An Etymological Dictionary of Astronomy and Astrophysics

A condition in which a dynamical system slightly displaced from its equilibrium configuration always tends to return to this configuration. → instability, → instability strip.

See also: Noun from adj. → stable.

A condition in which a dynamical system slightly displaced from its equilibrium configuration always tends to return to this configuration. → instability, → instability strip.

See also: Noun from adj. → stable.

Physics: 1) Having the ability to react to a disturbing force by maintaining or regaining position or condition.
2) Incapable of becoming a different isotope or element by radioactive decay.

Etymology (EN): M.E., from O.Fr. estable, from L. stabilis “firm, steadfast,” literally “able to stand,” from stem of stare “to stand;” cognate with Pers. istâdan “to stand” (Mid.Pers. êstâtan; O.Pers./Av. sta- “to stand, stand still; set;” Av. hištaiti; cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” Goth. standan; PIE base *sta- “to stand”).

Etymology (PE): Pâydâr “stable, firm” literally “having feet,” from pâ(y) “foot; step” (Mid.Pers. pâd, pây; Av. pad- “foot;” cf. Skt. pat; Gk. pos, genitive podos; L. pes, genitive pedis; P.Gmc. *fot; E. foot; Ger. Fuss; Fr. pied; PIE *pod-/*ped-) + dâr present stem of dâštan “to have, hold, maintain, possess” (Mid.Pers. dâštan; O.Pers./Av. root dar- “to hold, keep back, maintain, keep in mind;” cf.
Skt. dhr-, dharma- “law;”
Gk. thronos “elevated seat, throne;” L. firmus “firm, stable;” Lith. daryti “to make;” PIE *dher- “to hold, support”).

Physics: 1) Having the ability to react to a disturbing force by maintaining or regaining position or condition.
2) Incapable of becoming a different isotope or element by radioactive decay.

Etymology (EN): M.E., from O.Fr. estable, from L. stabilis “firm, steadfast,” literally “able to stand,” from stem of stare “to stand;” cognate with Pers. istâdan “to stand” (Mid.Pers. êstâtan; O.Pers./Av. sta- “to stand, stand still; set;” Av. hištaiti; cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” Goth. standan; PIE base *sta- “to stand”).

Etymology (PE): Pâydâr “stable, firm” literally “having feet,” from pâ(y) “foot; step” (Mid.Pers. pâd, pây; Av. pad- “foot;” cf. Skt. pat; Gk. pos, genitive podos; L. pes, genitive pedis; P.Gmc. *fot; E. foot; Ger. Fuss; Fr. pied; PIE *pod-/*ped-) + dâr present stem of dâštan “to have, hold, maintain, possess” (Mid.Pers. dâštan; O.Pers./Av. root dar- “to hold, keep back, maintain, keep in mind;” cf.
Skt. dhr-, dharma- “law;”
Gk. thronos “elevated seat, throne;” L. firmus “firm, stable;” Lith. daryti “to make;” PIE *dher- “to hold, support”).

An equilibrium state of a system in which if a small perturbation away from equilibrium is applied, the system will return to its equilibrium state. An example is a pendulum hanging straight down. If it is pushed slightly, it will experience a force back toward the equilibrium position. It may oscillate around the equilibrium position for a while, but it will finally regain its equilibrium position. → unstable equilibrium.

See also: → stable; → equilibrium.

An equilibrium state of a system in which if a small perturbation away from equilibrium is applied, the system will return to its equilibrium state. An example is a pendulum hanging straight down. If it is pushed slightly, it will experience a force back toward the equilibrium position. It may oscillate around the equilibrium position for a while, but it will finally regain its equilibrium position. → unstable equilibrium.

See also: → stable; → equilibrium.

A nuclide that is not → radioactive and therefore does not spontaneously undergo → radioactive decay.

See also: → stable; → nuclide.

A nuclide that is not → radioactive and therefore does not spontaneously undergo → radioactive decay.

See also: → stable; → nuclide.

1. A long stick used to help in walking. → Sharafeddin’s staff.
   

2. A body of persons, as employees, charged with carrying out the work of an establishment or executing some undertaking. → staff astronomer.

Etymology (EN): M.E. staf; O.E. stæf “walking stick, rod used as a weapon, pastoral staff;” sense of “group of military officers that assists a commander” attested from 1702; cf.
O.N. stafr, M.Du. staf, O.H.G. stab, Ger. Stab, M.Du. stapel “pillar, foundation;” PIE base *stebh- “to support, place firmly on, fasten; post, stem;” cognate with Av. stabra- “strong, firm” and other Iranian words, as below.

Etymology (PE): 1) Cubdast “hand stick,” from cub “staff, stick,” Mid.Pers. côp “wood, stick” + dast, → hand.

2. Estab, from Av. stabra- “strong, firm;” O.Pers. sta^mb- “to revolt;” Mid.Pers. stabr “strong, firm;” Mod.Pers. ustâm “column” [Steingass], Lori esi “tent pole,” setabr “strong, big, thick, dense,” setanbé “strong, powerful,” estam, setam “oppression;” cf. Skt. stabh- “support,” stambh- “to support, fix firmly;” Gk. stephein “to tie around, encircle,” astemphes “firm, rigid;” Lith. stebas “staff, pillar.”

1. A long stick used to help in walking. → Sharafeddin’s staff.
   

2. A body of persons, as employees, charged with carrying out the work of an establishment or executing some undertaking. → staff astronomer.

Etymology (EN): M.E. staf; O.E. stæf “walking stick, rod used as a weapon, pastoral staff;” sense of “group of military officers that assists a commander” attested from 1702; cf.
O.N. stafr, M.Du. staf, O.H.G. stab, Ger. Stab, M.Du. stapel “pillar, foundation;” PIE base *stebh- “to support, place firmly on, fasten; post, stem;” cognate with Av. stabra- “strong, firm” and other Iranian words, as below.

Etymology (PE): 1) Cubdast “hand stick,” from cub “staff, stick,” Mid.Pers. côp “wood, stick” + dast, → hand.

2. Estab, from Av. stabra- “strong, firm;” O.Pers. sta^mb- “to revolt;” Mid.Pers. stabr “strong, firm;” Mod.Pers. ustâm “column” [Steingass], Lori esi “tent pole,” setabr “strong, big, thick, dense,” setanbé “strong, powerful,” estam, setam “oppression;” cf. Skt. stabh- “support,” stambh- “to support, fix firmly;” Gk. stephein “to tie around, encircle,” astemphes “firm, rigid;” Lith. stebas “staff, pillar.”

A professional astronomer who works within a specified observatory or research group.

See also: → staff; → astronomer.

A professional astronomer who works within a specified observatory or research group.

See also: → staff; → astronomer.

A single step or phase in an ongoing process.

Etymology (EN): M.E., from O.Fr. estage “a story or floor of a building, stage for performance,” from V.L. *staticum “a place for standing,” from L. statum, p.p. of stare “to stand.”

Etymology (PE): Gâmé, from gâm “step, pace” (related to âmadan “to come”); Mid.Pers. gâm “step, stride, pace;”
O.Pers. gam- “to come; to go;” Av. gam- “to come; to go,” jamaiti “goes;” cf. Skt. gamati “goes;” Gk. bainein “to go, walk, step;” L. venire “to come;” Tocharian A käm- “to come;” O.H.G. queman “to come;” E. come; PIE stem *gwem- “to go, come.”

A single step or phase in an ongoing process.

Etymology (EN): M.E., from O.Fr. estage “a story or floor of a building, stage for performance,” from V.L. *staticum “a place for standing,” from L. statum, p.p. of stare “to stand.”

Etymology (PE): Gâmé, from gâm “step, pace” (related to âmadan “to come”); Mid.Pers. gâm “step, stride, pace;”
O.Pers. gam- “to come; to go;” Av. gam- “to come; to go,” jamaiti “goes;” cf. Skt. gamati “goes;” Gk. bainein “to go, walk, step;” L. venire “to come;” Tocharian A käm- “to come;” O.H.G. queman “to come;” E. come; PIE stem *gwem- “to go, come.”

The state or condition of not flowing or running.
→ stagnation point, → stagnation pressure.

Etymology (EN): L. stagnatum, stagnatus, p.p. of stagnare “to stagnate,” from stagnatum “standing water,” from PIE root *stag- “to seep drip.”

Etymology (PE): Nâravâni, literally “not flowing,” from nâ- negation prefix, → un-, + ravân “flowing, running,” pr.p. of raftan “to go, walk; to flow;” (Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- “to go; to attack”).

The state or condition of not flowing or running.
→ stagnation point, → stagnation pressure.

Etymology (EN): L. stagnatum, stagnatus, p.p. of stagnare “to stagnate,” from stagnatum “standing water,” from PIE root *stag- “to seep drip.”

Etymology (PE): Nâravâni, literally “not flowing,” from nâ- negation prefix, → un-, + ravân “flowing, running,” pr.p. of raftan “to go, walk; to flow;” (Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- “to go; to attack”).

A point where the → flow  → velocity is → zero. For example a point around an obstacle where a → flow tube splits into two portions.

See also: → stagnation; → point.

A point where the → flow  → velocity is → zero. For example a point around an obstacle where a → flow tube splits into two portions.

See also: → stagnation; → point.

The sum of → static pressure and → dynamic pressure in the → Bernoulli equation.

See also: → stagnation; → pressure.

The sum of → static pressure and → dynamic pressure in the → Bernoulli equation.

See also: → stagnation; → pressure.

An uncastrated adult male horse, especially one used for breeding.

Etymology (EN): M.E. stalon, from O.Fr. estalon, “uncastrated male horse,” cognate with O.H.G. stal “stable,” cf. O.H.G. stall “stand, place, stable, stall,” Ger. Stall “stable,” Stelle “place”), from PIE root *stel- “to put, stand,” with derivatives referring to a standing object or place; akin to Pers. istâdan “to stand,” → station.

Etymology (PE): Nariyân, from nar “male,” → masculine.

An uncastrated adult male horse, especially one used for breeding.

Etymology (EN): M.E. stalon, from O.Fr. estalon, “uncastrated male horse,” cognate with O.H.G. stal “stable,” cf. O.H.G. stall “stand, place, stable, stall,” Ger. Stall “stable,” Stelle “place”), from PIE root *stel- “to put, stand,” with derivatives referring to a standing object or place; akin to Pers. istâdan “to stand,” → station.

Etymology (PE): Nariyân, from nar “male,” → masculine.

To have or maintain an upright position, supported by one’s feet; rise to one’s feet (OxfordDictionaries.com).

Etymology (EN): M.E. standen, from O.En. standan “occupy a place; stand firm; stay, be, exist; oppose, resist attack; stand up, be on one’s feet;” cognate with O.Norse standa, O.Saxon and Gothic standan, O.H.G. stantan, Du. staan, Ger. stehen, cognate with Pers. istâdan, as below.

Etymology (PE): Istâdan “to stand,” from Mid.Pers. êstâtan; O.Pers./Av. sta- “to stand, stand still; set;”
Av. hištaiti; cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” Goth. standan; PIE base *sta- “to stand.”

To have or maintain an upright position, supported by one’s feet; rise to one’s feet (OxfordDictionaries.com).

Etymology (EN): M.E. standen, from O.En. standan “occupy a place; stand firm; stay, be, exist; oppose, resist attack; stand up, be on one’s feet;” cognate with O.Norse standa, O.Saxon and Gothic standan, O.H.G. stantan, Du. staan, Ger. stehen, cognate with Pers. istâdan, as below.

Etymology (PE): Istâdan “to stand,” from Mid.Pers. êstâtan; O.Pers./Av. sta- “to stand, stand still; set;”
Av. hištaiti; cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” Goth. standan; PIE base *sta- “to stand.”

Any set of conditions that describe the normal, desired, or ideal state of something, and that serves a basis for representing or evaluating actual examples of this thing.

Etymology (EN): M.E., from O.Fr. estandart “banner, standard,” probably from Frankish *standord; cf. Ger. Standort “standing-point,” from standan “to stand,” cognate with Pers. istâdan, as below, with the second component conformed to -ard.

Etymology (PE): Estândé, literally “made stand, fixed,” p.p. istândan transitive verb of istâdan, “to → stand.”

