An Etymological Dictionary of Astronomy and Astrophysics

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.

Endowed with sensation; having perception through the senses. Responding to a stimulus.

Etymology (EN): From M.Fr. sensitif, from M.L. sensitivus “capable of sensation,” from L. sensus, p.p. of sentire “feel perceive,” → sense.

Etymology (PE): hessmand, from hess, → sense + -mand possession suffix.

1. The required brightness for an object in order to be detected by an observing instrument.
   A highly sensitive telescope can detect dim objects, while a telescope with low sensitivity can detect only bright ones.
   

2. Of a radio receiver or similar device, the minimum input signal required to produce a specified output signal having a specified signal-to-noise ratio.

See also: State noun from → sensitive.

A device that receives and responds to a signal or stimulus (light, temperature, radiation level, or the like) by transmitting to a control equipment with the required degree of accuracy. → wavefront sensor.

See also: → sense; → -or.

1. Grammar: A grammatical unit of one or more words that expresses an independent statement, question, request, command, exclamation, etc., and that typically has a subject as well as a predicate, as in John is here. or Is John here? (Dictionary.com).
   

2. Law: An authoritative decision; a judicial judgment or decree, especially the judicial determination of the punishment to be inflicted on a convicted criminal: Knowledgeable sources say that the judge will announce the sentence early next week (Dictionary.com).

Etymology (EN): M.E., from O.Fr. sentence “judgment, decision; statement of authority,” from L. sententia “thought, opinion; judgment,” also “a thought expressed,” from sentientem, p.p. of sentire “be of opinion, feel, perceive.”

Etymology (PE): Sahân, related to soxan, → speech and pâsox, → response; Mid.Pers saxwan “speech, word;” O.Pers. θaⁿh- “to declare, say;” Av. səngh- (sanh-) “to declare;”
Proto-Ir. *sanh- “to declare, explain;” cf. Skt. śams- “to praise, recite;” L. censere “to estimate, think; decide;” PIE *kens- “to announce, proclaim” (Cheung 2007).

Pertaining to or of the nature of a → sentence.

See also: From L. sententi(a), → sentence, + → -al.

Same as → propositional logic.

See also: → sentential; → logic.

1. Detached, disconnected, disjoined.
   

2. To set apart; disconnect; dissociate.

See also: → separation.

General: The act or process of separating. The place at which a division or parting occurs.
Astro.: The angular distance between the two components of a visual binary or optical double star.

Etymology (EN): M.E., from O.Fr. separation, from L. separationem, from separare “to pull apart,” from se- “apart” + parare “make ready, prepare.”

Etymology (PE): Jodâyi state noun of jodâ “separate,” from Mid.Pers. yut “separate, different;” Av. yuta- “separate, apart.”

The energy required to remove a particle (a proton or a neutron) from a particular atomic nucleus.

See also: → separation; → energy.

A person or thing that separates. → decimal point.

See also: → separate; → -or.

1. General: Something that divides or separates. Plural: separatrices.
   

2. Math.: Any mark that separates digits in groups, such as the decimal point placed at the left of a decimal fraction, to separate it from the whole number which it follows.
   

3. Physics: A bundle of magnetic field lines which creates separate plasma regions.

Etymology (EN): From L. separatrix “she that separates,” → separation; -trix a suffix.

Etymology (PE): Jodâgar, from jodâ “separate,” → separation,

• -gar, → -or.

1. General: The following of one thing after another; succession; something that follows; connected line of events, ideas, etc.
   

2. Math.: A set of quantities that are ordered in some way, such as a₁, a₂, a₃, …. A sequence is said to be known if a formula can be given for any particular term using the preceding terms or using its position in the sequence. Special types of sequences are commonly called → progressions. The terms of a sequence, when written as an indicated sum, form a → series.

