An Etymological Dictionary of Astronomy and Astrophysics

A list of 100 compact groups of galaxies that were identified by a systematic search of the → Palomar Observatory Sky Survey red prints. Each group contains four or more galaxies, has an estimated mean surface brightness brighter than 26.0 magnitude per arcsec² and satisfies an isolation criterion.

See also: Hickson, Paul, 1982, ApJ 255, 382; → compact; → group.

A list of 100 compact groups of galaxies that were identified by a systematic search of the → Palomar Observatory Sky Survey red prints. Each group contains four or more galaxies, has an estimated mean surface brightness brighter than 26.0 magnitude per arcsec² and satisfies an isolation criterion.

See also: Hickson, Paul, 1982, ApJ 255, 382; → compact; → group.

Being out of sight; concealed.

Etymology (EN): From M.E., from O.E. hydan, from W.Gmc. *khuthjanan, from PIE keudh- (cf. Gk. keuthein “to hide, conceal”), from base (s)keu- “to cover, conceal.”

Etymology (PE): Penhân “hidden,” from Mid.Pers. pad nihân, from pad “to, at, for, in”
(from O.Pers. paity; Av. paiti “to, toward, in, at;” cf.
Skt. práti; Gk. poti) + nihân “concealment, secrecy, hiding place” (Mod.Pers. nahân), from Proto-Iranian *ni-dāna-, from ni- “down; into,” → ni- (PIE),

• dā- “to put; to establish; to give” (dadâiti “he gives;” cf. Skt. dadâti “he gives;” Gk. didomi “I give;” L. do “I give;” PIE base *do- “to give”).

Being out of sight; concealed.

Etymology (EN): From M.E., from O.E. hydan, from W.Gmc. *khuthjanan, from PIE keudh- (cf. Gk. keuthein “to hide, conceal”), from base (s)keu- “to cover, conceal.”

Etymology (PE): Penhân “hidden,” from Mid.Pers. pad nihân, from pad “to, at, for, in”
(from O.Pers. paity; Av. paiti “to, toward, in, at;” cf.
Skt. práti; Gk. poti) + nihân “concealment, secrecy, hiding place” (Mod.Pers. nahân), from Proto-Iranian *ni-dāna-, from ni- “down; into,” → ni- (PIE),

• dā- “to put; to establish; to give” (dadâiti “he gives;” cf. Skt. dadâti “he gives;” Gk. didomi “I give;” L. do “I give;” PIE base *do- “to give”).

Same as → missing mass, → non-luminous matter, or → dark matter.

See also: → hidden; → mass.

Same as → missing mass, → non-luminous matter, or → dark matter.

See also: → hidden; → mass.

A theory based on the hypothesis that the discrepancies with respect to classical reality found in → quantum mechanics stem from our lack of knowledge about the observed system (→ EPR paradox). According to this hypothesis, the system should be described by additional quantum parameters, of still unknown nature, but different from position, velocity, spin, etc. The hidden variable theory has been ruled out by the violation of → Bell’s inequality for all theories with local property, as suggested by the → Aspect experiment.

See also: → hidden; → variable.

A theory based on the hypothesis that the discrepancies with respect to classical reality found in → quantum mechanics stem from our lack of knowledge about the observed system (→ EPR paradox). According to this hypothesis, the system should be described by additional quantum parameters, of still unknown nature, but different from position, velocity, spin, etc. The hidden variable theory has been ruled out by the violation of → Bell’s inequality for all theories with local property, as suggested by the → Aspect experiment.

See also: → hidden; → variable.

Of, belonging to, or characteristic of a hierarchy. → hierarchical clustering;
→ hierarchical cosmology; → hierarchical multiple system; → hierarchical structure formation.

See also: → hierarchy; → -al.

Of, belonging to, or characteristic of a hierarchy. → hierarchical clustering;
→ hierarchical cosmology; → hierarchical multiple system; → hierarchical structure formation.

See also: → hierarchy; → -al.

A model in which a system of self-gravitating particles will gradually aggregate into larger and larger gravitationally bound groups and clusters.

See also: → hierarchical; → clustering.

A model in which a system of self-gravitating particles will gradually aggregate into larger and larger gravitationally bound groups and clusters.

See also: → hierarchical; → clustering.

A cosmology characterized by clustering of galaxy clusters in increasingly larger systems.

See also: → hierarchical; → cosmology.

A cosmology characterized by clustering of galaxy clusters in increasingly larger systems.

See also: → hierarchical; → cosmology.

A → multiple star system in which the stars can be divided into two groups, each of which traverses a larger orbit around the system’s center of mass. Each of these smaller groups must also be hierarchical, which means that they must be divided into smaller subgroups which themselves are hierarchical, and so on. Hierarchical multiple systems have long-term dynamical stability.

See also: → hierarchical; → multiple; → system.

A → multiple star system in which the stars can be divided into two groups, each of which traverses a larger orbit around the system’s center of mass. Each of these smaller groups must also be hierarchical, which means that they must be divided into smaller subgroups which themselves are hierarchical, and so on. Hierarchical multiple systems have long-term dynamical stability.

See also: → hierarchical; → multiple; → system.

A cosmological → structure formation model in which the smallest gravitationally bound structures (→ quasars and galaxies) form first, followed by → groups, → galaxy clusters, and → superclusters of galaxies.

See also: → hierarchical; → structure; → formation.

A cosmological → structure formation model in which the smallest gravitationally bound structures (→ quasars and galaxies) form first, followed by → groups, → galaxy clusters, and → superclusters of galaxies.

See also: → hierarchical; → structure; → formation.

A triple star system in which the (inner) binary is orbited by a third body in a much wider orbit. → hierarchical multiple system.

See also: → hierarchical; → stellar; → system.

A triple star system in which the (inner) binary is orbited by a third body in a much wider orbit. → hierarchical multiple system.

See also: → hierarchical; → stellar; → system.

A system in which the components are organized in increasingly larger structures.