Any set of conditions that describe the normal, desired, or ideal state of something, and that serves a basis for representing or evaluating actual examples of this thing.

Etymology (EN): M.E., from O.Fr. estandart “banner, standard,” probably from Frankish *standord; cf. Ger. Standort “standing-point,” from standan “to stand,” cognate with Pers. istâdan, as below, with the second component conformed to -ard.

Etymology (PE): Estândé, literally “made stand, fixed,” p.p. istândan transitive verb of istâdan, “to → stand.”

A hypothetical vertical distribution of atmospheric temperature, pressure, and density that, by international agreement, is taken to be representative of the atmosphere for purposes of pressure altimeter calibrations, aircraft performance calculations, aircraft and missile design, ballistic tables, etc.

See also: → standard; → atmosphere.

A hypothetical vertical distribution of atmospheric temperature, pressure, and density that, by international agreement, is taken to be representative of the atmosphere for purposes of pressure altimeter calibrations, aircraft performance calculations, aircraft and missile design, ballistic tables, etc.

See also: → standard; → atmosphere.

An astronomical object, belonging to some class, that has a known luminosity. In principle, by comparing the known luminosity to the observed brightness, the distance to the object can be derived. The four major primary distance indicators are Cepheids, supernovae, novae, and RR Lyrae variables. The secondary distance indicators include H II regions, globular clusters, brightest red and blue stars. → primary calibrator; → secondary calibrator.

See also: → standard; → candle.

An astronomical object, belonging to some class, that has a known luminosity. In principle, by comparing the known luminosity to the observed brightness, the distance to the object can be derived. The four major primary distance indicators are Cepheids, supernovae, novae, and RR Lyrae variables. The secondary distance indicators include H II regions, globular clusters, brightest red and blue stars. → primary calibrator; → secondary calibrator.

See also: → standard; → candle.

The conventional → Big Bang model, which is based on two assumptions: the → cosmological principle of homogeneity and isotropy leading to the → Robertson-Walker metric, and → Einstein’s field equations of general relativity along with familiar properties of matter. This model is a remarkably successful operating hypothesis describing the evolution of the Universe from 1/100 second after the initial event through to the present day. It provides explanations for several basic problems such as: → Hubble’s law of recession of galaxies, interpreted in terms of the expansion of the Universe; the abundances of the → light elements, in excellent agreement with the predictions of → primordial nucleosynthesis; and the thermal spectrum and angular isotropy of the → cosmic microwave background (CMB) radiation, as expected from a hot, dense early phase of expansion. For a non-standard model, see → ekpyrotic Universe.

See also: → standard; → cosmology.

The conventional → Big Bang model, which is based on two assumptions: the → cosmological principle of homogeneity and isotropy leading to the → Robertson-Walker metric, and → Einstein’s field equations of general relativity along with familiar properties of matter. This model is a remarkably successful operating hypothesis describing the evolution of the Universe from 1/100 second after the initial event through to the present day. It provides explanations for several basic problems such as: → Hubble’s law of recession of galaxies, interpreted in terms of the expansion of the Universe; the abundances of the → light elements, in excellent agreement with the predictions of → primordial nucleosynthesis; and the thermal spectrum and angular isotropy of the → cosmic microwave background (CMB) radiation, as expected from a hot, dense early phase of expansion. For a non-standard model, see → ekpyrotic Universe.

See also: → standard; → cosmology.

The most widely used measure of dispersion of a frequency distribution. It is equal to the positive square root of the → variance. Same as → standard error. Not to be confused with the
→ root mean square error.

See also: → standard; → deviation.

The most widely used measure of dispersion of a frequency distribution. It is equal to the positive square root of the → variance. Same as → standard error. Not to be confused with the
→ root mean square error.

See also: → standard; → deviation.

A particular date and time that specifies the reference system to which celestial coordinates are referred. From 1984 the → Julian year is used, as denoted by the prefix J, e.g. J2000.0.

See also: → standard; → epoch.

A particular date and time that specifies the reference system to which celestial coordinates are referred. From 1984 the → Julian year is used, as denoted by the prefix J, e.g. J2000.0.

See also: → standard; → epoch.

Same as → standard deviation.

See also: → standard; → error.

Same as → standard deviation.

See also: → standard; → error.

The accepted but possibly incomplete theoretical framework which usually describes a set of phenomena. For example, the model that describes the origin of the Universe, or the model concerned with the processes in the interior of the Sun.

See also: → standard; → model.

The accepted but possibly incomplete theoretical framework which usually describes a set of phenomena. For example, the model that describes the origin of the Universe, or the model concerned with the processes in the interior of the Sun.

See also: → standard; → model.

The theory developed since the 1970s, which is based on the theories and discoveries since the 1930s, and aims at explaining the fundamental structure of matter. According to the standard model, everything in the universe is made from a few basic building blocks called fundamental particles, governed by four fundamental forces. The particles occur in two basic types, called quarks and leptons. Three of the four fundamental forces (except gravity) and their carrier particles are included in the Standard Model. The Standard Model has successfully explained almost all experimental results and precisely predicted a wide variety of phenomena. Over time and through many experiments, the Standard Model has become established as a well-tested physics theory.

See also: → standard; → model; → particle; → physics.

The theory developed since the 1970s, which is based on the theories and discoveries since the 1930s, and aims at explaining the fundamental structure of matter. According to the standard model, everything in the universe is made from a few basic building blocks called fundamental particles, governed by four fundamental forces. The particles occur in two basic types, called quarks and leptons. Three of the four fundamental forces (except gravity) and their carrier particles are included in the Standard Model. The Standard Model has successfully explained almost all experimental results and precisely predicted a wide variety of phenomena. Over time and through many experiments, the Standard Model has become established as a well-tested physics theory.

See also: → standard; → model; → particle; → physics.

Stars for which accurate color indices and/or magnitudes exist, defining a standard system.

See also: → standard; → star.

Stars for which accurate color indices and/or magnitudes exist, defining a standard system.

See also: → standard; → star.

Photometric system used as a reference.

See also: → standard; → system.

Photometric system used as a reference.

See also: → standard; → system.

1. The most commonly used definition is temperature of 273.15 K (0 °C) and pressure of 1 → atmosphere.
   

2. Chemistry: Temperature of 273.15 K (0 °C) and pressure of 10⁵  → pascal (Pa)s (1 → bar). International Union of Pure and Applied Chemistry (IUPAC) recommends that the former use of the pressure of 1 atm as standard pressure (equivalent to 1.01325 × 10⁵ Pa) should be discontinued.

See also: → standard; → temperature; → pressure.

1. The most commonly used definition is temperature of 273.15 K (0 °C) and pressure of 1 → atmosphere.
   

2. Chemistry: Temperature of 273.15 K (0 °C) and pressure of 10⁵  → pascal (Pa)s (1 → bar). International Union of Pure and Applied Chemistry (IUPAC) recommends that the former use of the pressure of 1 atm as standard pressure (equivalent to 1.01325 × 10⁵ Pa) should be discontinued.

See also: → standard; → temperature; → pressure.

The time in any of the 24 internationally agreed time zones into which the Earth’s surface is divided. The primary zone is centered on the Greenwich meridian (0° longitude).

See also: → standard; → time.

The time in any of the 24 internationally agreed time zones into which the Earth’s surface is divided. The primary zone is centered on the Greenwich meridian (0° longitude).

See also: → standard; → time.

Photometric values of selected stars in a standard system.

See also: → standard; → value.

Photometric values of selected stars in a standard system.

See also: → standard; → value.

A wave produced by the simultaneous transmission of two similar wave motions in opposite directions. Same as stationary wave.

Etymology (EN): Standing verbal adjective from stand, cognate with Pers. istâdan, as below; → wave.

Etymology (PE): Istân pr.p. of istâdan “to stand;” Mid.Pers. êstâtan; O.Pers./Av. sta- “to stand, stand still; set;” Av. hištaiti; cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” O.N. standa, Goth. standan, O.H.G. stantan, Swed. stå, Du. staan, Ger. stehen; O.E. standan; PIE base *sta- “to stand;” mowj, → wave.

A wave produced by the simultaneous transmission of two similar wave motions in opposite directions. Same as stationary wave.

Etymology (EN): Standing verbal adjective from stand, cognate with Pers. istâdan, as below; → wave.

Etymology (PE): Istân pr.p. of istâdan “to stand;” Mid.Pers. êstâtan; O.Pers./Av. sta- “to stand, stand still; set;” Av. hištaiti; cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” O.N. standa, Goth. standan, O.H.G. stantan, Swed. stå, Du. staan, Ger. stehen; O.E. standan; PIE base *sta- “to stand;” mowj, → wave.

A huge mass of hot gas whose radiation is provided by its internal → thermonuclear reactions.
A star represents a → hydrodynamic equilibrium between two opposing forces, the inward → gravitational force, which is attempting to make the mass collapse and the pressure caused by the generation of nuclear energy. Below a certain mass (0.08 → solar masses), the central pressures and temperatures are insufficient to trigger the → hydrogen fusion (→ brown dwarf). Stars have a variety of masses and sizes. → Massive stars are less common than → low-mass stars (→ initial mass function).
→ Star formation results from → gravitational collapse of → molecular clouds (→ fragmentation; → pre-stellar core; → protostar; → accretion). After leaving the → main sequence, they
pass through several evolutionary stages (e.g., → red giant, → supergiant, → white dwarf, → supernova, → neutron star) depending on their initial masses. See also: → internal structure of stars; → spectral classification; → luminosity class; → variable star; → multiple star. The term star is sometimes loosely applied to objects that do not comply with the above specifications, but are evolutionary products of stars, such as neutron stars and white dwarfs. For ancient civilizations a star was anything appearing in the night sky, apart from perhaps the Moon.

Etymology (EN): M.E. sterre, O.E. steorra; cf. O.S. sterro, O.N. stjarna, O.Fris. stera, Du. ster, O.H.G. sterro, Ger. Stern, Goth. stairno;
cognate with Gk. aster, astron, L. stella
(Fr. étoile, Sp. esterella, It. stella), Bret. sterenn, Pers. setâré, as below.

Etymology (PE): Setâré, variants star, estâr, estâré, and probably axtar, → astro-, (Lori, Laki) âsâra, (Laki) hasâra, (Tabari) essâra, (Baluci) istâr, (Ossetic) st’aly, (i)sthalu, (Tâti) usdurâ; Mid.Pers. stârag, stâr; Av. star-; cf. Skt. stár-, tāra-, tārakā- “star;” akin to Gk. and L., as above; PIE base *ster- “star.”

A huge mass of hot gas whose radiation is provided by its internal → thermonuclear reactions.
A star represents a → hydrodynamic equilibrium between two opposing forces, the inward → gravitational force, which is attempting to make the mass collapse and the pressure caused by the generation of nuclear energy. Below a certain mass (0.08 → solar masses), the central pressures and temperatures are insufficient to trigger the → hydrogen fusion (→ brown dwarf). Stars have a variety of masses and sizes. → Massive stars are less common than → low-mass stars (→ initial mass function).
→ Star formation results from → gravitational collapse of → molecular clouds (→ fragmentation; → pre-stellar core; → protostar; → accretion). After leaving the → main sequence, they
pass through several evolutionary stages (e.g., → red giant, → supergiant, → white dwarf, → supernova, → neutron star) depending on their initial masses. See also: → internal structure of stars; → spectral classification; → luminosity class; → variable star; → multiple star. The term star is sometimes loosely applied to objects that do not comply with the above specifications, but are evolutionary products of stars, such as neutron stars and white dwarfs. For ancient civilizations a star was anything appearing in the night sky, apart from perhaps the Moon.

Etymology (EN): M.E. sterre, O.E. steorra; cf. O.S. sterro, O.N. stjarna, O.Fris. stera, Du. ster, O.H.G. sterro, Ger. Stern, Goth. stairno;
cognate with Gk. aster, astron, L. stella
(Fr. étoile, Sp. esterella, It. stella), Bret. sterenn, Pers. setâré, as below.