Etymology (EN): M.E., from O.Fr. sequence “answering verses,” from M.L. sequentia “a following, a succession,” from L. sequentem (nominative sequens), pr.p. 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): Peyâyé, literally “that follows; a subsequent event,” from pey “after; step,” related to pâ “foot” (Mid.Pers. pâd, pây, Av. pad-, Skt. pat, Gk. pos, gen. podos, L. pes, gen. pedis, P.Gmc. *fot, E. foot, Ger. Fuss, Fr. pied; PIE *pod-/*ped-) + ây- present stem of âmadan “to come, arrive, become”
(Av. ay- “to go, to come,” aēiti “goes;” O.Pers. aitiy “goes;” Skt. e- “to come near,” eti “arrival;” L. ire “to go;” Goth. iddja “went,” Lith. eiti “to go;” Rus. idti “to go”) +-é nuance suffix.
Rešté “thread, line, rope, row,” p.p. of reštan, risidan “to spin;” Mid.Pers. rištag “rope, string, thread;” Av. uruuaēs- “to turn around,” uruuaēsa- “vortex in water;” cf. Skt. vréśī- “an appellation of waters;” Gk. rhiknos “crooked;” Lith. rišti “tie, bind;” O.H.G. rīho “knee-bend.”

Following in order of time or place.

See also: Adj. from → sequence.

The formation of second-generation stars in a → molecular cloud, as triggered by the presence of → massive stars. The observation that some nearby → OB associations contain distinct, spatially separate subgroups of → OB stars in a sequence of monotonically changing age led Blaauw (1964, ARA&A 2, 213) to suggest that star formation in fact occurs in sequential bursts during the lifetimes of the corresponding molecular clouds. The first quantitative model of this mechanism was presented by Elmegreen and Lada (1977, ApJ 214, 725), who showed that the powerful ultraviolet photons of the massive star create an → ionization front which advances in the molecular cloud and is preceded by a → shock front. The compressed neutral gas lying between the ionization and shock fronts is gravitationally unstable and collapses in time-scales of a few million years to form a new generation of massive stars. The propagation of successive births of OB groups would produce a chain of associations presenting a gradient of age. Elmegreen and Lada estimated the propagation velocity to be 5 km s^-1. For a region with a length larger than 100 pc, this would imply an age difference of the order of 20 million years between the extremities. See also → stimulated star formation, → triggered star formation; → collect and collapse model.

See also: → sequential; → star formation.

1. Anything published, broadcast, etc., in short installments at regular intervals, as a novel appearing in successive issues of a magazine.
   

   2. Pertaining to, arranged in, or consisting of a series (Dictionary.com).

See also: → series; → -al.

1. Math.: A sequence of numbers or mathematical expressions such as the n-th term may be written down in general form, and any particular term (say, the r-th) may be obtained by substituting r for n; e.g. xⁿ is the general term of the series 1, x, x², x³, …, xⁿ.
   

2. Electricity: An arrangement of the components, as resistors, connected along a single path, so the same current flows through all of the components. Compare → parallel.
   

3. → spectral series; → Lyman-alpha series.

Etymology (EN): From L. series “row, chain, series,” from serere “to join, link, bind together,” from PIE base *ser- “to line up, join.”

Etymology (PE): Seri, loan from Fr., as above.
Rise “string, thread, series,” variant of rešte, → sequence.

1. Of or relating to a matter of importance.
   

   2. Having important or dangerous possible consequences.

Etymology (EN): From M.E., from O.Fr. serios “grave, earnest” and directly from Late L. seriosus, from L. serius “weighty, important, grave,” probably from a PIE root *swer- “slow, heavy;” cf. Lith. sveriu, sverti “to weigh, lift,” svarus “heavy, weighty;” O.E. swaer “heavy,” Ger. schwer “heavy,” Gothic swers “honored, esteemed,” literally “weighty”).

Etymology (PE): Padel, from Balôci padel “serious,” of unknown origin.

The Serpent. An inconspicuous, irregular constellation situated on both sides of → Ophiuchus. The constellation is divided into two unequal parts, originally called Serpens Caput “Serpent’s Head” at 15h 30m right ascension, 15° north declination, and Serpens Cauda “Serpent’s Body” at 18h 30m right ascension, 0° declination. The brightest star, Alpha Serpentis, is of second magnitude.
Abbreviation: Ser; Genitive: Serpentis.

Etymology (EN): From L. serpens “snake,” from pr.p. of serpere “to creep,” from PIE *serp- “to crawl;” cf. Skt. sarp- “to creep, crawl,” sárpati “creeps,” sarpá- “serpent;” Gk. herpein “to creep,” herpeton “serpent;” Alb. garper “serpent.”

Etymology (PE): Mâr “snake, serpent;” Mid.Pers. mâr “snake;” Av. mairya- “snake, serpent.”