Etymology (EN): From O.Fr. ierarchie, from M.L. hierarchia “ranked division of angels,” from Gk. hierarchia “rule of a high priest,” from hierarches “high priest, leader of sacred rites,” from ta hiera “the sacred rites” (neut. pl. of hieros “sacred”) + archein “to lead, rule.”

Etymology (PE): Pâygân, from pâyé “step, rank, degree,” from pây, pâ “foot, step,” from
Mid.Pers. pâd, pây; Av. pad- “foot” (cf. Skt. pat; Gk. pos, gen. podos; L. pes, gen. pedis; P.Gmc. *fot; E. foot; Ger. Fuss; Fr. pied; PIE *pod-/*ped-) + -gân suffix forming plural entities, from Mid.Pers. -gânag, -gâna, from Proto-Iranian *kāna-ka-.

A system in which the components are organized in increasingly larger structures.

Etymology (EN): From O.Fr. ierarchie, from M.L. hierarchia “ranked division of angels,” from Gk. hierarchia “rule of a high priest,” from hierarches “high priest, leader of sacred rites,” from ta hiera “the sacred rites” (neut. pl. of hieros “sacred”) + archein “to lead, rule.”

Etymology (PE): Pâygân, from pâyé “step, rank, degree,” from pây, pâ “foot, step,” from
Mid.Pers. pâd, pây; Av. pad- “foot” (cf. Skt. pat; Gk. pos, gen. podos; L. pes, gen. pedis; P.Gmc. *fot; E. foot; Ger. Fuss; Fr. pied; PIE *pod-/*ped-) + -gân suffix forming plural entities, from Mid.Pers. -gânag, -gâna, from Proto-Iranian *kāna-ka-.

A hypothetical, neutral → elementary particle which plays a key role in the → standard model
of → particle physics. This massive particle, whose mass is estimated to be about 125 GeV (→ giga → electron-volts)
and a zero → spin, carries the → Higgs field. In the current version of the → electroweak theory,
→ W boson and → Z boson and all the fundamental constituents (→ quarks and → leptons) get their masses by interacting with the Higgs boson.
The Higgs boson is produced by the fusion of two → gluons via a triangular loop of virtual top quarks. In the decay process, a loop of virtual top quarks allows the Higgs boson to decay into two photons. The particle’s discovery was announced by → CERN in July 2012.

See also: Named after the Scottish physicist Peter Ware Higgs (1929-), one of the researchers who theorized the existence of this particle in 1964. In fact three groups of physicists almost simultaneously published their results on this subject:
François Englert and Robert Brout in August 1964; Peter Higgs in October 1964;
and Gerald Guralnik, Carl Hagen, and Tom Kibble in November 1964; → boson.

A hypothetical, neutral → elementary particle which plays a key role in the → standard model
of → particle physics. This massive particle, whose mass is estimated to be about 125 GeV (→ giga → electron-volts)
and a zero → spin, carries the → Higgs field. In the current version of the → electroweak theory,
→ W boson and → Z boson and all the fundamental constituents (→ quarks and → leptons) get their masses by interacting with the Higgs boson.
The Higgs boson is produced by the fusion of two → gluons via a triangular loop of virtual top quarks. In the decay process, a loop of virtual top quarks allows the Higgs boson to decay into two photons. The particle’s discovery was announced by → CERN in July 2012.

See also: Named after the Scottish physicist Peter Ware Higgs (1929-), one of the researchers who theorized the existence of this particle in 1964. In fact three groups of physicists almost simultaneously published their results on this subject:
François Englert and Robert Brout in August 1964; Peter Higgs in October 1964;
and Gerald Guralnik, Carl Hagen, and Tom Kibble in November 1964; → boson.

A → scalar field supposed to be responsible for the genesis of → inertial mass. According to the → standard model of → particle physics, the Higgs field appeared
10^-36 to 10^-12 seconds after the → Big Bang, during the → electroweak epoch, when the temperature dropped below a critical threshold. The Higgs field permeates all space, and through its interaction with the fundamental particles it provides those particles with a mass. Any particle that does not interact with the Higgs field, such as the → photon,
will be mass-less.

See also: → Higgs boson; → field.

A → scalar field supposed to be responsible for the genesis of → inertial mass. According to the → standard model of → particle physics, the Higgs field appeared
10^-36 to 10^-12 seconds after the → Big Bang, during the → electroweak epoch, when the temperature dropped below a critical threshold. The Higgs field permeates all space, and through its interaction with the fundamental particles it provides those particles with a mass. Any particle that does not interact with the Higgs field, such as the → photon,
will be mass-less.

See also: → Higgs boson; → field.

In the → standard model of → particle physics, a mechanism postulated to endow mass to
→ elementary particles. Simply put, a background field, called the → Higgs field, becomes locally distorted whenever a particle moves through it. The distortion generates the particle’s mass.

See also: → Higgs boson; → mechanism.

In the → standard model of → particle physics, a mechanism postulated to endow mass to
→ elementary particles. Simply put, a background field, called the → Higgs field, becomes locally distorted whenever a particle moves through it. The distortion generates the particle’s mass.

See also: → Higgs boson; → mechanism.

1. Situated above the ground or exceeding the common degree or measure.
   

2. Exceeding the common degree or measure; strong; intense.
   

3. Meteo.: An area of high pressure, referring to a maximum of atmospheric pressure. Same as → anticyclone (Fr. haute pression).

Etymology (EN): M.E. heigh, variants hegh, hey, heh; O.E. heh, heah “of great height, lofty, tall,” (cf. Du. hoog, O.H.G. hoh, Ger. hoch, Goth. hauhs “high;” also Ger. Hügel “hill”); from PIE *koukos “hill.”

Etymology (PE): Boland “high,” variants bâlâ “up, above, high, elevated, height,” borz “height, magnitude” (it occurs also in the name of the mountain chain Alborz),
Lori dialect berg “hill, mountain;” Mid.Pers. buland “high;” O.Pers. baršan- “height;” Av. barəz- “high, mount,” barezan- “height;” cf. Skt. bhrant- “high;” L. fortis “strong” (Fr. & E. force); O.E. burg, burh “castle, fortified place,” from P.Gmc. *burgs “fortress;” Ger. Burg “castle,” Goth. baurgs “city,” E. burg, borough, Fr. bourgeois, bourgeoisie, faubourg); PIE base *bhergh- “high.”