Etymology (PE): Setâré, variants star, estâr, estâré, and probably axtar, → astro-, (Lori, Laki) âsâra, (Laki) hasâra, (Tabari) essâra, (Baluci) istâr, (Ossetic) st’aly, (i)sthalu, (Tâti) usdurâ; Mid.Pers. stârag, stâr; Av. star-; cf. Skt. stár-, tāra-, tārakā- “star;” akin to Gk. and L., as above; PIE base *ster- “star.”

A listing of stars usually ordered by right ascension with observational data elements such as coordinates, magnitude, distance, proper motion, and so on.

See also: → star; → catalog.

A listing of stars usually ordered by right ascension with observational data elements such as coordinates, magnitude, distance, proper motion, and so on.

See also: → star; → catalog.

A chart or map showing the relative apparent positions of the stars as viewed from the Earth.

Etymology (EN): → star; chart, from M.Fr. charte “card, map,” from L. charta “leaf of paper, tablet,” from Gk. khartes “layer of papyrus.”

Etymology (PE): Negâré, from negâr “picture, figure,” from negâštan→ Pictor; setâregân plural of setâré→ star.

A chart or map showing the relative apparent positions of the stars as viewed from the Earth.

Etymology (EN): → star; chart, from M.Fr. charte “card, map,” from L. charta “leaf of paper, tablet,” from Gk. khartes “layer of papyrus.”

Etymology (PE): Negâré, from negâr “picture, figure,” from negâštan→ Pictor; setâregân plural of setâré→ star.

1. A group of stars held together by the mutual → gravitational attraction of its members, which are physically related through common origin. They are of two types: → open clusters and → globular clusters.
   

2. A → bound stellar agglomeration for which the age of the stars exceeds the → crossing time (Giels & Portegies Zwart, 2010, MNRAS Letters, astro-ph/1010.1720). See also → stellar association

See also: → star; → cluster.

1. A group of stars held together by the mutual → gravitational attraction of its members, which are physically related through common origin. They are of two types: → open clusters and → globular clusters.
   

2. A → bound stellar agglomeration for which the age of the stars exceeds the → crossing time (Giels & Portegies Zwart, 2010, MNRAS Letters, astro-ph/1010.1720). See also → stellar association

See also: → star; → cluster.

The number of stars that appear in a given region of sky, usually counted on a photographic plate or CCD image.

See also: → star; → count.

The number of stars that appear in a given region of sky, usually counted on a photographic plate or CCD image.

See also: → star; → count.

The relative motion of two groups of stars in the Galaxy moving in opposite directions.

See also: → star; → drift.

The relative motion of two groups of stars in the Galaxy moving in opposite directions.

See also: → star; → drift.

The process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy, star formation includes the study of the interstellar medium and
molecular clouds as precursors to the star formation process as well as the study of young stellar objects.

See also: → star; → formation.

The process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy, star formation includes the study of the interstellar medium and
molecular clouds as precursors to the star formation process as well as the study of young stellar objects.

See also: → star; → formation.

The degree to which stars form in a system, such as a → molecular cloud or a → galaxy. It is given by the ratio of the total mass of stars to the initial gas mass: ε_SFE = M_stars / (M_stars + M_gas).

See also: → star formation; → efficiency.

The degree to which stars form in a system, such as a → molecular cloud or a → galaxy. It is given by the ratio of the total mass of stars to the initial gas mass: ε_SFE = M_stars / (M_stars + M_gas).

See also: → star formation; → efficiency.

The → star formation rate as a function of time.

See also: → star; → formation; → history.

The → star formation rate as a function of time.

See also: → star; → formation; → history.

The premature termination of star formation process in some galaxies.
The ultimate quenching of star formation is caused by stripping of the gas reservoir which will finally turn into stars.
A wide variety of mechanisms have been proposed to provide quenching. For example, → major mergers can transform spiral galaxies into ellipticals, and may also quench future star formation by ejecting the → interstellar medium from the galaxy via starburst, → active galactic nucleus, or shock-driven winds. In rich clusters, where merging is less efficient because of the large relative velocities of galaxies,
rapid encounters or fly-bys may cause the formation of a bar and growth of a spheroidal component instead of larger scale star formation. Also, cold gas can be stripped out of the galaxy both by tidal forces
and ram pressure in the intracluster medium. Similarly, the hot halo that provides future fuel for cooling and star
formation may be efficiently stripped in dense environments, thus quenching further star formation (see, e.g., Kimm et al., 2009, MNRAS 394, 1131, arXiv:0810.2794).

See also: → star; → formation; → quench.

The premature termination of star formation process in some galaxies.
The ultimate quenching of star formation is caused by stripping of the gas reservoir which will finally turn into stars.
A wide variety of mechanisms have been proposed to provide quenching. For example, → major mergers can transform spiral galaxies into ellipticals, and may also quench future star formation by ejecting the → interstellar medium from the galaxy via starburst, → active galactic nucleus, or shock-driven winds. In rich clusters, where merging is less efficient because of the large relative velocities of galaxies,
rapid encounters or fly-bys may cause the formation of a bar and growth of a spheroidal component instead of larger scale star formation. Also, cold gas can be stripped out of the galaxy both by tidal forces
and ram pressure in the intracluster medium. Similarly, the hot halo that provides future fuel for cooling and star
formation may be efficiently stripped in dense environments, thus quenching further star formation (see, e.g., Kimm et al., 2009, MNRAS 394, 1131, arXiv:0810.2794).

See also: → star; → formation; → quench.

The rate at which a molecular cloud or a galaxy is currently converting gas into stars. It is given by the ratio of the number of stars to the star formation time-scale.

See also: → star formation; → rate.

The rate at which a molecular cloud or a galaxy is currently converting gas into stars. It is given by the ratio of the number of stars to the star formation time-scale.

See also: → star formation; → rate.

A region in the → interstellar medium where processes of → star formation are going on or have occurred in the past.

See also: → star; → formation; → region.

A region in the → interstellar medium where processes of → star formation are going on or have occurred in the past.

See also: → star; → formation; → region.

The time necessary for a star to form. It depends inversely on the stellar mass.

See also: → star formation; → time scale.

The time necessary for a star to form. It depends inversely on the stellar mass.

See also: → star formation; → time scale.

A → main sequence→ B-type star that orbits the → supermassive black hole candidate → Sgr A* in the → Galactic center. The star S2, which is bright enough for making detailed measurements, has a highly elliptical, 16-year-period orbit around Sgr A*. Near → pericenter at 120 → astronomical units, ~ 1400 → Schwarzschild radii, the star has an orbital speed of ~ 7650 km s^-1, such that the first-order effects of → special relativity and → general relativity have become detectable with current capabilities (Auber et al., 2018, A&A 615, L15).

See also: → star.

A → main sequence→ B-type star that orbits the → supermassive black hole candidate → Sgr A* in the → Galactic center. The star S2, which is bright enough for making detailed measurements, has a highly elliptical, 16-year-period orbit around Sgr A*. Near → pericenter at 120 → astronomical units, ~ 1400 → Schwarzschild radii, the star has an orbital speed of ~ 7650 km s^-1, such that the first-order effects of → special relativity and → general relativity have become detectable with current capabilities (Auber et al., 2018, A&A 615, L15).

See also: → star.

Same as → stellar system.

See also: → stellar; → system.

Same as → stellar system.

See also: → stellar; → system.

A curved → path left by a star on an → imaging detector attached to a → telescope when the telescope does not keep up with the → rotation of the → Earth.

See also: → star; → trail.

A curved → path left by a star on an → imaging detector attached to a → telescope when the telescope does not keep up with the → rotation of the → Earth.

See also: → star; → trail.

A galaxy that is located on the → galaxy main sequence in the plane relating → star formation rates to total stellar masses.

See also: → star; → formation; → galaxy.

A galaxy that is located on the → galaxy main sequence in the plane relating → star formation rates to total stellar masses.

See also: → star; → formation; → galaxy.

A region in which → star formation is going on.

See also: → star; → formation; → region.

A region in which → star formation is going on.

See also: → star; → formation; → region.

Simultaneous formation of a large number of stars in a region of a galaxy at an exceptionally high rate, compared to the usual star formation rates seen in most galaxies.

See also: → star; → burst.

Simultaneous formation of a large number of stars in a region of a galaxy at an exceptionally high rate, compared to the usual star formation rates seen in most galaxies.

See also: → star; → burst.

A galaxy showing a short-lived intense period of star formation that is unsustainable over the → Hubble time due to the limited supply of gas within a galaxy. Starburst galaxies were first classified by Searle & Sargent (1972) and Searle et al. (1973), based on the blue colors produced by the → massive stars formed during the burst. In the local Universe, starbursts create approximately 10% of the radiant energy and 20% of the massive stars. At z = 1, starburst characteristics are found in 15% of galaxies, presumably attributable to the greater amounts of gas typically present in young galaxies and increased galactic interactions. The starburst’s impact on a galaxy and the surrounding → intergalactic medium is primarily due to the consumption of gas that fuels the burst and the feedback from massive stars formed in the burst (McQuinn et al. 2010, astro-ph/1008.1589).

See also: → starburst; → galaxy.

A galaxy showing a short-lived intense period of star formation that is unsustainable over the → Hubble time due to the limited supply of gas within a galaxy. Starburst galaxies were first classified by Searle & Sargent (1972) and Searle et al. (1973), based on the blue colors produced by the → massive stars formed during the burst. In the local Universe, starbursts create approximately 10% of the radiant energy and 20% of the massive stars. At z = 1, starburst characteristics are found in 15% of galaxies, presumably attributable to the greater amounts of gas typically present in young galaxies and increased galactic interactions. The starburst’s impact on a galaxy and the surrounding → intergalactic medium is primarily due to the consumption of gas that fuels the burst and the feedback from massive stars formed in the burst (McQuinn et al. 2010, astro-ph/1008.1589).

See also: → starburst; → galaxy.

The → splitting of spectral lines of atoms and molecules
due to the presence of an external electric field, which slightly changes the → energy levels of the atom. → Zeeman effect.

See also: Named after Johannes Stark (1874-1957), a German physicist, and Physics Nobel Prize laureate (1919); → effect.

The → splitting of spectral lines of atoms and molecules
due to the presence of an external electric field, which slightly changes the → energy levels of the atom. → Zeeman effect.

See also: Named after Johannes Stark (1874-1957), a German physicist, and Physics Nobel Prize laureate (1919); → effect.

An astrophysical phenomenon that occurs when the → crust of a → neutron star undergoes a sudden adjustment, analogous to an → earthquake on Earth. Starquakes are thought to be caused by huge → stresses exerted on the surface of the neutron star produced by twists in the ultra-strong interior → magnetic fields. They are thought to be the source of the intense → gamma-ray bursts that come from → soft gamma repeaters.

See also: → star; → quake.

An astrophysical phenomenon that occurs when the → crust of a → neutron star undergoes a sudden adjustment, analogous to an → earthquake on Earth. Starquakes are thought to be caused by huge → stresses exerted on the surface of the neutron star produced by twists in the ultra-strong interior → magnetic fields. They are thought to be the source of the intense → gamma-ray bursts that come from → soft gamma repeaters.

See also: → star; → quake.

A phenomenon similar to a → sunspot but occurring on the surface of a star
other than Sun. Due to spatial resolution constraints, starspots so far observed are in general much larger than those on the Sun, up to about 30% of the stellar surface may be covered, corresponding to sizes 100 times greater than those on the Sun.

See also: → star; → spot.

A phenomenon similar to a → sunspot but occurring on the surface of a star
other than Sun. Due to spatial resolution constraints, starspots so far observed are in general much larger than those on the Sun, up to about 30% of the stellar surface may be covered, corresponding to sizes 100 times greater than those on the Sun.

See also: → star; → spot.

A prefix attached to the name of a practical electrical unit indicating that it is part of the → CGS electrostatic system, e.g. statcoulomb, statvolt. These units are also indicated by the notation → esu (as in “volt esu”).

See also: Combining form representing → electrostatic, → -stat

A prefix attached to the name of a practical electrical unit indicating that it is part of the → CGS electrostatic system, e.g. statcoulomb, statvolt. These units are also indicated by the notation → esu (as in “volt esu”).

See also: Combining form representing → electrostatic, → -stat

A unit of → electric charge in the electrostatic → cgs system of units; equal to the charge that exerts a force of 1 → dyne on an equal charge at a distance of 1 cm under vacuum; equal to 3.3356 x 10^-10 → coulombs. Same as → electrostatic unit (esu).