A mathematical function that describes how the → intensity  I of a → galaxy varies with distance R from its center. It is given by: (dln I/dln R) = -(b/n)(R/R_e)^1/n. The constant b is chosen such that R_e is the → effective radius;
n is the Sérsic index. The Sérsic profile is a generalization of → de Vaucouleurs law. Setting n = 4 gives the de Vaucouleurs profile.

See also: J. L. Sérsic, 1963, Boletin de la Asociacion Argentina de Astronomia, Vol. 6, p.41; → profile.

To render assistance; be of use. To have definite use.

See also: Verbal form of service, → server.

General: Something that serves or is used in serving.
Computers: A computer software application that carries out some task on behalf of users. When users connect to a server, they can access programs, files, and other information from the server. Common servers are Web, mail, and database servers. A single computer can have several different server programs running on it.

Etymology (EN): Server, agent noun from serve, from M.E. serven, from O.Fr. servir “to serve,” from L. servire “to serve,” originally “be a slave,” related to servus “slave;” cognate with Av. har- “to guard, watch,” harətar- “guardian,” hāra- “caring for;” Mid./Mod.Pers. zinhâr “protection, security; beware! mind!”

Etymology (PE): Zâvar “attendant, servant” (Dehxodâ), zâvari “attendance, service” (Dehxodâ), maybe related to Skt. sev- “to attend upon, serve,” sevā- “service, attendance, worship,” sevati “serves, attends,” sevaka- “attendant, servant, follower.”

1. A helpful act by somebody for somebody else as a job, duty, or favor.
   

2. A system or organization supplying some public demand, e.g. transportation, telephone, health.
   

3. A facility providing maintenance and repair.

Etymology (EN): M.E., from O.Fr. servise, from L. servitium “slavery, servitude,” from servus “slave,” servire “to serve,” originally “be a slave;”
cognate with Av. har- “to guard, watch,” → server.

Etymology (PE): Zâvari “service” (Dehxodâ) → server.

Observation approved by the selection committee of an observatory which is carried out by the staff astronomers of the observatory.

See also: → service; → observation.

1. Math.: A finite or infinite collection of objects in which order has no significance. Members of a set are often referred to as elements and the notation a ∈ A is used to denote that a is an element of a set A. The study of sets and their properties is the object of set theory.
   

2. To pass below the horizon.
   → moonset; → sunset.
   

3a) (tr.v.) To put (something or someone) in a particular place.

3b) To adjust a device to a desired position.

Etymology (EN): 1) M.E. sette, from O.Fr. sette “sequence,” variant of secte, from M.L. secta “religious group, sect,” from L. secta “manner, following, school of thought,” literally “something to follow, pathway, course of conduct, school of thought,” from sectari “to pursue, accompany,” “a way, road,” from sequi “to follow,” → sequence.

2. M.E. setten, O.E. settan “cause to sit, put in some place, fix firmly;”
   cf. O.N. setja, O.Fris. setta, Du. zetten, Ger. setzen.

Etymology (PE): 1) Hangard, from Mid.Pers. hangart “whole, complete,” hangartik “complete,” hangartênitan “to collect, assemble,” from *hamkard- literally “created, cut together,” from han- variant of ham- “together,” cognate with L.L. insimul “at the same time,” from in- intensive prefix + simul “together, at the same time” (cf. Gk. homos “same,” Mod./Mid.Pers.
ham- “together, with,” O.Pers./Av. ham-, Skt. sam-; also O.Pers./Av. hama- “one and the same,” Skt. sama-; originally identical with PIE numeral *sam- “one,” from *som-).

(→ com-),

• gard, variant of kard “created, cut,” Mid.Pers. kirrēnītan, kirrēn- “to cut, create,” cf. fragard “section, chapter,” Av. karət- “to cut,” kərəntaiti “cuts, breaks,” with fraca- “to create, bring forth,”
  karəta- “knife, dagger” (Mid.Pers. kârt “knife;” Mod.Pers. kârd “knife”), Skt. kart- “to cut, split, break,” krti- “sword, knife;” PIE base *(s)kert- “to cut;” Hitt. kartae- “to cut;” Arm. kherthem “to skin;” L. cortex “bark of a tree,” corium “skin, leather;”
  O.H.G. scrinden “to split;” Lith. kertu “to fell, cut down.”
  

