujerdi boleng “piece, section,” ultimately from *brin-ka- (probable contracted forms Lari peng and pengi “portion or part of anything”), related to boridan “to cut off;” Mid.Pers. brīn-, blyn-, britan, brinitan “to cut off,” Av. brī- “to shave, shear,” brin- (with prefix pairi-);
cf. Skt. bhrī- “to hurt, injure,” bhrinanti “they hurt;” PIE base bhrei- “to cut, pierce.”

A large telescope mirror consisting of smaller mirror segments designed to act as a single, larger reflecting surface. Because current monolithic mirrors cannot be constructed larger than about eight meters in diameter, the use of segmented mirrors is a key component for larger aperture telescopes.

See also: Segmented, p.p. of → segment (v.);
→ mirror.

To separate or set apart from others or from the main body or group; isolate.

Etymology (EN): From M.E. segregat, from L. segregatus, p.p. of segregare “separate from the flock, isolate, divide,” from se- “apart from” + greg-, ablative of grex, gregis “herd, flock, crowd,” cf. Gk. gergera “swarm, flock;” maybe related to Old Khotanese -gris- in hamgris- “to assemble.”

Etymology (PE): Savâyidan, from savâ “separate, apart;” probably related to
Mid.Pers. s’w- “to crush,” sây- “to rub, wear, tear;” cf. Kurd. (Hawramân) sawa, Roshani sêw-/sêwt, Bartangi siw-/siwd, Yazghulani saw-/sed, Bajui sâw-/sâwd “to rub, smear, grind”
(Cheung 2007); Mod.Pers. sây-/sudan “to rub, wear, tear, grind, dissolve;” cf. Skt. śā- “to sharpen, whet.”

The act or practice of segregating. The state or condition of being segregated. → mass segregation.

See also: Verbal noun of → segregate.

Of, subject to, or caused by → vibrations of the → Earth. → seismic wave.

See also: From seism, → seismo-, + → -ic.

An → elastic wave generated in the → Earth by an → impulse such as an → earthquake or an
→ explosion.
Seismic waves may travel either along or near the Earth’s surface or through the Earth’s interior.

See also: → seismic; → wave.

A combining form meaning “earthquake;” → seismology, → seismograph, etc.

Etymology (EN): From Gk. seismo- combining form of seismos “shock, earthquake,” from seiein “to shake.”

Etymology (PE): Larzé-, from larzé “shaking, trembling,” from larzidan “to tremble, shiver;” Mid.Pers. larzidan “to shake, tremble;” Manichean Mid.Pers. rarz- “to shiver with fever;” Proto-Iranian *rarz- “to shake, tremble.”

An instrument that detects, magnifies, and records → seismic waves, especially those caused by → earthquakes or → explosions.

See also: → seismo-; → -graph.

The branch of geophysics that is concerned with the study of earthquakes and measurement of the mechanical properties of the Earth.

See also: → seismo-; → -logy.

Not → often; rarely.

Etymology (EN): M.E., from O.E. seldum, variant of seldan; cognate with Ger. selten, Goth. silda-, Dutch zelden.

Etymology (PE): Adverb from perz, → rare.

To choose from among several.

Etymology (EN): From L. selectus, p.p. of seligere “to choose out, gather apart,” from se- “apart” + legere “to gather, select.”

Etymology (PE): Gozidan “to select, choose;” Mid.Pers. vicitan, wizidan, wizin- “to choose, select, discriminate,” related to cin-, cidan “to gather, collect;” Av. vicidāi- “to discern,” viciθa- “separation, discernment;” from vi- “apart, away from” (O.Pers. viy- “apart, away;” cf. Skt. vi- “apart, asunder, away, out;” L. vitare “to avoid, turn aside”)

• kay- “to choose;” cf. Skt. ci- “to gather, heap up,” cinoti “gathers.”

Any of a set of rules specifying the relationships between the → quantum numbers that characterize the initial and final states of a quantum-mechanical system in a → discrete transition. Transitions that do not agree with the selection rules are called → forbidden and have considerably lower probability. There are several types of selection rules (→ rigorous selection rule,
→ LS coupling, etc.)
for → electric dipole transition (→ permitted), → magnetic dipole (forbidden), electric → quadrupole (forbidden), etc.

See also: Selection, verbal noun of → select;
→ rule.

Absorption which varies with the wavelength of radiation incident upon an absorbing substance.

Etymology (EN): Selective, verbal noun of → select;
→ absorption.