See also: → stat-; → coulomb.

A unit of → electric charge in the electrostatic → cgs system of units; equal to the charge that exerts a force of 1 → dyne on an equal charge at a distance of 1 cm under vacuum; equal to 3.3356 x 10^-10 → coulombs. Same as → electrostatic unit (esu).

See also: → stat-; → coulomb.

1a) The → condition of a → system characterized by a particular set of values for its properties.
1b) The → phase of matter; → solid, → liquid, or → gas.
See also:
→ eigenstate, → energy state, → equation of state, → equation of state parameter, → equilibrium state, → excited state, → ground state, → Hartle-Hawking initial state, → Hoyle state, → macroscopic state, → macrostate, → metastable state, → microscopic state, → microstate, → normal state, → quantum state, → singlet state, → solid state, → solid state physics, → steady state theory, → triplet state, → virial equation of state.

2a) To declare definitely or specifically.

2b) To set forth formally in speech or writing (to state a hypothesis).

2c) To set forth in proper or definite form (Dictionary.com).

Etymology (EN): M.E. stat, partly from M.Fr. estat, partly from
L. status “manner of standing, position, condition,” noun of action from p.p. stem of stare “to stand;” cognate with Pers. istâdan “to stand,” as below.

The sense of “declare in words” (1640s) comes from the notion of “placing
(something on the record), setting in a position.”

Etymology (PE): Estât, from istâdan “to stand;” Mid.Pers. êstâtan; O.Pers./Av. sta- “to stand, stand still; to set” (Sogd. ôštât “to stand”);
Av. hištaiti; cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” Goth. standan; PIE base *sta- “to stand”);

hâlat, from Ar. Hâlat “state, quality.”

1a) The → condition of a → system characterized by a particular set of values for its properties.
1b) The → phase of matter; → solid, → liquid, or → gas.
See also:
→ eigenstate, → energy state, → equation of state, → equation of state parameter, → equilibrium state, → excited state, → ground state, → Hartle-Hawking initial state, → Hoyle state, → macroscopic state, → macrostate, → metastable state, → microscopic state, → microstate, → normal state, → quantum state, → singlet state, → solid state, → solid state physics, → steady state theory, → triplet state, → virial equation of state.

2a) To declare definitely or specifically.

2b) To set forth formally in speech or writing (to state a hypothesis).

2c) To set forth in proper or definite form (Dictionary.com).

Etymology (EN): M.E. stat, partly from M.Fr. estat, partly from
L. status “manner of standing, position, condition,” noun of action from p.p. stem of stare “to stand;” cognate with Pers. istâdan “to stand,” as below.

The sense of “declare in words” (1640s) comes from the notion of “placing
(something on the record), setting in a position.”

Etymology (PE): Estât, from istâdan “to stand;” Mid.Pers. êstâtan; O.Pers./Av. sta- “to stand, stand still; to set” (Sogd. ôštât “to stand”);
Av. hištaiti; cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” Goth. standan; PIE base *sta- “to stand”);

hâlat, from Ar. Hâlat “state, quality.”

1. Something stated.
   

2. A communication or declaration in speech or writing, setting forth facts, particulars, etc.
   

3. A single sentence or assertion (Dictionary.com).

See also: Verbal noun of → state (v.)

1. Something stated.
   

2. A communication or declaration in speech or writing, setting forth facts, particulars, etc.
   

3. A single sentence or assertion (Dictionary.com).

See also: Verbal noun of → state (v.)

A person who is experienced in the art of government or versed in the administration of government affairs (Dictionary.com).

See also: From state’s man, translation of Fr. homme d’Etat; → state; → man.

A person who is experienced in the art of government or versed in the administration of government affairs (Dictionary.com).

See also: From state’s man, translation of Fr. homme d’Etat; → state; → man.

1. Pertaining to bodies, forces, charges, etc. that act in equilibrium; at rest; stationary.
   

2. Pertaining to → statistics.

Etymology (EN): From Mod.L. statica, from Gk. statikos “causing to stand,”
from stem of histanai “to cause to stand,” cognate with Pers. istâdan “to stand,” as below.

Etymology (PE): Istâ “standing, static,” from istâdan “to stand” (Mid.Pers. êstâtan;
O.Pers./Av. sta- “to stand, stand still; set;” Av. hištaiti; cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” Goth. standan; PIE base *sta- “to stand”).

1. Pertaining to bodies, forces, charges, etc. that act in equilibrium; at rest; stationary.
   

2. Pertaining to → statistics.

Etymology (EN): From Mod.L. statica, from Gk. statikos “causing to stand,”
from stem of histanai “to cause to stand,” cognate with Pers. istâdan “to stand,” as below.

Etymology (PE): Istâ “standing, static,” from istâdan “to stand” (Mid.Pers. êstâtan;
O.Pers./Av. sta- “to stand, stand still; set;” Av. hištaiti; cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” Goth. standan; PIE base *sta- “to stand”).

The state of a rigid body which is not moving at all.
The conditions for static equilibrium are: 1) the sum of the external forces on the object must equal zero, and 2) the sum of the → torques must equal zero. See also → dynamic equilibrium and → mechanical equilibrium.

See also: → static; → equilibrium.

The state of a rigid body which is not moving at all.
The conditions for static equilibrium are: 1) the sum of the external forces on the object must equal zero, and 2) the sum of the → torques must equal zero. See also → dynamic equilibrium and → mechanical equilibrium.

See also: → static; → equilibrium.

Same as → stationary limit.

See also: → static; → limit.

Same as → stationary limit.

See also: → static; → limit.

In → fluid mechanics, the → pressure felt by an object suspended in a → fluid and moving with it. This pressure is called static because the object is not moving relative to the fluid. See also → dynamic pressure.

See also: → static; → pressure.

In → fluid mechanics, the → pressure felt by an object suspended in a → fluid and moving with it. This pressure is called static because the object is not moving relative to the fluid. See also → dynamic pressure.

See also: → static; → pressure.

A closed Universe of finite volume with a constant radius of curvature.

See also: → static; → Universe.

A closed Universe of finite volume with a constant radius of curvature.

See also: → static; → Universe.

The branch of → mechanics which studies the laws of composition of forces and the conditions of equilibrium of material bodies under the action of forces.

Etymology (EN): → static; → -ics.

Etymology (PE): Istâyik, from istâ, → static + -ik, → -ics.

The branch of → mechanics which studies the laws of composition of forces and the conditions of equilibrium of material bodies under the action of forces.

Etymology (EN): → static; → -ics.

Etymology (PE): Istâyik, from istâ, → static + -ik, → -ics.

A stopping place for trains or other land vehicles, for the transfer of freight or passengers. → space station.

Etymology (EN): M.E., from O.Fr. station, from L. stationem (nominative statio) “a standing, job, position,” related to stare “to stand,” cognate with Pers. istâdan “to stand,” as below.

Etymology (PE): Istgâh “standing place,” from ist present stem of istâdan “to stand” (Mid.Pers. êstâtan; O.Pers./Av. sta- “to stand, stand still; set;” Av. hištaiti; cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” Goth. standan; PIE base *sta- “to stand”) + gâh “place; time” (Mid.Pers. gâh, gâs “time;” O.Pers. gāθu-; Av. gātav-, gātu- “place, throne, spot;” cf. Skt. gâtu- “going, motion; free space for moving; place of abode;” PIE *gwem- “to go, come”).

A stopping place for trains or other land vehicles, for the transfer of freight or passengers. → space station.

Etymology (EN): M.E., from O.Fr. station, from L. stationem (nominative statio) “a standing, job, position,” related to stare “to stand,” cognate with Pers. istâdan “to stand,” as below.

Etymology (PE): Istgâh “standing place,” from ist present stem of istâdan “to stand” (Mid.Pers. êstâtan; O.Pers./Av. sta- “to stand, stand still; set;” Av. hištaiti; cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” Goth. standan; PIE base *sta- “to stand”) + gâh “place; time” (Mid.Pers. gâh, gâs “time;” O.Pers. gāθu-; Av. gātav-, gātu- “place, throne, spot;” cf. Skt. gâtu- “going, motion; free space for moving; place of abode;” PIE *gwem- “to go, come”).

Having a fixed, unchanging position; motionless. geostationary orbit

Etymology (EN): M.E. from L. stationarius, in classical L., “of a military station,” from statio, → station.

Etymology (PE): Isatvar, from ist present stem of istâdan “to stand” (Mid.Pers. êstâtan; O.Pers./Av. sta- “to stand, stand still; set;” Av. hištaiti; cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” Goth. standan; PIE base *sta- “to stand”) + -var suffix of possession, variant
-ur (Mid.Pers. -uwar, -war;
from O.Pers. -bara, from bar- “to bear, carry”).

Having a fixed, unchanging position; motionless. geostationary orbit

Etymology (EN): M.E. from L. stationarius, in classical L., “of a military station,” from statio, → station.

Etymology (PE): Isatvar, from ist present stem of istâdan “to stand” (Mid.Pers. êstâtan; O.Pers./Av. sta- “to stand, stand still; set;” Av. hištaiti; cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” Goth. standan; PIE base *sta- “to stand”) + -var suffix of possession, variant
-ur (Mid.Pers. -uwar, -war;
from O.Pers. -bara, from bar- “to bear, carry”).

A → black hole with zero → angular momentum, that does not rotate.

See also: → stationary; → black hole.

A → black hole with zero → angular momentum, that does not rotate.

See also: → stationary; → black hole.

A property of → space-time outside a → rotating black hole, which consists
of a surface which geometrically bounds the → ergosphere outward. At the stationary limit a particle would have to move with the local light velocity in order to appear stationary to a distant observer. This is because the space here is being dragged at exactly the speed of light relative to the rest of space. Outside this limit space is still dragged, but at a rate less than the speed of light. Also known as → static limit.

See also: → stationary; → limit; → surface.

A property of → space-time outside a → rotating black hole, which consists
of a surface which geometrically bounds the → ergosphere outward. At the stationary limit a particle would have to move with the local light velocity in order to appear stationary to a distant observer. This is because the space here is being dragged at exactly the speed of light relative to the rest of space. Outside this limit space is still dragged, but at a rate less than the speed of light. Also known as → static limit.

See also: → stationary; → limit; → surface.

Electronics: A random noise whose intensity remains constant with time.

See also: → stationary; → noise.

Electronics: A random noise whose intensity remains constant with time.

See also: → stationary; → noise.

An orbit in which the satellite revolves about the primary at the angular rate at which the primary rotates on its axis. From the primary, the satellite thus appears to be stationary over a point on the primary.

See also: → stationary; → orbit.

An orbit in which the satellite revolves about the primary at the angular rate at which the primary rotates on its axis. From the primary, the satellite thus appears to be stationary over a point on the primary.

See also: → stationary; → orbit.

Mechanics: The condition of a body or system at rest.

See also: → stationary; → phase.

Mechanics: The condition of a body or system at rest.

See also: → stationary; → phase.

1. Math.: For a → function y = f(x), a point at which the → tangent to the graph is horizontal. In other words, a point where the → slope is zero: dy/dx = 0.
   

2. Of a planet, the position at which the rate of change of its apparent → right ascension is momentarily zero.

See also: → stationary; → point.

1. Math.: For a → function y = f(x), a point at which the → tangent to the graph is horizontal. In other words, a point where the → slope is zero: dy/dx = 0.
   

2. Of a planet, the position at which the rate of change of its apparent → right ascension is momentarily zero.

See also: → stationary; → point.

An artificial satellite in a synchronous orbit. → geostationary orbit

See also: → stationary; → satellite.

An artificial satellite in a synchronous orbit. → geostationary orbit

See also: → stationary; → satellite.

A → time series if it obeys the following criteria: 1) Constant → mean over time (t). 2) Constant → variance for all t, and 3) The → autocovariance function between X_t1 and X_t2 only depends on the interval t₁ and t₂.

See also: → stationary; → time; → series.

A → time series if it obeys the following criteria: 1) Constant → mean over time (t). 2) Constant → variance for all t, and 3) The → autocovariance function between X_t1 and X_t2 only depends on the interval t₁ and t₂.

