helium shell burning
suzeš-e puste-ye heliom
Fr.: combustion de la coquille d'hélium
A stage in the evolution of an → asymptotic giant branch star, when all the helium in the core is fused into carbon and oxygen. No more fusion takes place in the core, and as a result the core contracts. The core contraction generates a sufficient temperature for fusing the surrounding layers of helium. Since helium shell burning is unstable, it causes → helium shell flashes.
helium shell flash
deraxš-e puste-ye heliomi
Fr.: flash de la couche d'hélium
A violent outburst of energy that occurs periodically in an → asymptotic giant branch star. It occurs when helium is being burnt in a thin shell surrounding the inner dense core of carbon and oxygen. → Helium shell burning is unstable, producing energy mainly in short intense flashes. The shell flash causes considerable expansion of the star followed by collapse, thus setting up deep convection. As a consequence, the → convective zone in the outer part of the star goes deeper and may → dredge-up carbon to the surface. See also → late thermal pulse; → very late thermal pulse; → AGB final thermal pulse.
Fr.: étoile d'hélium
An → evolved star which has lost most or all of its hydrogen-rich envelope, leaving just a core of helium.
Fr.: calibration hélium-argon
A wavelength calibration of astronomical spectra using a helium-argon light source.
lâmp-e heliyom-ârgon (#)
Fr.: lampe hélium-argon
A comparison light source containing the known spectral lines of helium and Argon.
A curve which lies on a cylinder or cone, so that its angle to a plane perpendicular to the axis is constant.
From L. helix "spiral," from Gk. helix (genitive helikos), related to eilein "to turn, twist, roll."
Picâr "that which twists," from pic, present stem of picidan "to twist, entwine, coil" + -ar agent noun suffix (on the model of parastâr, padidâr, dustâr, xâstâr).
Helix Nebula (NGC 7293)
Fr.: Nébuleuse de l'Hélice
A large and bright → planetary nebula in the constellation → Aquarius. Its apparent diameter is about half the size of the full Moon, corresponding to about 2.5 → light-years for a distance of about 700 light-years. It is the nearest bright planetary nebulae to Earth and one of the most spectacular examples of such objects. The Helix Nebula possibly consists of at least two separate disks with outer rings and filaments. The brighter inner disk seems to be expanding at about 100,000 km/h and to have taken about 12,000 years to form. High-resolution observations of the inner edge of the Helix's main ring have revealed thousands of cometary knots of gas with faint tails extending away from the central star. The knots have masses similar to the Earth, but are typically the size of our Solar system. The comet-like shape of the knots results from the steady evaporation of gas from the knots, produced by the strong winds and ultraviolet radiation from the central star of the nebula.The origin of the knots is currently not well understood.
Fr.: bassin de Hallas
One of the largest identified → impact craters both on → Mars and within the → Solar System. Hellas spans more than 2000 km across in the → southern hemisphere, a region that is much more heavily cratered and higher in average elevation than the northern hemisphere. The depth of Hellas from its bottom to its inner rim is more than 4 km. In comparison, the depth of the Grand Canyon in the United States is roughly 1.6 km, that is 2.5 times smaller! The western part of the Hellas basin contains the lowest point on Mars, about 8.2 km below the Mars datum or Martian "sea level." The formation of the impact structure is believed to have taken place in the early Noachian epoch, between 3.9 and 4.6 billion years ago (Planetary Science Institute webpage).
Hellas refers to the classical name for Greece; → basin.
1) Any of various protective head coverings worn by soldiers,
policemen, firemen, etc.
From M.Fr. helmet, diminutive of helme "helmet," from Frank. *helm (cf. O.H.G. helm "helmet"); PIE base *kel- "to cover, to hide;" cf. Av. sar- "shelter;" Laki šârd "hidden, hiddenly," šârden "to hide;" Kurd. šâr-, šârdinawa "to hide;" Skt. śárman- "cover, protection, refuge;" L. celare "to conceal;" Goth. huljan "to cover, conceal;" O.H.G. helan "to hide."
Xud "helmet," from O.Pers. xaudā- "hat, cap," tigra-xauda- "wearing the pointed cap" (as is shown in the sculpture of Skunkha the Scythian at Behistan); Av. xaoδa- "hat, cap, helmet;" Ossetic xodä; Arm. (borrowed) xoir "headband."