A type of scattering that occurs when certain → particles are more effective at scattering a particular → wavelength of light, as in → Rayleigh scattering.

See also: → selective; → scattering.

Same as → horizontal eclipse.

Etymology (EN): From Gk. selene “Moon,” related to sela “light, brightness, flame,”

• helion, → sun.

Etymology (PE): → horizontal eclipse.

Same as → horizontal eclipse.

Etymology (EN): From Fr. selenelion, contraction of → selenehelion.

Etymology (PE): → horizontal eclipse.

Referring to or pertaining to the center of the Moon.

Etymology (EN): Formed on the model of → geocentric, from seleno- combining form of Gk. selene “moon” + -centric, from → center + → -ic.

Etymology (PE): Mâh-markazi, from mâh, → moon, + markazi, from markaz, → center, + -i, → -ic.

A parameter representing the product of the → gravitational constant by the → lunar mass. It is 49.03 x 10¹¹ m³ s^-2.

See also: → selenocentric; → gravitational; → constant.

Topographic description and charting of the surface of the Moon.

Etymology (EN): From seleno- combining form of Gk. selene “moon” + → -graphy.

Etymology (PE): Mâh-negâri, from mâh, → moon + negâri,
→ -graphy.

A combining form of self with a range of related meanings.

Etymology (EN): From M.E., from O.E. self, seolf, sylf “one’s own person, same;” cf. O.Fris. self, Du. zelf, O.H.G. selb, Ger. selbst.

Etymology (PE): Xod-, from xod; Mid.Pers. xwad “self; indeed;”
Av. hva- “self, own.”

The decrease in the radiation from a material caused by the absorption of a part of the radiation by the material itself.

See also: → self-; → absorption.

The → state or → condition of being aware of one’s own → personality or → individuality.

See also: → self-; → awareness.

The characteristic of a system of masses, such as a star, kept together by mutual gravity.

See also: → self; → gravitate.

The → gravitational attraction of a system of masses, such of a planet, that allows the system to be held together by their mutual gravity.
Self-gravity between atoms allows a → star to hold together, despite tremendous temperature and pressure. Similarly, to be considered a → planet, a body must have enough mass so that its self-gravity pulls it into a near-spherical shape.

See also: → self-; → gravity.

The inductance associated with an isolated electric circuit that is characteristic of the circuit’s physical design.

See also: → self-; → inductance.

The generation of a voltage in a circuit due to self-inductance, the polarity of which tends to oppose the changing current in the circuit.

See also: → self-; → induction.

A model of → accretion disk around a → pre-main sequence star or a → protostar in which the outer parts of the disk are geometrically flat, in contrast to a → flared disk. Inward of a certain radius (0.5-1 AU from the star) the dust in the disk evaporates. Because the dust is the main source of opacity and the gas in the disk is usually optically thin, the irradiation burns a hole in the disk. Moreover, the inner rim puffs up, similarly to the case of flared disks. The difference lies in the outer parts. The inner rim casts its shadow over the disk all the way out. Since the disk thickness is almost constant, no photons can reach the surface of the disk and the outer parts of the disk remain shadowed by the inner rim and the midplane temperatures decrease accordingly. This model explains the observed
→ spectral energy distribution of some pre-main sequence stars, such as HD 101412. It also accounts for the observed weak → far infrared→ excess, weak or no → PAH emission, and weak or no [O I] emission.

Etymology (EN): → self-; → shadow; → disk.

Etymology (PE): Gerdé, → disk; xod-, → self-; pardé, → screen.

The phenomenon whereby the → photodissociation
transitions of a molecule in interstellar clouds become → optically thick, so that the molecule in question is “shielded” by
other molecules against dissociating stellar → far-ultraviolet (FUV) photons. In the case of → molecular hydrogen (H₂),
when the → column density exceeds 10¹⁴ cm^-2,
the UV absorption bands become optically thick, and H₂ undergoes
self-shielding. More specifically, all of the photons that could lead to UV photodissociation are absorbed by H₂ in the outer layers of the cloud, hence protecting the H₂ within the cloud. Self-shielding occurs in → diffuse interstellar clouds exposed to the interstellar → radiation field or in → molecular clouds in proximity to sources of UV photons. Dust can also absorb UV photons, further limiting the photodissociation, but it dominates only when the local UV radiation field is unusually intense relative to the density of the cloud.

See also: → self-; → shield; → -ing.