See also: → stationary; → time; → series.

Same as → standing wave.

See also: → stationary; → wave.

Same as → standing wave.

See also: → stationary; → wave.

Of, pertaining to, consisting of, or based on → statistics.

See also: Statistic, from → statistics + → -al.

Of, pertaining to, consisting of, or based on → statistics.

See also: Statistic, from → statistics + → -al.

The process of collecting, manipulating, analyzing, and interpreting quantitative data to uncover underlying causes, patterns, and relationships between variables.

See also: → statistical; → analysis.

The process of collecting, manipulating, analyzing, and interpreting quantitative data to uncover underlying causes, patterns, and relationships between variables.

See also: → statistical; → analysis.

A state in which the average density of atoms per cubic centimeter in any atomic state does not change with time and in which, statistically, energy is equally divided among all degrees of freedom if classical concepts prevail.

See also: → statistical; → equilibrium.

A state in which the average density of atoms per cubic centimeter in any atomic state does not change with time and in which, statistically, energy is equally divided among all degrees of freedom if classical concepts prevail.

See also: → statistical; → equilibrium.

An assumed statement about the way a → random variable is distributed. A statistical hypothesis generally specifies the form of the → probability distribution or the values of the parameters of the distribution. The statement may be true or false. See also → null hypothesis.

See also: → statistical; → hypothesis.

An assumed statement about the way a → random variable is distributed. A statistical hypothesis generally specifies the form of the → probability distribution or the values of the parameters of the distribution. The statement may be true or false. See also → null hypothesis.

See also: → statistical; → hypothesis.

A method of making decision between rejecting or not rejecting a → null hypothesis on the basis of a set of observations.

See also: → statistical; → hypothesis; → test.

A method of making decision between rejecting or not rejecting a → null hypothesis on the basis of a set of observations.

See also: → statistical; → hypothesis; → test.

The process of inferring certain facts about a → statistical population from results found in a → sample.

See also: → statistical; → inference.

The process of inferring certain facts about a → statistical population from results found in a → sample.

See also: → statistical; → inference.

A law that governs the behavior of a system consisting of a large number of particles and which differs from the laws obeyed by each of the particles making up the macroscopic system. See also → dynamical law.

See also: → statistical; → law.

A law that governs the behavior of a system consisting of a large number of particles and which differs from the laws obeyed by each of the particles making up the macroscopic system. See also → dynamical law.

See also: → statistical; → law.

→ statistical physics.

See also: → statistical; → mechanics.

→ statistical physics.

See also: → statistical; → mechanics.

The mean parallax of a group of stars that are all at approximately the same distance, as determined from their radial velocities and proper motions.

See also: → statistical; → parallax.

The mean parallax of a group of stars that are all at approximately the same distance, as determined from their radial velocities and proper motions.

See also: → statistical; → parallax.

The branch of physics that applies methods of → probability theory and → statistics to the
behavior of large numbers of microscopic particles (such as molecules, atoms, or subatomic particles) in order to explain and predict the overall properties of the system composed of such particles.

See also: → statistical; → physics.

The branch of physics that applies methods of → probability theory and → statistics to the
behavior of large numbers of microscopic particles (such as molecules, atoms, or subatomic particles) in order to explain and predict the overall properties of the system composed of such particles.

See also: → statistical; → physics.

Any collection of individuals or items from which → samples are drawn. See also → finite population, → infinite population.

See also: → statistical; → population.

Any collection of individuals or items from which → samples are drawn. See also → finite population, → infinite population.

See also: → statistical; → population.

Same as → statistical mechanics.

See also: → statistical; → thermodynamics.

Same as → statistical mechanics.

See also: → statistical; → thermodynamics.

1. Statistics: A number assigned to each value or range of values of a given quantity, giving the number of times this value or range of values is found to be observed.
   

2. Statistical mechanics: A multiplicative factor in the expression for the probability of finding a system in a given
   quantum state. Usually the number of degenerate substates
   contained in the state.

See also: → statistical; → weight.

1. Statistics: A number assigned to each value or range of values of a given quantity, giving the number of times this value or range of values is found to be observed.
   

2. Statistical mechanics: A multiplicative factor in the expression for the probability of finding a system in a given
   quantum state. Usually the number of degenerate substates
   contained in the state.

See also: → statistical; → weight.

A branch of applied mathematics that deals with the collection and interpretation of quantitative data and the use of probability theory to estimate population parameters.

Etymology (EN): From Ger. Statistik “political science,” from Mod.L. statisticus (collegium) “state affairs,” from It. statista “person skilled in statecraft,” from stato “state,” ultimately from L. status “position, form of government;” cognate with Pers. ist-, istâdan “to stand” (Mid.Pers. êstâtan; O.Pers./Av. sta- “to stand, stand still; set;” Av. hištaiti; cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” Goth. standan; PIE base *sta- “to stand”).

Etymology (PE): Âmâr “computation, arithmetic; statistics,” from âmârdan “to reckon, to calculate,” related to ošmârdan, šomârdan, šomordan “to count, to calculate,”
mar, mâr- “count, reckon, measure,”
bimar “countless,” nahmâr “great, large, big;” Mid.Pers. âmâr “calculating, reckoning;”
Av. base mar- “to have in mind, remember, recall,” hišmar-; cf.
Skt. smr-, smarati “to remember, he remembers,” L. memor, memoria, Gk. mermera “care,” martyr “witness.”

A branch of applied mathematics that deals with the collection and interpretation of quantitative data and the use of probability theory to estimate population parameters.

Etymology (EN): From Ger. Statistik “political science,” from Mod.L. statisticus (collegium) “state affairs,” from It. statista “person skilled in statecraft,” from stato “state,” ultimately from L. status “position, form of government;” cognate with Pers. ist-, istâdan “to stand” (Mid.Pers. êstâtan; O.Pers./Av. sta- “to stand, stand still; set;” Av. hištaiti; cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” Goth. standan; PIE base *sta- “to stand”).

Etymology (PE): Âmâr “computation, arithmetic; statistics,” from âmârdan “to reckon, to calculate,” related to ošmârdan, šomârdan, šomordan “to count, to calculate,”
mar, mâr- “count, reckon, measure,”
bimar “countless,” nahmâr “great, large, big;” Mid.Pers. âmâr “calculating, reckoning;”
Av. base mar- “to have in mind, remember, recall,” hišmar-; cf.
Skt. smr-, smarati “to remember, he remembers,” L. memor, memoria, Gk. mermera “care,” martyr “witness.”

1. The natural height of a human or animal in an upright position.
   

   2. An achieved level; status (The FreeDictionary.com).

Etymology (EN): M.E., from from O.Fr. stature, estature “build, structure,” from L. statura “height, size of body, size,” from PIE root *sta- “to stand, make or be firm,” cf. Pers. ist-, istâdan “to stand,” → opposition.

Etymology (PE): Bašn “stature, height; the body;” Mid.Pers. bašn “the top;” O.Pers. baršan- “height,” variant borz “height, magnitude” (it occurs also in the name of the mountain chain Alborz), related to boland “high,” bâlâ “up, above, high, elevated, height,” berg “mountain, hill;” Mid.Pers. buland “high;”
Av. barəz- “high, mount,” barezan- “height;” cf. Skt. bhrant- “high;” L. fortis “strong” (Fr. & E. force); O.E. burg, burh “castle, fortified place;”
Ger. Burg “castle,” Goth. baurgs “city,” E. burg, borough, Fr. bourgeois, bourgeoisie, faubourg); PIE base *bhergh- “high.”

1. The natural height of a human or animal in an upright position.
   

   2. An achieved level; status (The FreeDictionary.com).

Etymology (EN): M.E., from from O.Fr. stature, estature “build, structure,” from L. statura “height, size of body, size,” from PIE root *sta- “to stand, make or be firm,” cf. Pers. ist-, istâdan “to stand,” → opposition.

Etymology (PE): Bašn “stature, height; the body;” Mid.Pers. bašn “the top;” O.Pers. baršan- “height,” variant borz “height, magnitude” (it occurs also in the name of the mountain chain Alborz), related to boland “high,” bâlâ “up, above, high, elevated, height,” berg “mountain, hill;” Mid.Pers. buland “high;”
Av. barəz- “high, mount,” barezan- “height;” cf. Skt. bhrant- “high;” L. fortis “strong” (Fr. & E. force); O.E. burg, burh “castle, fortified place;”
Ger. Burg “castle,” Goth. baurgs “city,” E. burg, borough, Fr. bourgeois, bourgeoisie, faubourg); PIE base *bhergh- “high.”

1. The position of an individual in relation to another or others, especially in regard to social or professional standing.
   

2. State or condition of affairs (Dictionary.com).

Etymology (EN): From L. status “condition, position, state, attitude” from p.p. stem of stare “to stand,” from PIE *ste-tu-, from root *sta- “to stand,” → state., + -tus suffix of action.

Etymology (PE): Estâté, from estat, → state, + nuance suffix -é.

1. The position of an individual in relation to another or others, especially in regard to social or professional standing.
   

2. State or condition of affairs (Dictionary.com).

Etymology (EN): From L. status “condition, position, state, attitude” from p.p. stem of stare “to stand,” from PIE *ste-tu-, from root *sta- “to stand,” → state., + -tus suffix of action.

Etymology (PE): Estâté, from estat, → state, + nuance suffix -é.

A flow in which the characterizing conditions, such as → streamlines or velocity at any given point, do not change with time.

Etymology (EN): → steady; → flow.

Etymology (PE): Tacân, → flow; pâyâ “steady, constant,” from
pâyidan “to stand firm, to be constant, steady,” from Mid.Pers. pattây-, pattutan “to last, endure, stay.”

A flow in which the characterizing conditions, such as → streamlines or velocity at any given point, do not change with time.

Etymology (EN): → steady; → flow.

Etymology (PE): Tacân, → flow; pâyâ “steady, constant,” from
pâyidan “to stand firm, to be constant, steady,” from Mid.Pers. pattây-, pattutan “to last, endure, stay.”

A → cosmological model according to which
the → Universe has no beginning and no end and maintains the same mean density, in spite of its observed expansion, by the continual
creation of matter throughout all space. The theory was first put forward by Sir James Jeans in about 1920 and again in revised form in 1948 by Hermann Bondi and Thomas Gold. It was further developed by Sir Fred Hoyle to deal with problems that had arisen in connection with the alternative → Big Bang model. Observations since the 1950s have produced much evidence
contradictory to the steady state theory and supportive of the Big Bang model. More specifically, the steady state theory attributed the → cosmic microwave background to → thermal radiation from → dust clouds, but this cannot account for a single → blackbody spectrum. Moreover, the steady state theory lacked a plausible mechanism for the creation of matter in space. See also → perfect cosmological principle.

See also: → steady; → state;
→ theory.

A → cosmological model according to which
the → Universe has no beginning and no end and maintains the same mean density, in spite of its observed expansion, by the continual
creation of matter throughout all space. The theory was first put forward by Sir James Jeans in about 1920 and again in revised form in 1948 by Hermann Bondi and Thomas Gold. It was further developed by Sir Fred Hoyle to deal with problems that had arisen in connection with the alternative → Big Bang model. Observations since the 1950s have produced much evidence
contradictory to the steady state theory and supportive of the Big Bang model. More specifically, the steady state theory attributed the → cosmic microwave background to → thermal radiation from → dust clouds, but this cannot account for a single → blackbody spectrum. Moreover, the steady state theory lacked a plausible mechanism for the creation of matter in space. See also → perfect cosmological principle.

See also: → steady; → state;
→ theory.

The vapor into which water is changed when boiled.

Etymology (EN): From M.E. steme, O.E. steam; cognate with Du. stoom, of unknown origin.

Etymology (PE): Boxâr, → vapor.

The vapor into which water is changed when boiled.

Etymology (EN): From M.E. steme, O.E. steam; cognate with Du. stoom, of unknown origin.

Etymology (PE): Boxâr, → vapor.

An engine in which the energy of hot → steam is converted into → mechanical power, especially an engine in which the force of expanding steam is used to drive one or more → pistons. The source of the steam is typically external to the part of the machine that converts the steam energy into → mechanical energy (Dictionary.com).