Meh “great, large” (Mid.Pers. meh, mas, Av. maz-, masan-, mazant- “great, important,” mazan- “greatness, majesty,” mazišta- “greatest,” cf. Skt. mah-, mahant-, Gk. megas, L. magnus; PIE *meg- “great”).
Por “much, very, too much; full” (Mid.Pers. purr “full;” O.Pers. paru- “much, many;” Av. parav-, pauru-, pouru-, from
par- “to fill;” PIE base *pelu- “full,” from *pel- “to be full;” cf. Skt. puru-; Gk. polus;
O.E. full).

1. Situated above the ground or exceeding the common degree or measure.
   

2. Exceeding the common degree or measure; strong; intense.
   

3. Meteo.: An area of high pressure, referring to a maximum of atmospheric pressure. Same as → anticyclone (Fr. haute pression).

Etymology (EN): M.E. heigh, variants hegh, hey, heh; O.E. heh, heah “of great height, lofty, tall,” (cf. Du. hoog, O.H.G. hoh, Ger. hoch, Goth. hauhs “high;” also Ger. Hügel “hill”); from PIE *koukos “hill.”

Etymology (PE): Boland “high,” variants bâlâ “up, above, high, elevated, height,” borz “height, magnitude” (it occurs also in the name of the mountain chain Alborz),
Lori dialect berg “hill, mountain;” Mid.Pers. buland “high;” O.Pers. baršan- “height;” Av. barəz- “high, mount,” barezan- “height;” cf. Skt. bhrant- “high;” L. fortis “strong” (Fr. & E. force); O.E. burg, burh “castle, fortified place,” from P.Gmc. *burgs “fortress;” Ger. Burg “castle,” Goth. baurgs “city,” E. burg, borough, Fr. bourgeois, bourgeoisie, faubourg); PIE base *bhergh- “high.”

Meh “great, large” (Mid.Pers. meh, mas, Av. maz-, masan-, mazant- “great, important,” mazan- “greatness, majesty,” mazišta- “greatest,” cf. Skt. mah-, mahant-, Gk. megas, L. magnus; PIE *meg- “great”).
Por “much, very, too much; full” (Mid.Pers. purr “full;” O.Pers. paru- “much, many;” Av. parav-, pauru-, pouru-, from
par- “to fill;” PIE base *pelu- “full,” from *pel- “to be full;” cf. Skt. puru-; Gk. polus;
O.E. full).

A high-precision echelle spectrograph built for exoplanet findings and installed on the ESO’s 3.6m telescope at La Silla Observatory in Chile. The first light was achieved in February 2003. HARPS has discovered dozens of exoplanets, making it the most successful planet finder behind the Kepler space observatory. HARPS can detect movements as small as 0.97 m s^-1 (3.5 km h^-1), with an effective precision of the order of 30 cm s^-1, and a → resolving power of 120,000 (Mayor et al., 2003, ESO Messengar 114, 20).

See also: → high; → accuracy; → radial; → velocity; → planet; → search; → -er.

A high-precision echelle spectrograph built for exoplanet findings and installed on the ESO’s 3.6m telescope at La Silla Observatory in Chile. The first light was achieved in February 2003. HARPS has discovered dozens of exoplanets, making it the most successful planet finder behind the Kepler space observatory. HARPS can detect movements as small as 0.97 m s^-1 (3.5 km h^-1), with an effective precision of the order of 30 cm s^-1, and a → resolving power of 120,000 (Mayor et al., 2003, ESO Messengar 114, 20).

See also: → high; → accuracy; → radial; → velocity; → planet; → search; → -er.

An array of → IACT telescopes for studying
cosmic → gamma rays in the 100 GeV to 100 TeV energy range.
The HESS observatory is located in Namibia, southern Africa, at an altitude of 1800 m, and the project is an international collaboration of more than 100 scientists from nine countries. In its Phase I, HESS used four telescopes each consisting of a light collector with a diameter of 13 m and a focal length of 15 m placed at the corners of a square 120 m apart. Each telescope is segmented into 380 round mirror facets of 60 cm diameter and uses a camera consisting of 960 closely packed → photomultiplier tubes. The first of the telescopes went into operation in Summer 2002. Phase II includes a fifth
telescope, called Large Cherenkov Telescope (LCT), of 27 m diameter, located in the centre of the initial array. This upgrade lowers the triggering threshold of the HESS array to about 20 GeV, thus broadening the energy window in which gamma-ray astronomy can be done, opening up more opportunities in astrophysical research (see, e.g., Bernlöhr et al. 2003, Astroparticle Physics 20, 111).

See also: H.E.S.S., short for High Energy Stereoscopic System, is also intended to pay homage to Victor F. Hess (1883-1964), an Austrian-American physicist who received the Nobel Prize in Physics in 1936 for his discovery of → cosmic rays.

An array of → IACT telescopes for studying
cosmic → gamma rays in the 100 GeV to 100 TeV energy range.
The HESS observatory is located in Namibia, southern Africa, at an altitude of 1800 m, and the project is an international collaboration of more than 100 scientists from nine countries. In its Phase I, HESS used four telescopes each consisting of a light collector with a diameter of 13 m and a focal length of 15 m placed at the corners of a square 120 m apart. Each telescope is segmented into 380 round mirror facets of 60 cm diameter and uses a camera consisting of 960 closely packed → photomultiplier tubes. The first of the telescopes went into operation in Summer 2002. Phase II includes a fifth
telescope, called Large Cherenkov Telescope (LCT), of 27 m diameter, located in the centre of the initial array. This upgrade lowers the triggering threshold of the HESS array to about 20 GeV, thus broadening the energy window in which gamma-ray astronomy can be done, opening up more opportunities in astrophysical research (see, e.g., Bernlöhr et al. 2003, Astroparticle Physics 20, 111).