1. Of a geometric figure, having a structure analogous or identical to its overall structure. → fractal.
   

2. The quality of a variable entity in which the shape does not change with time, such as a
   → self-similar process.

See also: → self-; → similar.

A process that is invariant in distribution under scaling of time. Schematically, images taken of such a process at different time scales will look similar.

See also: → self-; → similar; → process.

The property of being → self-similar.

See also: → self-; → similarity.

An empirical relation between the → refractive index of a medium and the wavelength of light passing through the medium:

n² - 1 = Σ (A_iλ²/(λ²

• λ_i²)),

where n is the refractive index at wavelength λ, and A_i and λ_i are constants.

See also: Named after Wolfgang Sellmeier who derived the equation in 1871;
→ equation.

1. Of, pertaining to, or arising from the different meanings of words or other signs and symbols.
   

2. Of or pertaining to → semantics.

Etymology (EN): From Fr. sémantique, from Gk. semantikos “significant,” from semainein “to show, signify, indicate by a sign,” from sema “sign.”

Etymology (PE): Cemârik, from cemâr, → meaning, + -ik, → -ic.

The study of the → meaning of signs or symbols, as opposed to their formal relations (→ syntactics).

See also: → semantic; → -ics.

Biology: The male reproductive fluid, containing spermatozoa in suspension. → inseminate, → insemination; → fecundate, → fecundation.

Etymology (EN): M.E., from L. semen “seed;” akin to serere “to sow;” cf. O.C.S. seme, O.H.G. samo; E. sow.

Etymology (PE): From Mid.Pers. šusar “semen; liquid, fluid;” Av. xšudra- “semen; liquid, fluid;” related to Pers. šostan/šuy- “to → wash.”

A combining form meaning “half,” freely prefixed to English words of any origin.

Etymology (EN): From L. semi- “half,” from PIE *semi-; cf. Skt. sāmi “half,” sāmi-krita- “half-done;” Gk. hemi- “half;” O.E. sam-; Goth. sami- “half.”

Etymology (PE): Nim, nimé “half,” from Mid.Pers. nêm, nêmag “half;”
Av. naēma- “half;” cf. Skt. néma- “half.”

A → spectral line for which the upper and lower → energy levels have different values of S, the total → spin angular momentum. These lines violate the quantum mechanical → selection rule under → LS coupling, ΔS = 0. For example, the Ca I λ6573 line results from transition between the upper → triplet state (³P₁) with a total
spin angular momentum S = 1 and the → ground state, a → singlet state (¹S₀, total spin angular momentum S = 0). A semi-forbidden line is marked by a right bracket following the atom name, i.e. Ca I], in the above-mentioned case.
Same as → interconnection line and → intersystem line.

See also: → semi-; → forbidden; → line.

An → atomic transition whose probability is reduced by a factor of the order of 10⁶ because of → selection rules. Same as → interconnection line.

See also: → semi-; → forbidden; → transition.

Half the length of the major axis of an ellipse; a standard element used to describe an elliptical orbit. see orbital elements

See also: → semi-; → major; → axis.

Any of various solid crystalline substances, such as germanium or silicon, which has conducting properties intermediate between metals and insulators.

See also: → semi-; → conductor.

In a semiconductor device, a region of transition between semiconducting regions of different electrical properties.

See also: → semiconductor; → junction.

An instability occurring in the region just outside the → convective core of a → massive star. The instability occurs when a → superadiabatic layer is stabilized by a chemical gradient. In fact, semiconvection takes place if → Schwarzschild’s criterion for convection is fulfilled but at the same time → Ledoux’s criterion is not fulfilled. The time-scale of semiconvection is the thermal time-scale, which is short compared to the nuclear time-scale in → main sequence stars but long compared to the time-scale of convection. However, semiconvection has a profound influence on the → post-main sequence star evolution. It affects the convective mixing above the hydrogen shell source, determines the appearance and extent of → blue loops in the → Hertzsprung-Russell diagram during core → helium burning, and is essential for defining the extent of the convective cores during core helium burning (See, e.g., N. Langer, 2012, Ann. Rev. Astron. Astrophys. 50, 107).

See also: → semi-; → convection.

A binary system whose secondary member fills its Roche lobe but whose primary member does not.

Etymology (EN): → semi-; detached, p.p. of detach, from O.Fr. destachier (Fr. détacher), from des- “apart,”

• -tachier (as in atachier “to attach”); → binary.

Etymology (PE): Dorin, → binary; nim-jodâ, from nim-→ semi- + jodâ “separate,” from Mid.Pers. yut “separate, different;” Av. yuta- “separate, apart.”