See also: → steam; → engine.

An engine in which the energy of hot → steam is converted into → mechanical power, especially an engine in which the force of expanding steam is used to drive one or more → pistons. The source of the steam is typically external to the part of the machine that converts the steam energy into → mechanical energy (Dictionary.com).

See also: → steam; → engine.

A strong → alloy of → iron containing up to 1.5 percent → carbon along with small amounts of other → chemical elements such as → manganese, → chromium, → nickel, and so forth.

Etymology (EN): O.E. style; cf. O.S. stehli, O.N., M.L.G. stal, Dan. staal, Swed. stål, M.Du. stael, Du. staal, O.H.G. stahal, Ger. Stahl.

Etymology (PE): Pulâd, variant fulâd, from Mid.Pers. pôlâwad, pôlâvat, loaned in Arm. polopat, polovat, maybe related to Skt. pavīra- “a weapon with metallic point, a spear, a lance.”

A strong → alloy of → iron containing up to 1.5 percent → carbon along with small amounts of other → chemical elements such as → manganese, → chromium, → nickel, and so forth.

Etymology (EN): O.E. style; cf. O.S. stehli, O.N., M.L.G. stal, Dan. staal, Swed. stål, M.Du. stael, Du. staal, O.H.G. stahal, Ger. Stahl.

Etymology (PE): Pulâd, variant fulâd, from Mid.Pers. pôlâwad, pôlâvat, loaned in Arm. polopat, polovat, maybe related to Skt. pavīra- “a weapon with metallic point, a spear, a lance.”

A balance used for weighing loads that has a two beams of different lengths.
The shorter beam has a hook or the like for holding the object to be weighed and the longer one supports a movable counterpoise that slides
to attain a balance.

Etymology (EN): → steel; yard, from M.E. yard(e), O.E. gerd “straight twig;” cognate with Du. gard, Ger. Gerte “rod.”

Etymology (PE): Qapân, from kapân “a large balance with one scale, being kept in equilibrium by a weight on the other end of the beam, a lever balance” (Steingass).

A balance used for weighing loads that has a two beams of different lengths.
The shorter beam has a hook or the like for holding the object to be weighed and the longer one supports a movable counterpoise that slides
to attain a balance.

Etymology (EN): → steel; yard, from M.E. yard(e), O.E. gerd “straight twig;” cognate with Du. gard, Ger. Gerte “rod.”

Etymology (PE): Qapân, from kapân “a large balance with one scale, being kept in equilibrium by a weight on the other end of the beam, a lever balance” (Steingass).

The constant of proportionality present in the → Stefan-Boltzmann law. It is equal to σ = 5.670 × 10^-8 W m^-2 K^-4 or 5.670 × 10^-5 erg cm^-2 s^-1 K^-4.

See also: → Stefan-Boltzmann law; → constant.

The constant of proportionality present in the → Stefan-Boltzmann law. It is equal to σ = 5.670 × 10^-8 W m^-2 K^-4 or 5.670 × 10^-5 erg cm^-2 s^-1 K^-4.

See also: → Stefan-Boltzmann law; → constant.

The flux of radiation from a blackbody is proportional to the fourth power of its absolute temperature: L = 4πR²σT⁴. Also known as Stefan’s law.

See also: Ludwig Eduard Boltzmann (1844-1906), an Austrian physicist, who made important contributions in the fields of statistical mechanics and statistical thermodynamics and Josef Stefan (1835-1893), an Austrian physicist; → law.

The flux of radiation from a blackbody is proportional to the fourth power of its absolute temperature: L = 4πR²σT⁴. Also known as Stefan’s law.

See also: Ludwig Eduard Boltzmann (1844-1906), an Austrian physicist, who made important contributions in the fields of statistical mechanics and statistical thermodynamics and Josef Stefan (1835-1893), an Austrian physicist; → law.

The → moment of inertia of a body about an arbitrary axis x’ is equal to the sum of its moment of inertia about axis x, passing through the center of mass of the body and parallel to axis x’, and the product of the mass M of the body by the square of the distance d between axes x and x’: I_x’ = I_x + Md².
Same as → parallel axis theorem.

See also: Named after Jakop Steiner (1796-1863), a Swiss mathematician who derived this statement; → theorem.

The → moment of inertia of a body about an arbitrary axis x’ is equal to the sum of its moment of inertia about axis x, passing through the center of mass of the body and parallel to axis x’, and the product of the mass M of the body by the square of the distance d between axes x and x’: I_x’ = I_x + Md².
Same as → parallel axis theorem.

See also: Named after Jakop Steiner (1796-1863), a Swiss mathematician who derived this statement; → theorem.

A small → main belt  → asteroid of size 5.9 x 4 km, discovered in 1969 by N. S. Chernykh.
It was visited by the → Rosetta space probe in 2008.

See also: Named after Karlis Šteins (1911-1983), a Latvian and Soviet astronomer.

A small → main belt  → asteroid of size 5.9 x 4 km, discovered in 1969 by N. S. Chernykh.
It was visited by the → Rosetta space probe in 2008.

See also: Named after Karlis Šteins (1911-1983), a Latvian and Soviet astronomer.

Same as → aberration of starlight .

See also: → stellar; → aberratio.

Same as → aberration of starlight .

See also: → stellar; → aberratio.

1. A large, loose grouping of 10 to 1000 stars that are of similar spectral type and share a common origin. The members move together through space, but have become gravitationally → unbound. Stellar associations are primarily identified by their common movement vectors and ages. → OB association; → T association; → R association.
   

2. An → unbound stellar agglomeration for which the age of the stars is smaller than the → crossing time (Giels & Portegies Zwart, 2010, MNRAS Letters, astro-ph/1010.1720). See also → star cluster.

See also: The concept of stellar association was first introduced by Viktor A. Ambartsumian (1908-1996), Armenian astrophysicist (1947, Stellar Evolution and Astrophysics, Armenian Acad. of Sci.; German translation, Abhandl. Sowjetischen Astron. Ser. 1. 33, 1951). → stellar; → association.

1. A large, loose grouping of 10 to 1000 stars that are of similar spectral type and share a common origin. The members move together through space, but have become gravitationally → unbound. Stellar associations are primarily identified by their common movement vectors and ages. → OB association; → T association; → R association.
   

2. An → unbound stellar agglomeration for which the age of the stars is smaller than the → crossing time (Giels & Portegies Zwart, 2010, MNRAS Letters, astro-ph/1010.1720). See also → star cluster.

See also: The concept of stellar association was first introduced by Viktor A. Ambartsumian (1908-1996), Armenian astrophysicist (1947, Stellar Evolution and Astrophysics, Armenian Acad. of Sci.; German translation, Abhandl. Sowjetischen Astron. Ser. 1. 33, 1951). → stellar; → association.

The branch of astronomy that deals with the study of stars, their physical properties, formation, and evolution. Same as → stellar astrophysics and → stellar physics.

See also: → stellar; → astronomy.

The branch of astronomy that deals with the study of stars, their physical properties, formation, and evolution. Same as → stellar astrophysics and → stellar physics.

See also: → stellar; → astronomy.

The field of → astrophysics concerned with the study of the physical characteristics of stars, more specifically their → internal structure, physical processes taking place in their interiors, atmospheres, → stellar winds, → mass loss, interaction with the → interstellar medium, as well as the physical laws governing → star formation. Same as → stellar physics and → stellar astronomy.

See also: → stellar; → astrophysics.

The field of → astrophysics concerned with the study of the physical characteristics of stars, more specifically their → internal structure, physical processes taking place in their interiors, atmospheres, → stellar winds, → mass loss, interaction with the → interstellar medium, as well as the physical laws governing → star formation. Same as → stellar physics and → stellar astronomy.

See also: → stellar; → astrophysics.

The outer envelope of gas and plasma that surrounds a star; characterized by pressure, temperature, density, chemical composition, and opacity at varying altitudes.

See also: → stellar; → atmosphere.

The outer envelope of gas and plasma that surrounds a star; characterized by pressure, temperature, density, chemical composition, and opacity at varying altitudes.

See also: → stellar; → atmosphere.

A model that computes the radiation field crossing the boundary layers of a star at all frequencies. The parameters used for the characterization of a stellar atmosphere model are: → effective temperature, → surface gravity, and → metallicity.

See also: → stellar; → atmosphere; → model.

A model that computes the radiation field crossing the boundary layers of a star at all frequencies. The parameters used for the characterization of a stellar atmosphere model are: → effective temperature, → surface gravity, and → metallicity.

See also: → stellar; → atmosphere; → model.

A bar-shaped accumulation of stars in galaxies, created by → density waves in a → spiral galaxy. → galactic bar, → barred spiral galaxy.

See also: → stellar; → bar.

A bar-shaped accumulation of stars in galaxies, created by → density waves in a → spiral galaxy. → galactic bar, → barred spiral galaxy.

See also: → stellar; → bar.

A → black hole with a mass in the range 3-30 → solar masses
representing the end-product of → massive star evolution. Since → neutron stars cannot have masses larger than 3 solar masses, compact objects more massive than this must be black holes. There is good observational evidence for the existence of stellar black holes, based in particular on dynamical measurements of the masses of compact objects in → transient X-ray sources. Same as → stellar-mass black hole.

See also: → stellar; → black; → hole.

A → black hole with a mass in the range 3-30 → solar masses
representing the end-product of → massive star evolution. Since → neutron stars cannot have masses larger than 3 solar masses, compact objects more massive than this must be black holes. There is good observational evidence for the existence of stellar black holes, based in particular on dynamical measurements of the masses of compact objects in → transient X-ray sources. Same as → stellar-mass black hole.

See also: → stellar; → black; → hole.

Any of the largest stellar assemblages consisting of the groupings of → star clusters, → stellar associations, and individual stars with sizes of 300-1000 → parsecs and ages of up to 100 millions years. Most stellar complexes are physical entities containing objects of common origin and are the birth places of most star clusters and associations. The brightest and youngest complexes are well-known stellar superstructures that outline the Galactic → spiral arms, and also include → H II regions, → giant molecular clouds, and → neutral hydrogen clouds (Efremov, Y. N., 1996, The Origins, Evolutions, and Densities of Binary Stars in Clusters, ASP Conf. Series, Vol. 90).

See also: → stellar; → complex.

Any of the largest stellar assemblages consisting of the groupings of → star clusters, → stellar associations, and individual stars with sizes of 300-1000 → parsecs and ages of up to 100 millions years. Most stellar complexes are physical entities containing objects of common origin and are the birth places of most star clusters and associations. The brightest and youngest complexes are well-known stellar superstructures that outline the Galactic → spiral arms, and also include → H II regions, → giant molecular clouds, and → neutral hydrogen clouds (Efremov, Y. N., 1996, The Origins, Evolutions, and Densities of Binary Stars in Clusters, ASP Conf. Series, Vol. 90).

See also: → stellar; → complex.

The number of stars born per unit area in the mass range log M to log M + d log M during the time interval t to t + dt. The integration of the creation function over time gives the → present-day mass function
(Miller & Scalo, 1797, ApJSS 41, 513).

See also: → stellar; → creation; → function.

The number of stars born per unit area in the mass range log M to log M + d log M during the time interval t to t + dt. The integration of the creation function over time gives the → present-day mass function
(Miller & Scalo, 1797, ApJSS 41, 513).

See also: → stellar; → creation; → function.

A steeply rising radial profile (→ cusp) in the number density of stars in the central region of a galaxy resulting from the gravitational influence of a central → supermassive black hole, as predicted by theoretical models. An important assumption of all cusp formation models is that the stellar cluster is in dynamical equilibrium in the black hole potential. This radial profile is usually characterized by a power law of the form n(r) ∝ r^-γ, with a slope that is steeper than that of a flat isothermal → core. For a single-mass stellar cluster, Bahcall & Wolf (1976) determined the dynamically
relaxed cusp will have γ = 7/4. The presence of such a cusp is important observationally because it may represent a simple test for black holes in stellar systems where dynamical mass estimates are difficult, such as in the cores of galaxies. In the case of the Milky Way, several attempts have been done to probe the presence of such a stellar cusp. However, the presence of the cusp is not confirmed. For example, based on the late-type stars alone, Do et al. (2009, ApJ 703, 1323),
show that γ is less than 1.0 at the 99.7% confidence level. This is consistent with the nuclear star cluster having no cusp, with a → core profile that is significantly flatter than that predicted by most cusp formation theories, and even allows for the presence of a central hole in the stellar distribution (See also Genzel et al., 2010, Rev.Mod.Phys. 82, 3121, also at astro-ph/1006.0064).