2. Forušodan, literally “to descend, go down,” from foru- “down, downward; below; beneath” (Mid.Pers. frôt “down, downward;” O.Pers. fravata “forward, downward;” cf. Skt. pravát- “a sloping path, the slope of a mountain”)

• šodan “to go, to pass; to become, to be, to be doing” (Mid.Pers. šudan, šaw- “to go;” Av. š(ii)auu-, šiyav- “to move, go,” šiyavati “goes,” šyaoθna- “activity; action; doing, working;” O.Pers. šiyav- “to go forth, set,” ašiyavam “I set forth;” cf. Skt. cyu- “to move to and fro, shake about; to stir,” cyávate “stirs himself, goes;” Gk. kinein “to move;” Goth. haitan “call, be called;” O.E. hatan “command, call;” PIE base *kei- “to move to and fro”).
  

3. Beštidan, from bešt past stem of Tabari/Mâzandarâni beštan “to place, put, set,” from intensive prefix be- + eštan, from heštan “to place, put,” → intercalate.

A collection of → nonempty subsets of a set A such that every element of A is in exactly one of the subsets. In other words, A is the → disjoint  → union of these subsets.

See also: → partition; → set.

The branch of mathematics that studies sets. Set theory is closely associated with the branch of mathematics known as logic. It was initiated by the German mathematician Georg Cantor (1845-1918).

See also: → set; → theory.

To install and configure hardware and software on a computer.

See also: Verb of → setup.

1. The act of setting; the appearance of a → celestial body below the → horizon. Opposite of → rising.
   

   2. A change in a software program or hardware device that adjusts it to the user’s preference. For example, date and time setting, language setting, display setting, font setting, etc.

See also: → set; → -ing.

Two graduated disks attached to the right ascension and declination axis of an equatorial mount used in amateur astronomy that help an observer find astronomical objects in the sky by their equatorial coordinates.

Etymology (EN): M.E.; O.E. settan “cause to sit, put in some place, fix firmly” (cf. O.N. setja, O.Fris. setta, Du. zetten, Ger. setzen); → circle.

Etymology (PE): Dâyeré, → circle; âmâj-giri “taking aim,” from âmâj “aim, target,” → point + giri “taking” (vebal 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”).

1. (tr.) To put in order; arrange in a desired state or condition.
   

2. (intr.) To come to rest or a halt. To become fixed in a particular place, direction, etc. (Dictionary.com).

Etymology (EN): M.E. set(t)len, O.E. setlan “to place,” derivative of setl “a seat; stall; position, abode;” related to sittan “to sit,” from Proto-Germanic *setla- (cognates: Middle Low German, Middle Dutch setel, Dutch zetel, German Sessel, Gothic sitls), from PIE *sedla- (cognates: L. sella “seat, chair,” O.C.S. sedlo “saddle,” O.E. sadol “saddle”), from root *sed-.

Etymology (PE): Niyâšidan, from Yidghda niâst- , Munji niôst- “to sit down;” Nâini âš-/âšis- “to become seated;” Baluci ništ, related to nešastan “to sit down,” → sit (see also → reside); ultimately from Proto-Ir. *had- “to sit, be setaed.”

A → galactic disk that has undergone → disk settling.

See also: → settle; → disk.

The act of a person or thing that settles. → disk settling, → dust settling.

See also: → settle; → -ing.

1. The process of connecting and preparing a software program, hardware device, or computer to function properly. Setup is a file commonly called “setup.exe” that is executed to begin installing a software program onto a computer. See also → set up.
   

   2. Equipment designed for a particular purpose; an apparatus.
      

   3. The fashion in which something is organized or arranged.

Etymology (EN): → set; → up.

Etymology (PE): Barbešt, from prefix bar-, → on-.

A → cardinal number between → six and → eight.

Etymology (EN): From M.E. seoven(e), seofne, seven, O.E. seofon, cognate with M.Du. seven, Du. zeven, O.H.G. sibun, Ger. sieben, related to Pers. haft, as below, from PIE *septm “seven.”

Etymology (PE): Haft, from Mid.Pers. haft, Av. hapta Skt. sapta, Gk. hepta, L. septem.

Relating to, or based on, the number 60.

Etymology (EN): From M.L. sexagesimalis, from L. sexagesimus “sixtieth,” from sexaginta “sixty.”

Etymology (PE): Šast-šasti, from Šast, → sixty.