See also: H.E.S.S., short for High Energy Stereoscopic System, is also intended to pay homage to Victor F. Hess (1883-1964), an Austrian-American physicist who received the Nobel Prize in Physics in 1936 for his discovery of → cosmic rays.

The latitude belt roughly between 60 and 90 degrees North and South. Also referred to as the polar region.

See also: → high; → latitude.

The latitude belt roughly between 60 and 90 degrees North and South. Also referred to as the polar region.

See also: → high; → latitude.

A galaxy or quasar having a → redshift larger than about 0.8, corresponding to a → look-back time half the present age of the Universe. The qualifier “high” is, however, relative and depends on context and authors’ assessment.

See also: → high; → redshift;
→ object.

A galaxy or quasar having a → redshift larger than about 0.8, corresponding to a → look-back time half the present age of the Universe. The qualifier “high” is, however, relative and depends on context and authors’ assessment.

See also: → high; → redshift;
→ object.

The state of the → tide when at its highest level.

Etymology (EN): → high; → tide.

Etymology (PE): Owpiš, from Persian Gulf dialects, literally “forward water,” from ow, variant of âb, → water, + piš “→ forward.”
Madd, loan from Ar.

The state of the → tide when at its highest level.

Etymology (EN): → high; → tide.

Etymology (PE): Owpiš, from Persian Gulf dialects, literally “forward water,” from ow, variant of âb, → water, + piš “→ forward.”
Madd, loan from Ar.

Also known as → high tide.

See also: → high; → water.

Also known as → high tide.

See also: → high; → water.

A branch of astrophysics that deals with objects emitting highly energetic radiation, such as X-ray astronomy, gamma-ray astronomy, and extreme ultraviolet astronomy, as well as neutrinos and cosmic rays.

See also: → high; → energy; → astrophysics.

A branch of astrophysics that deals with objects emitting highly energetic radiation, such as X-ray astronomy, gamma-ray astronomy, and extreme ultraviolet astronomy, as well as neutrinos and cosmic rays.

See also: → high; → energy; → astrophysics.

Cosmic rays which typically have energies in the range 10¹⁵ to 10²⁰ electron volts. For the most part, they are protons and other atomic nuclei, and come from distant cosmos, perhaps even from outside our own Galaxy.

See also: → high; → energy; → cosmic; → ray.

Cosmic rays which typically have energies in the range 10¹⁵ to 10²⁰ electron volts. For the most part, they are protons and other atomic nuclei, and come from distant cosmos, perhaps even from outside our own Galaxy.

See also: → high; → energy; → cosmic; → ray.

A neutrino produced in high-energy particle collisions, such as those occurring when → cosmic rays strike atoms
in the Earth’s → atmosphere. Their energy range expands from a few → MeVs up to tenths of a → peta- (P) → electron-volts.

See also: → high; → energy; → neutrino.

A neutrino produced in high-energy particle collisions, such as those occurring when → cosmic rays strike atoms
in the Earth’s → atmosphere. Their energy range expands from a few → MeVs up to tenths of a → peta- (P) → electron-volts.

See also: → high; → energy; → neutrino.

A rare class of → H II regions in the → Magellanic Clouds. In contrast to the typical H II regions of the Magellanic Clouds, which are extended structures (sizes of several arc minutes corresponding to more than 50 pc, powered by a large number of exciting stars), HEBs are very dense and small regions (~ 4" to 10" in diameter corresponding to ~ 1-3 pc). They have a higher degree of → excitation ([O III] 5007Å /Hβ) with respect to the typical H II regions, and are, in general, heavily affected by local → dust. They are powered by a relatively smaller number of → massive stars.

See also: → high; → excitation; → blob.

A rare class of → H II regions in the → Magellanic Clouds. In contrast to the typical H II regions of the Magellanic Clouds, which are extended structures (sizes of several arc minutes corresponding to more than 50 pc, powered by a large number of exciting stars), HEBs are very dense and small regions (~ 4" to 10" in diameter corresponding to ~ 1-3 pc). They have a higher degree of → excitation ([O III] 5007Å /Hβ) with respect to the typical H II regions, and are, in general, heavily affected by local → dust. They are powered by a relatively smaller number of → massive stars.

See also: → high; → excitation; → blob.

A star whose mass exceeds 8 solar masses. Same as → massive star. → intermediate-mass star; → low-mass star.

See also: → high; → mass; → star.

A star whose mass exceeds 8 solar masses. Same as → massive star. → intermediate-mass star; → low-mass star.

See also: → high; → mass; → star.

A member of one of the two main classes of → X-ray binary systems where one of the components is a neutron star or a black hole and the other one a → massive star. HMXBs emit relatively → hard X-rays and usually show regular pulsations, no X-ray bursts, and often X-ray eclipses. Their X-ray luminosity is much larger than their optical luminosity. In our Galaxy HMXBs are found predominantly in the → spiral arms and within the → Galactic disk in young → stellar populations less than 10⁷ years old. One of the most famous HMXB is Cygnus X-1 which was the first stellar-mass black hole discovered. See also: → low-mass X-ray binary.

See also: → high; → mass; → X-ray; → binary.

A member of one of the two main classes of → X-ray binary systems where one of the components is a neutron star or a black hole and the other one a → massive star. HMXBs emit relatively → hard X-rays and usually show regular pulsations, no X-ray bursts, and often X-ray eclipses. Their X-ray luminosity is much larger than their optical luminosity. In our Galaxy HMXBs are found predominantly in the → spiral arms and within the → Galactic disk in young → stellar populations less than 10⁷ years old. One of the most famous HMXB is Cygnus X-1 which was the first stellar-mass black hole discovered. See also: → low-mass X-ray binary.

See also: → high; → mass; → X-ray; → binary.