See also: → stellar; → cusp.

A steeply rising radial profile (→ cusp) in the number density of stars in the central region of a galaxy resulting from the gravitational influence of a central → supermassive black hole, as predicted by theoretical models. An important assumption of all cusp formation models is that the stellar cluster is in dynamical equilibrium in the black hole potential. This radial profile is usually characterized by a power law of the form n(r) ∝ r^-γ, with a slope that is steeper than that of a flat isothermal → core. For a single-mass stellar cluster, Bahcall & Wolf (1976) determined the dynamically
relaxed cusp will have γ = 7/4. The presence of such a cusp is important observationally because it may represent a simple test for black holes in stellar systems where dynamical mass estimates are difficult, such as in the cores of galaxies. In the case of the Milky Way, several attempts have been done to probe the presence of such a stellar cusp. However, the presence of the cusp is not confirmed. For example, based on the late-type stars alone, Do et al. (2009, ApJ 703, 1323),
show that γ is less than 1.0 at the 99.7% confidence level. This is consistent with the nuclear star cluster having no cusp, with a → core profile that is significantly flatter than that predicted by most cusp formation theories, and even allows for the presence of a central hole in the stellar distribution (See also Genzel et al., 2010, Rev.Mod.Phys. 82, 3121, also at astro-ph/1006.0064).

See also: → stellar; → cusp.

The field of astrophysics that describes systems of many → point mass particles whose mutual gravitational interactions determine their orbits. Theses systems include → star clusters, → globular clusters, and galaxies (→ galaxy) consisting of about 10²-10³, 10⁴-10⁶, and up to about 10¹² members respectively. Stellar dynamics deals with systems in which each member contributes importantly to the overall gravitational field and is usually concerned with the statistical properties of many orbits. It can be compared to the → kinetic theory of gases developed in the late 19th century. In contrast, → celestial mechanics deals with systems where the gravitational force of a massive planet or star determines the orbits of its satellites.

See also: → stellar; → dynamics.

The field of astrophysics that describes systems of many → point mass particles whose mutual gravitational interactions determine their orbits. Theses systems include → star clusters, → globular clusters, and galaxies (→ galaxy) consisting of about 10²-10³, 10⁴-10⁶, and up to about 10¹² members respectively. Stellar dynamics deals with systems in which each member contributes importantly to the overall gravitational field and is usually concerned with the statistical properties of many orbits. It can be compared to the → kinetic theory of gases developed in the late 19th century. In contrast, → celestial mechanics deals with systems where the gravitational force of a massive planet or star determines the orbits of its satellites.

See also: → stellar; → dynamics.

The gradual changes in physical state (spectrum, luminosity, temperature) and chemical composition that occurs during the life of a star.

See also: → stellar; → evolution.

The gradual changes in physical state (spectrum, luminosity, temperature) and chemical composition that occurs during the life of a star.

See also: → stellar; → evolution.

The process whereby large quantities of → energy and → momentum are released into the gas surrounding → star formation regions in galaxies. More specifically, → massive stars
inject → energy, → mass, and → metals back to the → interstellar medium through → stellar winds and → supernova explosions. Feedback inhibits further star formation either by removing gas from the galaxy, or by heating it to temperatures that are too high to form new stars.

Observations reveal feedback in the form of → galactic-scale outflows of gas in galaxies with high → star formation rates,
especially in the → early Universe. Feedback in faint, low-mass galaxies (→ low-mass galaxy) probably facilitated the escape of ionizing radiation from galaxies when the Universe was about 500 million years old, so that the hydrogen between galaxies changed from neutral to ionized, a process called → reionization (Dawn K. Erb, 2015, Nature, 9 July).

See also: → stellar; → feedback.

The process whereby large quantities of → energy and → momentum are released into the gas surrounding → star formation regions in galaxies. More specifically, → massive stars
inject → energy, → mass, and → metals back to the → interstellar medium through → stellar winds and → supernova explosions. Feedback inhibits further star formation either by removing gas from the galaxy, or by heating it to temperatures that are too high to form new stars.

Observations reveal feedback in the form of → galactic-scale outflows of gas in galaxies with high → star formation rates,
especially in the → early Universe. Feedback in faint, low-mass galaxies (→ low-mass galaxy) probably facilitated the escape of ionizing radiation from galaxies when the Universe was about 500 million years old, so that the hydrogen between galaxies changed from neutral to ionized, a process called → reionization (Dawn K. Erb, 2015, Nature, 9 July).

See also: → stellar; → feedback.

That part of a star which lies below the photosphere.

See also: → stellar; → interior.

That part of a star which lies below the photosphere.

See also: → stellar; → interior.

The total amount of energy emitted by a star per unit time. According to the → Stefan-Boltzmann law, the stellar luminosity is given by: L = 4πR²σT_eff⁴, where R_* is radius, σ is the → Stefan-Boltzmann constant, and T_eff is → effective temperature. A star’s luminosity depends, therefore, on two factors, its size and its surface temperature. Stellar luminosity is measured either in ergs per second or in units of → solar luminosity or in → absolute magnitude. See also → luminosity class.

See also: → stellar; → luminosity.

The total amount of energy emitted by a star per unit time. According to the → Stefan-Boltzmann law, the stellar luminosity is given by: L = 4πR²σT_eff⁴, where R_* is radius, σ is the → Stefan-Boltzmann constant, and T_eff is → effective temperature. A star’s luminosity depends, therefore, on two factors, its size and its surface temperature. Stellar luminosity is measured either in ergs per second or in units of → solar luminosity or in → absolute magnitude. See also → luminosity class.

See also: → stellar; → luminosity.

The → magnetic field associated with a star. Magnetic fields are common among stars of solar and lower masses. So far definitive detections of fields in stars with masses ~1.5 Msun have, for the most part, been made for objects having anomalous chemical abundances (e.g., the → chemically peculiar A and B stars). Recently, however, observations of cyclic variability in the properties of → stellar winds from luminous → OB stars have been interpreted as evidence for the presence of large-scale magnetic fields in the surface layers and atmospheres of these objects (→ magnetic massive star). These inferences have been bolstered by the unambiguous measurement of a weak (~ 360 G) field in the chemically normal B1 IIIe star → Beta Cephei. These results suggest that magnetic fields of moderate strength might be more prevalent among → hot stars than had previously been thought.

At the present time, the origin of magnetism in massive stars is not well understood.

If the magnetic field of a hot star is produced by → dynamo effect in the → convective core, then a mechanism for transporting the field to the stellar surface must be identified. The finite electrical conductivity of the envelope leads to the outward diffusion of any fields contained therein, but only over an extended period of time. Estimates indicate that for stars more massive than a few solar masses, the resistive diffusion time across the radiative interior exceeds the → main sequence lifetime. Another possibility is that dynamo fields are advected from the core to the surface by rotation-induced → meridional circulation (MacGregor & Cassinelli, 2002, astro-ph/0212224).

See also: → stellar; → magnetic; → field.

The → magnetic field associated with a star. Magnetic fields are common among stars of solar and lower masses. So far definitive detections of fields in stars with masses ~1.5 Msun have, for the most part, been made for objects having anomalous chemical abundances (e.g., the → chemically peculiar A and B stars). Recently, however, observations of cyclic variability in the properties of → stellar winds from luminous → OB stars have been interpreted as evidence for the presence of large-scale magnetic fields in the surface layers and atmospheres of these objects (→ magnetic massive star). These inferences have been bolstered by the unambiguous measurement of a weak (~ 360 G) field in the chemically normal B1 IIIe star → Beta Cephei. These results suggest that magnetic fields of moderate strength might be more prevalent among → hot stars than had previously been thought.

At the present time, the origin of magnetism in massive stars is not well understood.

If the magnetic field of a hot star is produced by → dynamo effect in the → convective core, then a mechanism for transporting the field to the stellar surface must be identified. The finite electrical conductivity of the envelope leads to the outward diffusion of any fields contained therein, but only over an extended period of time. Estimates indicate that for stars more massive than a few solar masses, the resistive diffusion time across the radiative interior exceeds the → main sequence lifetime. Another possibility is that dynamo fields are advected from the core to the surface by rotation-induced → meridional circulation (MacGregor & Cassinelli, 2002, astro-ph/0212224).

See also: → stellar; → magnetic; → field.

1. The quantity of mass contained in a star. It is usually expressed in terms of the → solar mass (Msun).
   

   2. A component of the total mass of a → galaxy represented by the mass of all its stars.

See also: → stellar; → mass.

1. The quantity of mass contained in a star. It is usually expressed in terms of the → solar mass (Msun).
   

   2. A component of the total mass of a → galaxy represented by the mass of all its stars.

See also: → stellar; → mass.

The metallicity derived from observations of stars in galaxies. It is mainly based on spectral → absorption lines in → ultraviolet (UV) and optical ranges. Stellar metallicity is a direct measure of the amount of metals in a galaxy, since large part of heavy elements lies in its stars.

See also: → stellar; → metallicity.

The metallicity derived from observations of stars in galaxies. It is mainly based on spectral → absorption lines in → ultraviolet (UV) and optical ranges. Stellar metallicity is a direct measure of the amount of metals in a galaxy, since large part of heavy elements lies in its stars.

See also: → stellar; → metallicity.

The → nuclear reaction process taking place inside stars, whereby → chemical elements are produced from pre-existing nuclei heavier than → hydrogen and → helium.

See also: → stellar; → nucleosynthesis.

The → nuclear reaction process taking place inside stars, whereby → chemical elements are produced from pre-existing nuclei heavier than → hydrogen and → helium.

See also: → stellar; → nucleosynthesis.

Any of a class of → astronomical objects which is thought to evolve into a → star or is a descendant of a star.

See also: → stellar; → object.

Any of a class of → astronomical objects which is thought to evolve into a → star or is a descendant of a star.

See also: → stellar; → object.

The apparent → difference in the → position
of a → celestial object as seen by an → observer from two widely separated → locations. The parallax of an object can be used to derive its → distance.
The relationship between the → parallax angle  p (measured in seconds of arc) and the distance d (measured in → astronomical units) is given by d = 206,264 / p. For a parallax angle p = 1’’, the distance to the star would correspond to 206,264 AU. By convention, the distance unit
→ parsec is defined to be equivalent to 206,264 AU. Therefore, the parallax relation takes the much simpler form: d (in pc) = 1/p (in seconds of arc). The first star whose parallax was measured was → 61 Cygni (Bessel, 1838).

See also:
→ annual parallax, → diurnal parallax, → dynamical parallax, → geocentric parallax, → heliocentric parallax, → horizontal parallax, → lunar parallax, → mean parallax, → parallactic ellipse, → parsec, → photometric parallax, → secular parallax, → solar parallax, → spectroscopic parallax, → statistical parallax, → trigonometric parallax.

See also: → stellar; → parallax.

The apparent → difference in the → position
of a → celestial object as seen by an → observer from two widely separated → locations. The parallax of an object can be used to derive its → distance.
The relationship between the → parallax angle  p (measured in seconds of arc) and the distance d (measured in → astronomical units) is given by d = 206,264 / p. For a parallax angle p = 1’’, the distance to the star would correspond to 206,264 AU. By convention, the distance unit
→ parsec is defined to be equivalent to 206,264 AU. Therefore, the parallax relation takes the much simpler form: d (in pc) = 1/p (in seconds of arc). The first star whose parallax was measured was → 61 Cygni (Bessel, 1838).

See also:
→ annual parallax, → diurnal parallax, → dynamical parallax, → geocentric parallax, → heliocentric parallax, → horizontal parallax, → lunar parallax, → mean parallax, → parallactic ellipse, → parsec, → photometric parallax, → secular parallax, → solar parallax, → spectroscopic parallax, → statistical parallax, → trigonometric parallax.

See also: → stellar; → parallax.