A number system whose base is 60. It originated with the ancient Sumerians around 2000 B.C., was transmitted to the Babylonians, and is still used in modified form for measuring time, angles, and geographic coordinates.

See also: → sexagesimal; → system.

The Sextant. An inconspicuous constellation located on the celestial equator at 10h 20m, 0° declination. Its brightest star, Alpha Sextantis is of 4th magnitude. It was introduced in the 17th century by Johannes Hevelius (1611-1687). Abbreviation: Sex; genitive: Sextantis.

See also: The name is L. for → sextant, an instrument that Hevelius made frequent use of in his observations.

An instrument used mainly in nautical astronomy to determine the angular distances of celestial bodies above the horizon. The sextant consists of a 60° graduated arc, or limb, a small telescope, and two mirrors. Only half of one of the mirrors, the horizon glass, is silvered and, like the telescope, it is fastened to the frame supporting the limb. The other mirror, the index mirror, moves with an index arm pivoted at the center of the arc. The index arm is equipped with a → vernier which moves along the limb. An object in direction S can be observed in the telescope through the un-silvered portion of the horizon glass. By moving the index arm, the second object S’ is made to coincide with S in the telescope. According to the law of reflection, the angle between S and S’ is double the angle between the mirrors. The angular distance between the objects can therefore be obtained from the measurement of angles between the planes of the mirrors.

Etymology (EN): From Mod.L. sextans, from L. sextans “a sixth,” from sex→ six.

Etymology (PE): Šešakân, from šešak “a sixth,” from šeš, → six + -ak, contraction of yak “one,” (Mid.Pers. êwak; Proto-Iranian *aiua-ka-; O.Pers. aiva- “one, alone;” Av. aēuua- “one, alone” (cf. Skt. éka- “one, alone, single;” Gk. oios “alone, lonely;” L. unus “one;” E. one) + -ân nuance suffix.

Any group or set of six. → Seyfert’s sextet.

Etymology (EN): From Latinized sestet, from It. sestetto, diminutive of sesto “sixth,” from L. sextus “sixth,” from sex,
→ six.

Etymology (PE): Šeštâyé, from šeš, → six,

• tâ “fold, plait, ply; piece, part” (Mid.Pers. tâg “piece, part”)
• -(y)é nuance suffix.

A group or combination of six objects, such as the → Mizar and the → Castor star systems.

See also: From sextuple, from L. sext(us) “sixth,” → six, + et

A member of an important class of → active galaxies
which are characterized by the presence of an intensely bright nucleus in the optical wavelengths (10⁹-10¹²Lsun) displaying emission spectral lines. The presence of these emission features, which are not seen in the spectra of normal galaxies,
indicates a very high degree of ionization. Moreover, the nucleus
radiates → non-thermal  → continuum emission extending over the whole electromagnetic spectrum. It is thought that a → massive black hole in the nucleus of a galaxy accretes gas (→ accretion) from its surrounding environment to power Seyfert galaxies. These galaxies
are divided into two types according to the width of their spectral lines. Type 1 Seyfert (Sy 1) galaxies have very broad emission lines (10³- 10⁴ km s^-1), while Type 2 Seyferts (Sy 2) show relatively narrow lines (several hundred km s^-1). These spectral differences may be the result of viewing the nucleus from different
angles. A Type 2 Seyfert galaxy may be a mostly edge-on view of matter spiraling in toward the supermassive black hole, whereas a Type 1 Seyfert provides a more pole-on view, allowing us to see the more turbulent region around the black hole.

See also: Named after Carl Keenan Seyfert (1911-1960), the American astronomer who first identified this type of galaxies in 1943; → galaxy.

A group of galaxies about 190 million → light-years away in
the head portion of the constellation → Serpens. The group appears to contain six members, but actually contains only four → interacting galaxies. A face-on → spiral galaxy lies in the distant background and appears only by chance aligned with the main group. Also, a prominent condensation is likely not a separate galaxy, but a → tidal tail of stars flung out by the galaxies’ gravitational interactions. The interacting galaxies are tightly packed into a region around 100,000 light-years across, comparable to the size of our own Milky Way galaxy, making this one of the densest known galaxy groups.

See also: Discovered by the American astronomer Carl Keenan Seyfert (1911-1960) using photographic plates made at the Barnard Observatory of Vanderbilt University. → sextet.