A laser beam with the output power in the range 10¹²-10¹⁵
watts/cm², capable of depositing kilo-joule order energies during nano-second time intervals in small volumes (about 1 mm³). High power lasers, which can produce temperatures of 10-50 million degrees and pressures of 10-100 million bars, are used to simulate astrophysical conditions in laboratories.

Etymology (EN): → high; → power;, → laser.

Etymology (PE): leyzer, → laser; por “much, many, full,” → full; tavân, → power.

A laser beam with the output power in the range 10¹²-10¹⁵
watts/cm², capable of depositing kilo-joule order energies during nano-second time intervals in small volumes (about 1 mm³). High power lasers, which can produce temperatures of 10-50 million degrees and pressures of 10-100 million bars, are used to simulate astrophysical conditions in laboratories.

Etymology (EN): → high; → power;, → laser.

Etymology (PE): leyzer, → laser; por “much, many, full,” → full; tavân, → power.

An observation that provides a particularly narrow, peaked image of a point source. → point spread function.

See also: → high; → resolution;
→ observation.

An observation that provides a particularly narrow, peaked image of a point source. → point spread function.

See also: → high; → resolution;
→ observation.

A population of neutral or partly ionized gas clouds in the → Galactic halo which are seen as high-altitude structures in the → atomic hydrogen  → 21 cm emission at high radial velocities (v_LSR > 100 km/sec). They have substantial neutral → column densities (> 10¹⁹ cm^-2) and their → metallicities range from 0.1 to about 1.0 times solar. The distances to the majority of them remain unknown. They may represent the continuing infall of matter onto the → Local Group.
See also → compact high-velocity clouds.

See also: → high; → velocity; → cloud.

A population of neutral or partly ionized gas clouds in the → Galactic halo which are seen as high-altitude structures in the → atomic hydrogen  → 21 cm emission at high radial velocities (v_LSR > 100 km/sec). They have substantial neutral → column densities (> 10¹⁹ cm^-2) and their → metallicities range from 0.1 to about 1.0 times solar. The distances to the majority of them remain unknown. They may represent the continuing infall of matter onto the → Local Group.
See also → compact high-velocity clouds.

See also: → high; → velocity; → cloud.

A mountainous or elevated region; → plateau.

Etymology (EN): → high; → land.

Etymology (PE): Kuhsâr “mountainous, hilly area,” from kuh, → mountain, + -sâr suffix denoting profusion, abundance, variant -zâr, → catastrophe.

A mountainous or elevated region; → plateau.

Etymology (EN): → high; → land.

Etymology (PE): Kuhsâr “mountainous, hilly area,” from kuh, → mountain, + -sâr suffix denoting profusion, abundance, variant -zâr, → catastrophe.

A → chemical element that is → geochemically characterized as having a strong → affinity to partition into → metals relative to → silicates.

The highly siderophile elements, → ruthenium (Ru), → rhodium (Rh), → palladium (Pd), → rhenium (Re), → osmium (Os), → iridium (Ir), → platinum (Pt), and → gold (Au), are of interest to planetary scientists because they give insights into the early history of → accretion and → differentiation. HSEs prefer to reside in the metal of planetary cores. Therefore, the HSEs found in planetary → mantles are considered to be overabundant relative to their known preferences for metal over silicate. Therefore, it has been inferred that processes other than → equilibrium partitioning have been responsible for establishing the abundances of → mantle siderophiles. A detailed understanding of the absolute → concentrations and relative abundances of the HSEs may therefore give important insights into the earliest history of a planet (Jones et al., 2003, Chemical Geology 196, 21).

Etymology (EN): From Gk. sidero-, from sideros “iron” + → -phile.

Etymology (PE): Âhandust, from âhan, → iron, + -dust, → -phile.

A → chemical element that is → geochemically characterized as having a strong → affinity to partition into → metals relative to → silicates.

The highly siderophile elements, → ruthenium (Ru), → rhodium (Rh), → palladium (Pd), → rhenium (Re), → osmium (Os), → iridium (Ir), → platinum (Pt), and → gold (Au), are of interest to planetary scientists because they give insights into the early history of → accretion and → differentiation. HSEs prefer to reside in the metal of planetary cores. Therefore, the HSEs found in planetary → mantles are considered to be overabundant relative to their known preferences for metal over silicate. Therefore, it has been inferred that processes other than → equilibrium partitioning have been responsible for establishing the abundances of → mantle siderophiles. A detailed understanding of the absolute → concentrations and relative abundances of the HSEs may therefore give important insights into the earliest history of a planet (Jones et al., 2003, Chemical Geology 196, 21).

Etymology (EN): From Gk. sidero-, from sideros “iron” + → -phile.

Etymology (PE): Âhandust, from âhan, → iron, + -dust, → -phile.

1. A long walk or march for recreational activity, military training, or the like.
   

   2. To walk or march a great distance, especially through rural areas, for pleasure, exercise, military training, or the like (Dictionary.com).

Etymology (EN): From E. dialectal hyke “to walk vigorously,” maybe a Northern form of hitch “to move or draw (something) with a jerk,” of unknown origin.

Etymology (PE): Vaniž, from Sangesari wəniž-/wəništ “to walk about, go round;” cf. Shughni näγ-, Roshani niγ-, naγên- “to turn round;” Book Pahlavi/Zoroastrian Mid.Pers. nâz-, nâž- “to roll, turn;” Mid.Pers. nâys- “be proud, delicate.”

1. A long walk or march for recreational activity, military training, or the like.
   

   2. To walk or march a great distance, especially through rural areas, for pleasure, exercise, military training, or the like (Dictionary.com).

Etymology (EN): From E. dialectal hyke “to walk vigorously,” maybe a Northern form of hitch “to move or draw (something) with a jerk,” of unknown origin.

Etymology (PE): Vaniž, from Sangesari wəniž-/wəništ “to walk about, go round;” cf. Shughni näγ-, Roshani niγ-, naγên- “to turn round;” Book Pahlavi/Zoroastrian Mid.Pers. nâz-, nâž- “to roll, turn;” Mid.Pers. nâys- “be proud, delicate.”

