An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



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Number of Results: 3105 Search : on
horizon system
  راژمان ِ افقی   
râžmân-e ofoqi

Fr.: coordonnées horizontales   

Same as → horizon coordinate system.

horizon; → system.

ofoqi (#)

Fr.: horizontal   

1) Of or pertaining to the → horizon.
2) At right angles to the → vertical; parallel to level ground.
See also:
blue horizontal branch star, → extreme horizontal branch star, → field horizontal branch star, → horizontal branch, → horizontal branch star, → horizontal eclipse, → horizontal parallax, → horizontal refraction, → red horizontal branch star, → supra-horizontal branch star, → zero age horizontal branch star.

From → horizon + → -al.

horizontal branch (HB)
  شاخه‌ی ِ افقی   
šâxe-ye ofoqi (#)

Fr.: branche horizontale   

A set of roughly horizontal points in the → Hertzsprung-Russell diagram of a typical → globular cluster. It displays a stage of stellar evolution which immediately follows the → red giant branch (RGB) in stars with an initial mass < 1.2 Msun. When the star's ascent of the RGB is terminated by the → helium flash, it moves down to the HB. The star's → effective temperature on the HB is higher than it was on the RGB, but the luminosity is considerably less than at the helium flash. Usually HB stars have two energy sources: in addition to the → helium burning in their cores, they experience → hydrogen fusion in a surrounding shell. The thickness of the shell determines the color of the HB stars. A thin shell, involving low → opacity, makes the star look blue. The HB domain encompasses a very large effective temperature range with several members: → extreme HB, → blue HB, → RR Lyrae, → red HB, and → red clump stars. The locations depend on many parameters, including stellar mass, metallicity, age, helium abundance, and rotation.

horizontal; → branch.

horizontal branch star
  ستاره‌ی ِ شاخه‌ی ِ افقی   
setâre-ye šâxe-ye ofoqi

Fr.: étoile de la branche horizontale   

A star lying on the → horizontal branch.

horizontal; → branch; → star.

horizontal eclipse
  ماه‌گرفت ِ افقی   
mâhgereft-e ofoqi

Fr.: selenelion   

A type of → lunar eclipse that occurs when both the Sun and the eclipsed Moon can be observed at the same time. This is possible only when lunar eclipse occurs just before sunset or just after sunrise. At that case, both bodies will appear just above the horizon at nearly opposite points in the sky. Also called → selenelion and → selenehelion.

horizontal; → eclipse.

horizontal parallax
  دیدگشت ِ افقی   
didgašt-e ofoqi

Fr.: parallaxe horizontale   

The angle under which the radius of the Earth at the place of observation would be seen from a celestial body when it is in the horizon (at the instant of rising or setting). The amount varies with the latitude since the Earth is not exactly spherical, and is greatest at equator.

horizontal; → parallax.

horizontal refraction
  شکست ِ افقی   
šekast-e ofoqi (#)

Fr.: réfraction horizontale   

The angular distance of an object below the horizon when it appears to lie on the horizon.

horizontal; → refraction.

horizontal scaling
  مرپلش ِ افقی   
marpeleš- ofoqi


In computer science, a scaling in which the processing power is increased/decreased by adding/removing nodes with similar resources. See also → vertical scaling.

horizontal; → scaling.

hot accretion flow
  تچان ِ فربال ِ داغ   
tacân-e farbâl-e dâq

Fr.: écoulement d'accrétion chaud   

A type of → accretion flow by a → compact object such as a → black hole which has a high → virial temperature, is → optically thick, and occurs at lower mass → accretion rates compared with → cold accretion flows. In a hot accretion flow with a very low mass accretion rate, the electron mean free path is very large, and so the accreting → plasma is nearly collisionless. In this type of accretion flow, thermal conduction transports the energy from the inner to the outer regions. As the gas temperature in the outer regions can be increased above the → virial temperature , the gas in the outer regions can escape from the gravitational potential of the central black hole and form outflows, significantly decreasing the mass accretion rate.

cold; → accretion; → flow.

hot electron diode
  دیود ِ الکترون ِ داغ   
diod-e elektron-e dâq

Fr.:diode à électrons chauds   

Same as → Schottky diode

hot; → electron; → diode.

Hubble classification
  رده‌بندی ِ هابل   
radebandi-ye Hubble (#)

Fr.: classification de Hubble   

The classification of galaxies according to their visual appearance into four basic types suggested by E. Hubble: → ellipticals (E), → spirals (S), → barred spirals (SB), and → irregulars (Ir). Later on a separate class of → lenticulars (S0) was appended as an intermediate type between ellipticals and spirals. The sequence starts with round elliptical galaxies (E0). Flatter galaxies are arranged following a number which is calculated from the ratio (a - b)/a, where a and b are the major and minor axes as measured on the sky. Ellipticals are divided into eight categories (E0, E1, ..., E7). Beyond E7 a clear disk is apparent in the lenticular (S0) galaxies. The sequence then splits into two parallel branches of disk galaxies showing spiral structure: ordinary spirals, S, and barred spirals, SB. The spiral and barred types are subdivided into Sa, Sb, Sc, and SBa, SBb, SBc, respectively. Along the sequence from Sa to Sc, the central bulge becomes smaller, while the spiral arms become more and more paramount. The original, erroneous idea that such arrangement of the galaxies might represent an evolutionary sequence led to the ellipticals being referred to as early-type galaxies, and the spirals and Irr I irregulars as late-type galaxies. See also → dwarf galaxy, → dwarf elliptical galaxy, → dwarf spheroidal galaxy.

