An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A member of the most extreme galaxies in terms of their luminosities and unusual hot → dust temperatures. The → infrared emission from HDOGs is dominated by obscured accretion onto a central → supermassive black hole (SMBH), in most cases without significant contribution from → star formation. The large contrast between the underlying → host galaxy and the hyper-luminous emission from the → active galactic nucleus (AGN) implies that either the SMBH is much more massive than expected for the stellar mass of its host, or is radiating well above its → Eddington limit. The most extreme of these remarkable systems known is → W2246-0526.

→ hot; → dust; → obscure; → galaxy.

Same as → Schottky diode

→ hot; → electron; → diode.

A giant, gaseous, Jupiter-like planet lying too close to its parent star and having an orbital period from a few days to a few weeks. The existence of hot Jupiters is usually interpreted in terms of planetary migration. These planets can, in principle, be formed at larger distances from their stars and migrate to the inner regions due to dynamical interaction with the proto-planetary disk.

→ hot; → Jupiter.

A relatively small, dense, and hot → molecular clump occurring in regions of → massive star formation. HMCs have diameters ≤ 0.1 pc, densities ≥ 10⁷ cm^-3, and temperatures ≥ 100 K. The densest hot cores are traced in → ammonia (NH₃) and possess densities of 10⁸ cm^-3, sizes down to 0.05 pc and temperatures of up to 250 K. Hot molecular cores are generally associated with → compact H II regions and → ultracompact H II regions. High angular resolution observations suggest that HMCs are internally heated by embedded sources, since temperature and density increases toward the center as expected if star formation is occurring close to the core center. Same as → hot core.

→ hot; → molecular; → core.

Of a → CCD detector, a pixel that has higher charge loss. Hot pixels are a type of noise affecting almost every CCD camera. They are caused by small contamination or production faults in the CCD sensor area.

→ hot; → pixel.

A compact, highly luminous region in a cataclysmic binary located in the accretion disk where the stream of material hits it.

→ hot; → spot.

A member of a class of stars having high → effective temperatures (above some 15,000 K); mainly → massive stars, but also including → exciting stars of → planetary nebulae, → white dwarfs, and → symbiotic stars.

→ hot; → star.

The 24th part of a day; 60 minutes.
An angular unit of right ascension, equivalent to 15°.

Hour, from M.E. houre, from O.Fr. hore, from L. hora "hour, time, season," from Gk. hora "any limited time," used of day, hour, season, year; cognate E. → year.

Sâ'at, from Ar.

A telescope based coordinate specifying the angle, in the equatorial plane, from the meridian to a plane containing the celestial object and the north and south celestial poles.

→ hour; → angle.

A great circle passing through an object and the → celestial poles intersecting the → celestial equator at right angles.

→ hour; → circle.

A device for measuring time; it consists of a glass container having two compartments from the uppermost of which a quantity of sand runs in an hour into the lower one through a narrow tube.

Hour glass, from → hour + → glass.

Sâat-e šeni, from sâat, → hour + šeni, adj. of šen "sand."

1) A building in which people live.
2) A building for any purpose. → greenhouse.

M.E. h(o)us, from O.E. hus "dwelling, shelter, house;" cf. O.N. hus; Du. huis; Ger. Haus .

Xâné "house," from Mid.Pers. xânak, xân, xôn; Aftari dialect kiye "house, home;" Xonsâri ki "house;" Anâraki xiya, Tâti Karingân kâ, Sangesari keh "house, home;" cf. L. cunae "cradle; bed;" Gk. kome "village;" PIE base *kei- "bed; to lie, to settle; beloved" (other cognates: O.E. ham "dwelling, house, village;" E. home; Ger. Heim; L. civis "townsman;" Fr. cité; E. city; Skt. śiva- "auspicious, dear").

1) To remain in one place in the air by rapidly beating the wings.
2) Computers: To place (a pointer) over a particular area of a computer screen (such as a hyperlink on a webpage) so as to cause a pop-up box to appear or other change to occur, without clicking a button on the device.

M.E. hoveren, frequentative of hoven "hover, tarry, linger," of unknown origin.

Parjâ zadan (on the model of darjâ zadan "to march in the same place, moving one's legs up and down without going forward"), from par zadan darjâ "to beat the wings at the same place," from par zadan "to beat the wings," from par "wing, → feather," zadan, → beat, + darjâ "in the same place," from dar, → in, + jâ, → place.

A vehicle capable of travelling over land or water on a cushion of air.

→ space; → craft.

The act of one who hovers.

→ hover; → -ing.

An → excited state in the → triple alpha process leading to the production of the most abundant → isotope of → carbon. The existence of this state is of extreme astrophysical importance concerning the → nucleosynthesis of ¹²C in stellar → cores:
⁴He + ⁴He ↔ ⁸Be,
⁸Be + ⁴He ↔ ¹²C^*,
¹²C^*→ ¹²C + γ.
The process proceeds as follows. First the unstable → ground state of ⁸Be is formed in the collision of two → alpha particles. Since ⁸Be exists roughly 7 x 10^-17 sec, it must fuse with an alpha particle before breaking up. However, the probability of three bodies merging simultaneously is extremely low. Hoyle showed that the ¹²C nucleus needs an excited state or resonance at 7.68 MeV to provide for a high reaction probability. The Hoyle state was soon found at 7.65 MeV with the predicted → spin and → parity.

In honor of the British astrophysicist Fred Hoyle (1915-2001), who predicted this state in 1953 (Hoyle et al. 1953, Physical Review 92, 1095); it was discovered by W. A. Fowler in 1957; → state.

Same as → Hertzsprung-Russell diagram.

Short for → Hertzsprung-Russell diagram.

Edwin Powell Hubble (1889-1953), the American astronomer who provided the observational evidence of the expansion of the Universe in 1929; → Hubble-Lemaitre law.

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-Lemaitre constant.

→ Hubble; → constant.