A generalization of Euclidean space in a way that extends methods of vector algebra from the two- and three-dimensional spaces to infinite-dimensional spaces.
Multi-dimensional space in which the eigenfunctions of quantum mechanics are represented by orthogonal unit vectors.

See also: Named after the German mathematician David Hilbert (1862-1943), recognized as one of the most influential mathematicians of the 19th and early 20th centuries for his numerous contributions to various areas of mathematics; → space.

A generalization of Euclidean space in a way that extends methods of vector algebra from the two- and three-dimensional spaces to infinite-dimensional spaces.
Multi-dimensional space in which the eigenfunctions of quantum mechanics are represented by orthogonal unit vectors.

See also: Named after the German mathematician David Hilbert (1862-1943), recognized as one of the most influential mathematicians of the 19th and early 20th centuries for his numerous contributions to various areas of mathematics; → space.

The asteroids found on the outer edge of the main asteroid belt in a 2:3 orbital resonance with Jupiter. The group is not an asteroid family since the members are not physically related. The group
consists of asteroids with semi-major axes between 3.70 AU and 4.20 AU, eccentricities less than 0.30, and inclinations less than 20°. It is dominated by D- and P-type asteroids.

See also: Named for the prototype 153 Hilda, discovered by
Johann Palisa (1848-1925) on November 2, 1875, and named Hilda after a daughter of his teacher, the astronomer Theodor von Oppolzer (1841-1886); → asteroid.

The asteroids found on the outer edge of the main asteroid belt in a 2:3 orbital resonance with Jupiter. The group is not an asteroid family since the members are not physically related. The group
consists of asteroids with semi-major axes between 3.70 AU and 4.20 AU, eccentricities less than 0.30, and inclinations less than 20°. It is dominated by D- and P-type asteroids.

See also: Named for the prototype 153 Hilda, discovered by
Johann Palisa (1848-1925) on November 2, 1875, and named Hilda after a daughter of his teacher, the astronomer Theodor von Oppolzer (1841-1886); → asteroid.

A natural elevation of the earth’s surface, smaller than a mountain.

Etymology (EN): M.E.; O.E. hyll, from P.Gmc. *khulnis (cf. M.Du. hille, Low Ger. hull “hill,” O.N. hallr “stone,” Goth. hallus “rock,” O.E. holm “rising land, island”), from PIE base *kel- “to rise, to be prominent” (cf. Skt. kuta- “summit, peak;” Mod.Pers. kutal, kotal high hill, the skirts of a hill;" Tabari dialect keti “hill; top of the head;
L. collis “hill,” culmen “top, summit,” cellere “raise,” celsus “high;” Gk. kolonos “hill,” kolophon “summit;” Lith. kalnas “mountain,” kalnelis “hill”).

Etymology (PE): Tappé “hill.”

A natural elevation of the earth’s surface, smaller than a mountain.

Etymology (EN): M.E.; O.E. hyll, from P.Gmc. *khulnis (cf. M.Du. hille, Low Ger. hull “hill,” O.N. hallr “stone,” Goth. hallus “rock,” O.E. holm “rising land, island”), from PIE base *kel- “to rise, to be prominent” (cf. Skt. kuta- “summit, peak;” Mod.Pers. kutal, kotal high hill, the skirts of a hill;" Tabari dialect keti “hill; top of the head;
L. collis “hill,” culmen “top, summit,” cellere “raise,” celsus “high;” Gk. kolonos “hill,” kolophon “summit;” Lith. kalnas “mountain,” kalnelis “hill”).

Etymology (PE): Tappé “hill.”

The spherical region around a → secondary in which the secondary’s gravity is more important for the motion of a particle about the secondary than the tidal influence of the → primary. The radius is described by the formula: r = a (m/3M)^1/3, where, in the case of the Earth, a is the semi-major axis of the orbit around the Sun, m is the mass of Earth, and M is the mass of the Sun. The Hill sphere for the Earth has a radius of 0.01
→ astronomical units (AU).
Therefore the Moon, lying at a distance of 0.0025 AU, is well within the Hill sphere of the Earth.

See also: Named for George William Hill (1838-1914), an American astronomer who described this sphere of influence; → sphere.

The spherical region around a → secondary in which the secondary’s gravity is more important for the motion of a particle about the secondary than the tidal influence of the → primary. The radius is described by the formula: r = a (m/3M)^1/3, where, in the case of the Earth, a is the semi-major axis of the orbit around the Sun, m is the mass of Earth, and M is the mass of the Sun. The Hill sphere for the Earth has a radius of 0.01
→ astronomical units (AU).
Therefore the Moon, lying at a distance of 0.0025 AU, is well within the Hill sphere of the Earth.

See also: Named for George William Hill (1838-1914), an American astronomer who described this sphere of influence; → sphere.

The condition for the stability of a → three-body system. Three-body systems exist widely in the → solar system and → extrasolar systems, including Sun-planet-moon systems, planets-star systems, and → triple star systems. This concept of stability was introduced by Hill (1878). He used the → Jacobi integral to construct bounds of motion for → conservative systems with time-independent → potentials, which was introduced to study the stability of the Moon in the Sun-Earth → restricted three-body problem. The stability is defined by the → zero-velocity surface based on the Jacobi integral. The concept of the Hill stability has been used by many researchers to study the stability of three-body systems. The studies include the Hill stability in the full → three-body problems, the hierarchical three body problems, and the restricted three body problems (See, e.g., S. Gong & J. Li, 2015, Astrophys Space Sci. 358,37).

See also: Hill, G.W.: Researches in the lunar theory. Am. J. Math. 1(2), 129-147 (1878); → stability.