Hubble; → classification.

Hubble constant
  پایا‌ی ِ هابل   
pâyâ-ye Hubble (#)

Fr.: constante de Hubble   

Hubble-Lemaitre constant.

Hubble; → constant.

Hubble-Lemaitre constant
  پایا‌ی ِ هابل-لومتر   
pâyâ-ye Hubble-Lemaître

Fr.: constante de Hubble-Lemaître   

The → Hubble parameter for the → present epoch. It is the constant of proportionality between the → recession velocities of galaxies and their distances from each other. The latest determinations using the → Hubble Space Telescope observations of → Cepheids give H0 = 72 ± 8 km s-1 Mpc-1 (W. L. Freedman et al., 2001, ApJ 553, 47, arXiv:astro-ph/0012376), the → WMAP observations yield 70.4 ± 1.3 km s-1 Mpc-1 (N. Jarosik et al., 2011, ApJS 192, 14, arXiv:1001.4744), and the → Planck Satellite observations give 67.3 ± 1.2 km s-1 Mpc-1 (Planck Collaboration, 2014, A&A 571, A16, arXiv:1303.5076). More recently, the Hubble constant was derived by a team of astronomers, using the NASA/ESA Hubble Space Telescope, with a 2.4% accuracy (Adam G. Reiss et al., 2016, arXiv:1604.01424). The new value, 73.2 km s-1 Mpc-1, suggests that the Universe is expanding between five and nine percent faster than previously calculated. The → Hubble law is only applicable for large distances (> 20 Mpc), when the proper motions of galaxies in groups and clusters cannot confuse the recession due to expansion.

Hubble; → Friedmann-Lemaitre Universe; → constant.

Hubble-Lemaitre tension
  تنش ِ پایای ِ هابل-لومتر   
taneš-e pâyâ-ye Hubble-Lemaître

Fr.: Tension sur la constante de Hubble-Lemaître   

The discrepancy between the value of the → Hubble-Lemaitre constant inferred from a ΛCDM fit (→ Lambda cold dark matter model) to the → cosmic microwave background (CMB) and local measurements. The Universe appears to be expanding much faster now than predicted even with our latest understanding of its initial conditions and contents. Based on the → Hubble Space Telescope observations, the Hubble-Lemaitre constant is very recently estimated to be 74.03 km s-1 Mpc-1. This value indicates that the Universe is expanding at a rate about 9% faster than that implied by the → Planck satellite's observations of the → early Universe, which give a value for the Hubble constant of 67.4 km s-1 Mpc-1. For discussion, see D'Arcy Kenworthy et al. (2019, ApJ 875, 145).

Hubble-Lamaitre constant; → tension.

Hubble-Sandage classification
  رده‌بندی ِ هابل-سندیج   
radebandi-ye Hubble-Sandage

Fr.: classification de Hubble-Sandage   

Same as the → Hubble classification.

Hubble; → Hubble-Sandage variable; → sequence.

Hugoniot curve
  خم ِ هوگونیو   
xam-e Hugoniot

Fr.: courbe de Hugoniot   

A curve, on the pressure versus specific volume plane, representing the locus of all the possible states that can be reached by a substance immediately after the passage of a single → shock wave. For each initial condition there is a different curve. No combustion occurs in the process and, therefore, the chemical composition of the medium does not change. See also → Rayleigh line; → Crussard curve.

Named after the French physicist Pierre Henri Hugoniot (1851-1887), who worked on fluid mechanics, especially flow properties before and after shock waves; → curve.

Humphreys-Davidson limit
  حد ِ همفریز-دیویدسون   
hadd-e Humphreys-Davidson

Fr.: limite de Humphreys-Davidson   

An empirical upper → luminosity boundary in the → H-R diagram. It consists of two sections, a sloping part and a horizontal part. The sloping part, which decreases with decreasing → effective temperature, corresponds roughly to the → Eddington limit. The horizontal part is the temperature-independent upper luminosity limit for late-type → hypergiants. It is thought that → massive stars above the Humphreys-Davidson limit encounter an → instability, possibly due to the opacity-modified Eddington limit, and experience high → mass loss episodes which prevent their evolution to cooler temperatures. → Luminous Blue Variable stars are examples of this high mass loss phase.

Named after Roberta M. Humphreys and Kris Davidson, who first dealt with this limit (1979, ApJ 232, 409); → limit.

Huygens Division
  شکاف ِ هویگنس   
šekâf-e Huygens

Fr.: division de Huygens   

In the system of → Saturn's rings, the gap at the inner edge of the → Cassini division at a distance of 117,680 km from the center of the planet with a width of 285-400 km.

Huygens; → division.

Huygens Region
  ناحیه‌ی ِ هویگنس   
nâhiye-ye Huygens

Fr.: région de Huygens   

The inner bright part of the → Orion Nebula, from which most of the radiation is emitted. It is about 5' across corresponding to 0.7 pc (for a distance of 440 pc). See O'Dell (2001, ARAA 39, 99).

Named after the Dutch astronomer Christiaan → Huygens (1629-1695), who sketched the appearance of the Orion Nebula. His drawing, the first such known sketch, was published in Systema Saturnium in 1659. First named such by O. Gingerich (1982, Ann. NY Acad. Sci. 395, 308); → region.

  هیدرش، آبش   
hidreš, âbeš (#)

Fr.: hydratation   

The process of combining with → water.

Verbal noun of → hydrate.

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