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.
Fr.: règle de Hund
An empirical rule stating that all orbitals of a given sublevel must be occupied by single electrons before pairing begins.
After the German physicist Friedrich Hund (1896-1997), known for his work on atoms and molecules. → rule.
The smallest three digit number in the decimal system and the smallest square of a two-digit number (10).
Hundred, from O.E. hundred "a counting of 100," from P.Gmc. *hunda- "hundred," as below, + *rath "reckoning, number."
Sad "hundred," from Mid.Pers. sad, sat, Av. sata- "hundred," satô.raocana- "with a hundred windows," satô.təmô.sata- "hundreds of hundred;" cf. Skt. śatá- "hundred;" Gk. hekaton; L. centum; Lith. simtas; P.Gmc. *hunda- "hundred" (Goth. hund; O.H.G. hunt); PIE *kmtom "hundred."
šekâridan (#), šekâr kardan (#)
M.E, hunten, from O.E. huntian "chase game," from hunta "hunter," and related to hentan "to pursue."
Šekâridan, šekâr kardan, from šekâr "hunt;" variant bešgar(d) "hunter, fowler; chase; game; place for hunting;" Parthian Mid.Pers. škr "to hunt, pursuit;" Sogdian škr-, (')škr- "to lead, take; pursue, persecute," prefixed 'pškr- "to chase;" Proto-Ir. *skar- "to pursue, drive, look for (the cattle);" + -gar, → -or.
šekârandé (#), šekârgar (#)
A person who hunts game or other wild animals for food or in sport (Dictionary.com).
Fr.: diagramme de Hunter
I. Hunter et al., 2009, A&A, 496, 841; → diagram.
An intense warm-core oceanic cyclone that originates in tropical latitudes; called a typhoon in the western Pacific Ocean. Sustained winds are 120 km per hr or higher.
Hurricane, from Sp. huracán, from Taino (the language of an extinct Arawakan Indian tribe of the West Indies) hurakán.
Tufand, from tufidan "to roar, to raise a tumult," tufân "storm, the roaring of the sea, the confused hum of men or animals." Is this Persian word related to Gk. typhon "whirlwind," personified as a giant, father of the winds?
1) tâveš; 2) tâvešidan
Fr.: 1) hâte; 2) se dépêcher, se presser
1) A state of urgency or eagerness; haste
Of uncertain origin.
Tâveš, from Tâleši tâveš "hurry, haste," tâvisté "to hurry up;" variant Kurd. (Mahâbâd) tus "hasty."
Fr.: division de 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.
Fr.: principe de Huygens
Every point of a → wavefront may be considered as a center of a secondary disturbance which gives rise to spherical wavelets, and the wavefront at any later instant may be regarded as the envelope of these wavelets. This statement suffices to account for the laws of → reflection and → refraction, and the approximately straight line propagation of light through large apertures, but it fails to account for → diffraction, the deviations from exact straight line propagation of light. Huygens' principle was later extended by Fresnel and led to the formulation of → Huygens-Fresnel principle, which is of great importance in the theory of diffraction.
Fr.: principe Huygens-Fresnel
A development of → Huygens' principle stating that every point on a → wavefront acts, at a given instant, as a source of outgoing secondary spherical waves. The secondary wavelets mutually interfere and the resulting net light amplitude at any position in the outgoing wavefront is the vector sum of the amplitudes of all the individual wavelets. Using this principle, Fresnel calculated with a high accuracy the distribution of light in → diffraction patterns. The Huygens-Fresnel principle was put on a firm theoretical basis by Kirchhoff and expressed as an integral derived from the → wave equation.
A young, nearby cluster of stars (spectral types A1-K) visible to the naked eye in the constellation → Taurus about 150 light-years away. Its individual stars (more than 200) appear to spread out in space. → Aldebaran is a foreground star in that region of the sky.
In Gk. mythology, a group of nymphs and sisters of Hyas, or else his daughters, and when Hyas died while hunting, killed by a lion or a boar, they grieved his death exceedingly, and turned into the stars called Hyades.
Huâdes, from Gk., as above.
Composed of two distinct races, breeds, varieties, species, or heterogeneous elements.
Hybrid, from L. hybrida, from ibrida "a crossbred animal," of unknown origin.
Doragé "hybrid," from do "two" + rag "lineage, race, stock; vein, blood vessel," from Mid.Pers. rag, from Proto-Iranian *raha-ka-, from *raha- "liquid, essence," + suffix -ka-; cf. Av. ranhā- "name of a mythical stream;" Skt. rása- "juice (of plants), liquid, essence," rásavant-, rasin- "juicy;" L. ros "dew;" Lith. ràsa "dew;" O.C.S. rosa "dew."
xorgereft-e doragé (#)
Fr.: éclipse solaire hybride
A rare type of solar eclipse where at some places the eclipse is annular while at other places it appears total. This duality comes about when the vertex of the Moon's umbral shadow pierces Earth's surface at some points, but falls short of the planet along other portions of the eclipse path. Hybrid eclipses are also known as → annular-total eclipses. In most cases, hybrid eclipses begin as annular, transform into total, and then revert back to annular before the end of their track. In rare instances, a hybrid eclipse may begin annular and end total, or vice versa (F. Espenak, NASA).
Fr.: étoile hybride
hidr- (#), âb- (#)
1) The Water Serpent. The longest and largest constellation in the sky,
stretching almost 7 hours of right ascension, and covering over 1300 square degrees,
from → Canis Minor to → Libra.
It lies south of → Cancer, → Leo,
and → Virgo, and is best seen in the northern hemisphere during
the months of February through May. Its brightest star is → Alphard.
Abbreviation: Hya; Genitive: Hydrae.
Hydra, from L. hydrus, from Gk. hydra "water-snake;" cf. Av. udra- "otter;" Skt. udrá- "otter;" O.H.G. ottar "otter;" O.E. otor, ottor; Ger. Otter; E. otter; Lith. udras, udra "otter;" akin to → water. In Gk. mythology, this constellation represents the gigantic nine-head water-snake which haunted the swamps of Lerna. Herakles was sent to destroy her as one of his twelve labours, but for each of her heads that he decapitated, two more sprang forth. So with the help of Iolaos, Herakles applied burning brands to the severed stumps, cauterizing the wounds and preventing regeneration. In the battle he also crushed a giant crab (→ Cancer) beneath his heel which had come to assist Hydra.
Hudrâ, from Gk. hydra, as above. → Hydrus (
Hydra I cluster
Fr.: amas de l'Hydre
A relatively poor → galaxy cluster at about 50 Mpc containing a pair of bright galaxies near its centre: NGC 3309 and NGC 3311. Also known as Abell 1060 (→ Abell catalog), Hydra I is the prototype of an evolved and dynamically relaxed cluster, being dominated by early-type galaxies and having a regular core shape.