The condition for the stability of a → three-body system. Three-body systems exist widely in the → solar system and → extrasolar systems, including Sun-planet-moon systems, planets-star systems, and → triple star systems. This concept of stability was introduced by Hill (1878). He used the → Jacobi integral to construct bounds of motion for → conservative systems with time-independent → potentials, which was introduced to study the stability of the Moon in the Sun-Earth → restricted three-body problem. The stability is defined by the → zero-velocity surface based on the Jacobi integral. The concept of the Hill stability has been used by many researchers to study the stability of three-body systems. The studies include the Hill stability in the full → three-body problems, the hierarchical three body problems, and the restricted three body problems (See, e.g., S. Gong & J. Li, 2015, Astrophys Space Sci. 358,37).

See also: Hill, G.W.: Researches in the lunar theory. Am. J. Math. 1(2), 129-147 (1878); → stability.

A process in which a → close encounter between a → tightly bound binary star system and a → supermassive black hole causes one binary component to become bound to the black hole and the other to be ejected at very high velocities, up to 4,000 km s^-1. → hypervelocity star.

See also: Hills, J. G, “Hyper-velocity and tidal stars from binaries disrupted by a massive Galactic black hole,” Nature 331, 687; → mechanism.

A process in which a → close encounter between a → tightly bound binary star system and a → supermassive black hole causes one binary component to become bound to the black hole and the other to be ejected at very high velocities, up to 4,000 km s^-1. → hypervelocity star.

See also: Hills, J. G, “Hyper-velocity and tidal stars from binaries disrupted by a massive Galactic black hole,” Nature 331, 687; → mechanism.

The tenth of Jupiter’s known satellites, 186 km in diameter revolving at a mean distance of 11,480,000 km from Jupiter. Discovered in 1904 by the Argentine-American astronomer Charles Dillon Perrine (1867-1951).

See also: Himalia was a nymph of the island of Rhodes. She was seduced by the god Zeus (Jupiter).

The tenth of Jupiter’s known satellites, 186 km in diameter revolving at a mean distance of 11,480,000 km from Jupiter. Discovered in 1904 by the Argentine-American astronomer Charles Dillon Perrine (1867-1951).

See also: Himalia was a nymph of the island of Rhodes. She was seduced by the god Zeus (Jupiter).

Same as → Indian numeral system.

See also: → numeral; → system.

Same as → Indian numeral system.

See also: → numeral; → system.

Etymology (EN): O.E. hype “hip,” akin to Du. heup, O.H.G. huf, Ger. Hüfte, Swed. höft, Goth. hups “hip,” of uncertain origin.

Etymology (PE): Šanj (Dehxodâ) “hip, buttock, thigh, haunch,” of unknown origin.

Etymology (EN): O.E. hype “hip,” akin to Du. heup, O.H.G. huf, Ger. Hüfte, Swed. höft, Goth. hups “hip,” of uncertain origin.

Etymology (PE): Šanj (Dehxodâ) “hip, buttock, thigh, haunch,” of unknown origin.

A → European Space Agency satellite, which was launched in August 1989 and operated until March 1993. It was the first space mission devoted to → astrometry with an unprecedented degree of accuracy. The telescope on Hipparcos had a main mirror of diameter 29 cm. Calculations from observations by the main instrument
generated the Hipparcos Catalogue of 118,218 stars charted with the
highest precision (published in 1997) containing positions, distances, → parallaxes, and → proper motions. An auxiliary star mapper pinpointed many more stars with lesser but still unprecedented accuracy, in the Tycho Catalogue of 1,058,332 stars. The Tycho 2 Catalogue, completed in 2000, brings the total to 2,539,913 stars, and includes 99% of all stars down to magnitude 11.
→ Gaia.

See also: Hipparcos, acronym for → High  → Precision  → Parallax  → Collecting → Satellite, chosen for its similarity to the name of the Greek astronomer Hipparchus of Nicaea (c. 190-125 BC), one of the most influential astronomers of antiquity, who compiled an extensive star catalogue in which he gave the positions of over 1,000 stars and also classified them according to their magnitude (on a scale of 1 to 6, brightest to faintest). Ptolemy later incorporated this information into his → Almagest. In addition, he discovered the → precession of the equinoxes.

A → European Space Agency satellite, which was launched in August 1989 and operated until March 1993. It was the first space mission devoted to → astrometry with an unprecedented degree of accuracy. The telescope on Hipparcos had a main mirror of diameter 29 cm. Calculations from observations by the main instrument
generated the Hipparcos Catalogue of 118,218 stars charted with the
highest precision (published in 1997) containing positions, distances, → parallaxes, and → proper motions. An auxiliary star mapper pinpointed many more stars with lesser but still unprecedented accuracy, in the Tycho Catalogue of 1,058,332 stars. The Tycho 2 Catalogue, completed in 2000, brings the total to 2,539,913 stars, and includes 99% of all stars down to magnitude 11.
→ Gaia.

See also: Hipparcos, acronym for → High  → Precision  → Parallax  → Collecting → Satellite, chosen for its similarity to the name of the Greek astronomer Hipparchus of Nicaea (c. 190-125 BC), one of the most influential astronomers of antiquity, who compiled an extensive star catalogue in which he gave the positions of over 1,000 stars and also classified them according to their magnitude (on a scale of 1 to 6, brightest to faintest). Ptolemy later incorporated this information into his → Almagest. In addition, he discovered the → precession of the equinoxes.

The smallest known moon orbiting the planet → Neptune, discovered in 2013. Hippocamp has an estimated diameter of only about 34 km and orbits close to → Proteus, the outer and the second largest of Neptune’s moons. The orbital → semi-major axes of the two moons differ by only 10%. Hippocamp is probably an ancient fragment of Proteus.
Billions of years ago a comet collision would have chipped off a chunk of Proteus. Images from the Voyager 2 space probe from 1989 show a large → impact crater on Proteus, whose size compares with Hippocamp’s (Showalter et al., 2019, Nature 566, 350).

See also: Formerly known as S/2004 N 1, Hippocamp is named after the sea creatures in Greek and Roman mythology. The mythological Hippocampus possesses the upper body of a horse and the lower body of a fish. The Roman god Neptune would drive a sea-chariot pulled by Hippocampi.

