An Etymological Dictionary of Astronomy and Astrophysics
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A parameter representing the product of the → gravitational constant by the → solar mass. It is 13.27 x 10¹⁹ m³ s^-2.

→ heliocentric; → gravitational; → constant.

A wavelength calibration of astronomical spectra using a helium-argon light source.

→ helium; → argon; → calibration.

A comparison light source containing the known spectral lines of helium and Argon.

→ helium; → argon; → lamp.

A → polygon with seven → angles and seven → sides.

→ hepta-; → -gon.

Math.: The Hermitian conjugate of an m by n matrix A is the n by m matrix A^* obtained from A by taking the → transpose and then taking the complex conjugate of each entry. Also called adjoint matrix, conjugate transpose. → Hermitian operator.

Hermitian, named in honor of the Fr. mathematician Charles Hermite (1822-1901), who made important contributions to number theory, quadratic forms, invariant theory, orthogonal polynomials, elliptic functions, and algebra. One of his students was Henri Poincaré; → conjugate.

→ frequency to wavelength conversion.

→ hertz; → meter; → conversion.

→ High Energy Stereoscopic System (H.E.S.S.).

→ H.E.S.S.; → collaboration.

A six-sided → polygon.

→ hexa-; → -gon;.

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.

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

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.

→ hierarchical; → structure; → formation.

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.

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 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.

→ high; → excitation; → blob.

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

→ high; → resolution; → observation.

The injection of holes in a semiconductor which can be produced by application of a sharp conducting point in contact with an n-type semiconductor.

→ hole; → injection.

A material system in which the → constraints can be expressed in the form of an equation relating the coordinates.

From Gk. → holo- "whole" + -nomic, related to nomos "law, managing, governing, custom," → -nomy; → system.

A → linear differential equation if the right-hand member is zero, Q(x) = 0, on interval I.

→ homogeneous; → linear; → differential; → equation.

A molecule that is composed of only one type of → chemical element, e.g. the → molecular hydrogen and → ozone.

→ homo-; → nuclear; → molecule.

1) An imaginary circle that delimits the sky and the Earth.
2) The fundamental great circle of the → horizon system, defined by the intersection of the → celestial sphere and a level plane passing through the observer. → celestial horizon.
3) In → Robertson-Walker models, the boundary separating objects already observed from those not yet observed, or the boundary separating objects observable from unobservable (J. Plebanski, A. Krasinski, 2006, An Introduction to General Relativity and Cosmology, Cambridge Univ. Press).
4) → cosmic horizon.
5) → event horizon.
See also:
→ apparent horizon, → artificial horizon, → astronomical horizon, → dip of the horizon, → distance to the horizon, → geometric horizon, → horizon coordinate system, → horizon problem, → horizon system, → particle horizon, → sea horizon, → sensible horizon, → sound horizon, → true horizon, → visible horizon.

From O.Fr. orizon, from orizonte, from L. horizontem (nom. horizon), from Gk. horizon kyklos "bounding circle," from horizein "bound, limit, divide, separate," from horos "boundary."

Ofoq, from Ar.

The coordinate system based on the position of the observer. The horizontal plane is the fundamental plane and the coordinates are → altitude and → azimuth.

→ horizon; → coordinate; → system.

A problem with the standard cosmological model of the Big Bang related to the observational fact that regions of the Universe that are separated by vast distances nevertheless have nearly identical properties such as temperature. This contradicts the fact that light moves with a finite speed and, as a result, certain events which occur in the Universe are completely independent of each other. Inflationary cosmology offers a possible solution.

→ horizon; → problem.