The precise measurement of a star’s brightness, usually through several specific wavelength bands.

See also: → stellar; → photometry.

The precise measurement of a star’s brightness, usually through several specific wavelength bands.

See also: → stellar; → photometry.

Same as → stellar astrophysics.

See also: → stellar; → physics.

Same as → stellar astrophysics.

See also: → stellar; → physics.

→ Population I star; → Population II star.

See also: → stellar; → population.

→ Population I star; → Population II star.

See also: → stellar; → population.

A theoretical model that reconstructs the integrated spectrum of → stellar populations from an empirical library of stellar spectra containing the range of types expected to be present in the sample. The light received from a given galaxy is emitted by a large number of stars that may have different masses, ages, and metallicities. Stellar population synthesis models are tools for interpreting the integrated light that we observe from the galaxies.

See also: → stellar; → population; → model.

A theoretical model that reconstructs the integrated spectrum of → stellar populations from an empirical library of stellar spectra containing the range of types expected to be present in the sample. The light received from a given galaxy is emitted by a large number of stars that may have different masses, ages, and metallicities. Stellar population synthesis models are tools for interpreting the integrated light that we observe from the galaxies.

See also: → stellar; → population; → model.

The expansion of a star followed by contraction so that its → surface temperature and → luminosity undergo periodic variation. Pulsation starts with a loss of → hydrostatic equilibrium, when, for example, a layer contracts. This layer heats up and becomes more opaque to radiation. Therefore, radiative diffusion slows down through the layer because of its increased → opacity and heat increases beneath it. Hence pressure rises below the layer. Eventually this increase in pressure starts to push the layer out. The layer expands, cools and becomes more transparent to radiation. Energy now escapes from below the layer and the pressure beneath the layer drops. The layer falls inward and the cycle starts over. See also → kappa mechanism; → gamma mechanism; → partial ionization zone; → pulsating star; → valve mechanism.

See also: → stellar; → pulsation.

The expansion of a star followed by contraction so that its → surface temperature and → luminosity undergo periodic variation. Pulsation starts with a loss of → hydrostatic equilibrium, when, for example, a layer contracts. This layer heats up and becomes more opaque to radiation. Therefore, radiative diffusion slows down through the layer because of its increased → opacity and heat increases beneath it. Hence pressure rises below the layer. Eventually this increase in pressure starts to push the layer out. The layer expands, cools and becomes more transparent to radiation. Energy now escapes from below the layer and the pressure beneath the layer drops. The layer falls inward and the cycle starts over. See also → kappa mechanism; → gamma mechanism; → partial ionization zone; → pulsating star; → valve mechanism.

See also: → stellar; → pulsation.

The spinning of a star about its axis, due to its angular momentum. Stars do not necessarily rotate as solid bodies, and their angular momentum may be distributed non-uniformly, depending on radius or latitude.Thus the equator of the star can rotate at a different angular velocity than the higher latitudes. These differences in the rate of rotation within a star may have a significant role in the generation of a stellar magnetic field.

See also: → stellar; → rotation.

The spinning of a star about its axis, due to its angular momentum. Stars do not necessarily rotate as solid bodies, and their angular momentum may be distributed non-uniformly, depending on radius or latitude.Thus the equator of the star can rotate at a different angular velocity than the higher latitudes. These differences in the rate of rotation within a star may have a significant role in the generation of a stellar magnetic field.

See also: → stellar; → rotation.

A physical model that describes the internal arrangement of a star in detail and makes detailed predictions about the luminosity, the color, and the future evolution of the star.

See also: → stellar; → structure.

A physical model that describes the internal arrangement of a star in detail and makes detailed predictions about the luminosity, the color, and the future evolution of the star.

See also: → stellar; → structure.

A set of → differential equations describing the physical properties of stars based on two main assumptions: a star is a perfect sphere and the net force on a macroscopic mass element is zero. If the effects of rotation and magnetism are ignored, these assumptions lead to a set of five differential equations.

See also: → stellar; → structure; → equation.

A set of → differential equations describing the physical properties of stars based on two main assumptions: a star is a perfect sphere and the net force on a macroscopic mass element is zero. If the effects of rotation and magnetism are ignored, these assumptions lead to a set of five differential equations.

See also: → stellar; → structure; → equation.

A system comprised of a group of stars bound by → gravitational attraction. Same as → star system.

See also: → stellar; → system.

A system comprised of a group of stars bound by → gravitational attraction. Same as → star system.

See also: → stellar; → system.

The steady flow of gas away from a star resulting in → mass loss. They range from gentle solar wind (2 x 10^-14 solar masses per year) to violent winds some 10 billions times stronger (10^-4 solar masses per year) for hot, massive stars.

See also: → stellar; → wind.

The steady flow of gas away from a star resulting in → mass loss. They range from gentle solar wind (2 x 10^-14 solar masses per year) to violent winds some 10 billions times stronger (10^-4 solar masses per year) for hot, massive stars.

See also: → stellar; → wind.

Same as → stellar black hole.

See also: → stellar; → mass; → black; → hole.

Same as → stellar black hole.

See also: → stellar; → mass; → black; → hole.

Math.:

A function f of a real variable defined on an interval [a,b] so that [a,b] can be divided into a finite number of sub-intervals on each of which f is a constant. The graph of a step function is a series of line segments resembling a set of steps.

Etymology (EN): Step, from M.E. steppen, O.E. steppan; cf. Du. stap, O.H.G. stapfo, Ger. stapfe “footprint;” → function.

Etymology (PE): Karyâ, → function; pellé “stair, step;” Mid.Pers. pylg “step,” pillagân “steps, staircase;” from *palak, from *padak, from pad-, → foot,

• relation suffix -ak.

Math.:

A function f of a real variable defined on an interval [a,b] so that [a,b] can be divided into a finite number of sub-intervals on each of which f is a constant. The graph of a step function is a series of line segments resembling a set of steps.

Etymology (EN): Step, from M.E. steppen, O.E. steppan; cf. Du. stap, O.H.G. stapfo, Ger. stapfe “footprint;” → function.

Etymology (PE): Karyâ, → function; pellé “stair, step;” Mid.Pers. pylg “step,” pillagân “steps, staircase;” from *palak, from *padak, from pad-, → foot,

• relation suffix -ak.

A group of five closely grouped galaxies (NGC 7317, 7318A, 7318B, 7319 and 7320) in the constellation → Pegasus. Four of the galaxies show essentially the same → redshift, suggesting that they are at the same distance from us. The fifth galaxy (NGC 7320) has a smaller redshift than the others, indicating it is much closer. This one is probably a foreground galaxy which happens to lie along the line of sight. The four distant galaxies seem to be colliding, showing serious distortions due to gravitational → tidal forces. The NASA → Spitzer Space Telescope has revealed the presence of a huge intergalactic → shock wave. Collisions play an important role in the life cycles of galaxies. → merging galaxies.

See also: Named after the French astronomer Edouard Stéphan (1837-1923), who discovered the group in 1877 at Marseilles Observatory, using the → Foucault’s reflector; → quintet.

A group of five closely grouped galaxies (NGC 7317, 7318A, 7318B, 7319 and 7320) in the constellation → Pegasus. Four of the galaxies show essentially the same → redshift, suggesting that they are at the same distance from us. The fifth galaxy (NGC 7320) has a smaller redshift than the others, indicating it is much closer. This one is probably a foreground galaxy which happens to lie along the line of sight. The four distant galaxies seem to be colliding, showing serious distortions due to gravitational → tidal forces. The NASA → Spitzer Space Telescope has revealed the presence of a huge intergalactic → shock wave. Collisions play an important role in the life cycles of galaxies. → merging galaxies.

See also: Named after the French astronomer Edouard Stéphan (1837-1923), who discovered the group in 1877 at Marseilles Observatory, using the → Foucault’s reflector; → quintet.

The solid angle subtended at the center of a sphere by an area on its surface numerically equal to the square of the radius. → square degree.

See also: From ste(reo)-, → stereo-

• → radian.

The solid angle subtended at the center of a sphere by an area on its surface numerically equal to the square of the radius. → square degree.

See also: From ste(reo)-, → stereo-

• → radian.

A combining form meaning “having and dealing with three dimensions of space; solid.”

Etymology (EN): From stereo a shortening of stereotype, from Fr. stéréotype (adj.) “printing by means of a solid plate of type,” from Gk. stereos “solid.”

Etymology (PE): Loan from Fr., as above.

A combining form meaning “having and dealing with three dimensions of space; solid.”

Etymology (EN): From stereo a shortening of stereotype, from Fr. stéréotype (adj.) “printing by means of a solid plate of type,” from Gk. stereos “solid.”

Etymology (PE): Loan from Fr., as above.

A device that allows two images of the sky taken at different times to be optically superimposed so that changes in star brightness or moving objects can be detected.

Etymology (EN): → stereo-; comparator, from L. comparare
“to place together, match,” from compar “alike, matching,” → com-; → partial

• -tor.

Etymology (PE): Ham-sanj-gar “comapartor,” from ham-, → com-,

• sanj stem of sanjidan “to compare” + -gar,
  → -or; → stereo-.

A device that allows two images of the sky taken at different times to be optically superimposed so that changes in star brightness or moving objects can be detected.

Etymology (EN): → stereo-; comparator, from L. comparare
“to place together, match,” from compar “alike, matching,” → com-; → partial

• -tor.

Etymology (PE): Ham-sanj-gar “comapartor,” from ham-, → com-,

• sanj stem of sanjidan “to compare” + -gar,
  → -or; → stereo-.

Of, relating to, or being a delineation of the form of a solid body on a plane.

See also: → stereography; → -ic

Of, relating to, or being a delineation of the form of a solid body on a plane.

See also: → stereography; → -ic

A graphical method of depicting three-dimensional geometrical objects in two dimensions. In a → planispheric astrolabe, it is the projection of a point of the celestial sphere onto the equatorial plane, as seen from one of the poles. The center of projection is the South pole for the northern hemisphere, and the North pole for the southern hemisphere. In this operation
the projection of any circle of the sphere remains a circle on the projection plane and moreover the projection does not alter angles.

See also: → stereographic; → projection

A graphical method of depicting three-dimensional geometrical objects in two dimensions. In a → planispheric astrolabe, it is the projection of a point of the celestial sphere onto the equatorial plane, as seen from one of the poles. The center of projection is the South pole for the northern hemisphere, and the North pole for the southern hemisphere. In this operation
the projection of any circle of the sphere remains a circle on the projection plane and moreover the projection does not alter angles.

See also: → stereographic; → projection

The process or art of depicting solid objects on a plane surface.

See also: → stereo- + → -graphy

The process or art of depicting solid objects on a plane surface.

See also: → stereo- + → -graphy

An optical instrument for viewing an overlapping pair of photographs (or perspective drawings) in order to see a three-dimensional image.

See also: → stereo-; → -scope.

An optical instrument for viewing an overlapping pair of photographs (or perspective drawings) in order to see a three-dimensional image.

See also: → stereo-; → -scope.

Incapable of producing offspring; not producing offspring (Dictionary.com).

Etymology (EN): M.Fr. stérile “not producing fruit,” from L. sterilis “barren, unproductive, unfruitful,” from PIE *ster- “stiff, rigid, firm, strong.”

Etymology (PE): Satarvan, literally “mule-like, resembling a mule,” from setar, variant of astar, → mule, + -van similarity and attribution suffix.

Incapable of producing offspring; not producing offspring (Dictionary.com).

Etymology (EN): M.Fr. stérile “not producing fruit,” from L. sterilis “barren, unproductive, unfruitful,” from PIE *ster- “stiff, rigid, firm, strong.”

Etymology (PE): Satarvan, literally “mule-like, resembling a mule,” from setar, variant of astar, → mule, + -van similarity and attribution suffix.

A hypothetical type of → neutrino which does not participate in the → weak interaction. It would arise only from ordinary neutrinos oscillating into a sterile form (singlet, right handed → helicity). The sterile neutrino is a candidate for the → dark matter. Sterile neutrinos might have been produced in primordial plasma in the → early Universe. The idea of sterile neutrino was first proposed by Bruno Pontecorvo (1967) in a paper which also discussed neutrino oscillations.

See also: → sterile; → neutrino.