The smallest known moon orbiting the planet → Neptune, discovered in 2013. Hippocamp has an estimated diameter of only about 34 km and orbits close to → Proteus, the outer and the second largest of Neptune’s moons. The orbital → semi-major axes of the two moons differ by only 10%. Hippocamp is probably an ancient fragment of Proteus.
Billions of years ago a comet collision would have chipped off a chunk of Proteus. Images from the Voyager 2 space probe from 1989 show a large → impact crater on Proteus, whose size compares with Hippocamp’s (Showalter et al., 2019, Nature 566, 350).

See also: Formerly known as S/2004 N 1, Hippocamp is named after the sea creatures in Greek and Roman mythology. The mythological Hippocampus possesses the upper body of a horse and the lower body of a fish. The Roman god Neptune would drive a sea-chariot pulled by Hippocampi.

A curve described by the → polar equation  r² = 4b (a - b sin²θ), where a and b are positive constants. For appropriate
values of a and b, the curve looks like the infinity symbol, ∞. See also → spheres of Eudoxus.

See also: Hippopede, literally “a horse’s foot,” denoting a “horse fetter (hobble),” from Gk. hippos, → horse, + -pede variant of -ped, combining form of pos,→ foot.

A curve described by the → polar equation  r² = 4b (a - b sin²θ), where a and b are positive constants. For appropriate
values of a and b, the curve looks like the infinity symbol, ∞. See also → spheres of Eudoxus.

See also: Hippopede, literally “a horse’s foot,” denoting a “horse fetter (hobble),” from Gk. hippos, → horse, + -pede variant of -ped, combining form of pos,→ foot.

A type of graphical representation, used in statistics, in which frequency distributions are illustrated by rectangles.

Etymology (EN): Histogram, from Gk. histo-, a combining form meaning “tissue,”
from histos “mast, loom, beam, warp, web,” literally “that which causes to stand,” from histasthai “to stand,” from PIE *sta- “to stand” (cf. Pers. ist-, istâdan “to stand;” O.Pers./Av. sta- “to stand, stand still; set;”
Skt. sthâ- “to stand;” L. stare “to stand;” Lith. statau “place;” Goth. standan); → -gram.

Etymology (PE): Nemudâr, → diagram + sotuni “column-like,” from sotun “column,” from Mid.Pers. stun, from O.Pers. stênâ “column,” Av. stuna-, Skt. sthuna- “column.”

A type of graphical representation, used in statistics, in which frequency distributions are illustrated by rectangles.

Etymology (EN): Histogram, from Gk. histo-, a combining form meaning “tissue,”
from histos “mast, loom, beam, warp, web,” literally “that which causes to stand,” from histasthai “to stand,” from PIE *sta- “to stand” (cf. Pers. ist-, istâdan “to stand;” O.Pers./Av. sta- “to stand, stand still; set;”
Skt. sthâ- “to stand;” L. stare “to stand;” Lith. statau “place;” Goth. standan); → -gram.

Etymology (PE): Nemudâr, → diagram + sotuni “column-like,” from sotun “column,” from Mid.Pers. stun, from O.Pers. stênâ “column,” Av. stuna-, Skt. sthuna- “column.”

Of, pertaining to, treating, or characteristic of → history or past events (Dictionary.com). → historical supernova.

See also: → history; → -al.

Of, pertaining to, treating, or characteristic of → history or past events (Dictionary.com). → historical supernova.

See also: → history; → -al.

A supernova event recorded in the course of history before the invention of the telescope. The well recorded supernovae of this small group are
SN 185, SN 1006, SN 1054 (→ Crab Nebula), SN 1181, SN 1572 (→ Tycho’s star), and SN 1604 (→ Kepler’s star).

See also: → historical; → supernova.

A supernova event recorded in the course of history before the invention of the telescope. The well recorded supernovae of this small group are
SN 185, SN 1006, SN 1054 (→ Crab Nebula), SN 1181, SN 1572 (→ Tycho’s star), and SN 1604 (→ Kepler’s star).

See also: → historical; → supernova.

1. The branch of knowledge dealing with past events.
   

2. The record and explanation of past events and times, especially in connection with a particular people, country, period, person, etc. See: → star formation history, → historical supernova.

Etymology (EN): History, from M.E. histoire, historie, from O.Fr. estoire, histoire, from L. historia “narrative, tale, story,” from Gk. historia “a learning or knowing by inquiry, record, account,” from historein “to inquire,” from histor “one who knows or sees, wise man, " from PIE *wid-tor-, from base *weid- “to know; to see;” cf. Pers. bin- “to see” (present stem of didan);
Mid.Pers. wyn-; O.Pers. vain- “to see;” Av. vaēn- “to see;”
Skt. veda “I know.” Related to Gk. idein “to see,” and to eidenai “to know,” → idea.

Etymology (PE): Târix, from Ar., itself, according to Abu Rayhân Biruni (973-1048, in Athar al-Baqqiya), loan from Mid.Pers. mâhrôz “date,” first Arabicized as murux, from which the infinitive taurix, and then târix.

1. The branch of knowledge dealing with past events.
   

2. The record and explanation of past events and times, especially in connection with a particular people, country, period, person, etc. See: → star formation history, → historical supernova.

Etymology (EN): History, from M.E. histoire, historie, from O.Fr. estoire, histoire, from L. historia “narrative, tale, story,” from Gk. historia “a learning or knowing by inquiry, record, account,” from historein “to inquire,” from histor “one who knows or sees, wise man, " from PIE *wid-tor-, from base *weid- “to know; to see;” cf. Pers. bin- “to see” (present stem of didan);
Mid.Pers. wyn-; O.Pers. vain- “to see;” Av. vaēn- “to see;”
Skt. veda “I know.” Related to Gk. idein “to see,” and to eidenai “to know,” → idea.

Etymology (PE): Târix, from Ar., itself, according to Abu Rayhân Biruni (973-1048, in Athar al-Baqqiya), loan from Mid.Pers. mâhrôz “date,” first Arabicized as murux, from which the infinitive taurix, and then târix.