Same as → semidetached binary.

Etymology (EN): → semi-; detached, p.p. of detach, from O.Fr. destachier (Fr. détacher), from des- “apart,”

• -tachier (as in atachier “to attach”); → system.

Etymology (PE): Râžmân, → system; nim-jodâ, from nim-→ semi- + jodâ “separate,” from Mid.Pers. yut “separate, different;” Av. yuta- “separate, apart.”

The angle at the observer subtended by the equatorial radius of the Sun, Moon, or a planet.

See also: → semi-; → diameter.

Describing an → equation or → formula that results from a → combination of → experiment and → theory.

See also: → semi-; → empirical.

Same as → Weizsacker formula.

See also: → semiempirical; → binding; → energy; → formula.

Half the → latus rectum.

For an ellipse, semilatus rectum has the expression l = b²/a, where a and b are semi-major and minor axes of the ellipse. It can also be expressed in terms of → eccentricity, e, as: l = a(1 - e²).

See also: → semi-; → latus rectum.

1. Of or pertaining to → signs.
   

2. Of or pertaining to → semiotics.

Etymology (EN): From Gk. semeiotikos “significant,” also “observant of signs,” adj. form of semeiosis “indication,” from semeion “to signal, interpret a sign,” from sema “sign.”

The study of linguistic and non-linguistic signs and symbols used in natural and artificially constructed languages. Semiotics is usually divided into three branches:

1. → pragmatics, 2) → semantics,
   and 3) → syntax.

See also: → semiotic; → -ics.

A type of giant or supergiant pulsating variable star, with intermediate or late spectra, showing noticeable periodicity in its light changes, accompanied or sometimes interrupted by various irregularities. Periods lie in the range from 20 to more than 2000 days, while the shapes of the light curve may be rather different and variable with each cycle. The amplitudes may be from several hundredths to several magnitudes (usually 1-2 magnitudes in the V filter). Examples are Betelgeuse, Antares, and Rasalgethi.
Semiregular variables are classified in several subtypes: SRA, SRB, SRC, and SRD.

See also: → semi-; → regular; → variable.

1. Older or elder (designating the older of two men bearing the same name, as a father whose son is named after him, often written as Sr. or sr. Compare → junior.
   

2a) Of higher or the highest rank or standing.

2b) (in American schools, colleges, and universities) Of or relating to students in their final year or to their class (Dictionary.com).

Etymology (EN): M.E., from L. senior “older,” comparative of senex (genitive senis) “old,” from PIE root *sen- “old;”
cf. Av. hana- “old;” O.Pers. hanata- “old age, lapse of time;” Skt. sanah “old;” Armenian hin “old;” Gk. enos “old, of last year;” Lith. senas “old,” senis “an old man;” Goth. sineigs “old,” O.Irish sen, O.Welsh hen “old.” sjunio

Etymology (PE): Mehtar “greater, elder, governor,” from meh “great, large, principla,” cognate with L. mas, → Big Bang, + comparative suffix -tar.

The perception or awareness of stimuli through the senses.

Etymology (EN): From M.L. sensationem (nominative sensatio), from L.L. sensatus “endowed with sense, sensible,” from L. sensus “feeling,” → sense.

Etymology (PE): Hess-kard, hesseš, verbal nouns of hess kardin, hessidan, → sense.

1. Any of the faculties, as sight, hearing, smell, taste, by which a person or animal obtains information about the physical world.
   
2. To perceive by the senses; become aware of.

Etymology (EN): M.E., from O.Fr. sens, from L. sensus “perception, feeling, undertaking,” from sentire “perceive, feel, know.”

Etymology (PE): Hess, loan from Ar. Hess kardan, hessidan infinitives from hess.

1. Capacity for sensation or feeling; responsiveness to sensory stimuli.
   
2. Physics: The realignment of a magnetic compass pointer along a magnetic field line after the pointer has been deflected.

See also: → sensible; → -ity.

That can be perceived by the senses.

Etymology (EN): M.E., from O.Fr. from L. sensibilis “having feeling, perceptible by the senses,” from sensus, p.p. of sentire “perceive, feel,” → sense.

Etymology (PE): Hess-pazir, from hess, → sense + pazir, → -able; hessidani, from hessidan (v.) + -i, → -able.

The great circle of the celestial sphere whose poles are the nadir and zenith. Same as astronomical horizon.

See also: → sensible; → horizon.