Fr.: grand jet en bulbe, ~ ~ en casque prussien
A large-scale → coronal feature with apparent → cusp, seen during a → solar eclipse. They usually arise from → sunspots and → active regions, so at the base of a helmet streamer one will often find a → prominence. They form magnetic loops that connect the sunspots and suspend material above the surface of the Sun. The magnetic field lines trap the material to form the streamers. The action of the → solar wind is at the origin of the peak feature.
Helmholtz free energy
kâruž-e âzâd-e Helmholtz
Fr.: énergie libre de Helmholtz
Of a system, the quantity whose decrease gives the maximum amount of external work which is performed when any physical or chemical process is carried out reversibly at constant temperature. It is defined by F = U - TS, where U is the → internal energy, T the → absolute temperature, and S the final → entropy.
Fr.: théorème de Helmholtz
A → decomposition theorem, whereby a continuous → vector field, F, can be broken down into the sum of a → gradient and a → curl term: F = -∇φ + ∇ xA, where φ is called the → scalar potential and A the → vector potential.
Fr.: courant de Helmi
A systematic trend in the motion of some → Galactic halo→ old stars thought to be a relic of the → merging of a dwarf satellite galaxy devoured by our Milky Way. Using kinematic data from the → Hipparcos satellite, Helmi et al. (1999, Nature 402, 53) found two halo star streams which share a common progenitor: a single coherent object disrupted during or soon after the Milky Way's formation, and which probably resembled the Fornax and Sagittarius dwarf spheroidal galaxies.
See also Helmi & White 1999, MNRAS 307, 495; → stream.
Fr.: centre d'assistance
A service in an organization or computer system where users are directed for technical support or assistance.
M.E., O.E. help (m.), helpe (f.) "assistance, succor," helpan "o help" (cf. O.N. hjialp, hjalpa, M.Du., Du. hulp, helpen, O.H.G. helfa, helfan, Ger. Hilfe, helfen); PIE base *kelb- "to help" (cf. Lith. selpiu "to support, help"); desk, from M.E. deske; M.L. desca, descus "table to write on," from L. discus "quoit, platter, dish," from Gk. diskos "disk, dish."
A mineral that is often found in meteorites. It is an oxide of iron (Fe2O3) that is similar to magnetite. It does not attract a magnet. When it is rubbed against an object harder than itself, it leaves a reddish-brown stain. Hematite is also sometimes called bloodstone.
From M.Fr. hematite, from L. hæmatites, from Gk. haimatites lithos "bloodlike stone," from haima (genitive haimatos) "blood" + -ites, → -ite, + lithos "stone."
A defect of the eyes in which sight is normal in the night or in a dim light but is abnormally poor or wholly absent in the day or in a bright light. Also called day blindness. Opposite of → nyctalopia
From N.L., from Gk hemeralop- (stem of hemeralops having such a condition, from hemer(a) "day" + al(aos) "blind" + -ops having such an appearance) + -ia a noun suffix.
nimsepehr (#), nimkoré (#)
Half of a sphere bounded by a great circle, especially one of the halves into which the earth or the celestial sphere is divided.
From L. hemisphærium, from Gk. hemisphairion, from hemi- "half," (from PIE base *semi-; cf. Skt. sami, L. semi-, O.H.G. sami- "half," and O.E. sam-) + sphaira, → sphere.
Henry Draper system
râžmân-e Henry Draper
Fr.: système de Henry Draper
A catalog of stars in which every star is classified by its stellar spectrum. This system is named for the astronomer Henry Draper, but was cataloged by Annie J. Cannon (225,300 stars), and later extended by Margaret W. Mayall.
Henry Draper (1837-1882), an American pioneer of astronomical spectroscopy who established the observing techniques and program for the work that would bear his name when published, seven years after his early death; → system.
Fr.: trajet de Henyey
A nearly horizontal path on the → Hertzsprung-Russell diagram that a → pre-main sequence star of small mass follows in an early stage of evolution after leaving the → Hayashi track and before reaching the → main sequence. During this stage the pre-main sequence star remains almost wholly in radiative equilibrium.
After Louis George Henyey (1910-1970), American astronomer. Henyey et al. (1955, PASP 67, 154).
A combining form meaning "seven."
From Gk. hepta "seven;" cognate with L. septem; Pers. haft, as below; Du. zeven, O.H.G. sibun, Ger. sieben, E. seven.
Haft-, from haft "seven;" Mid.Pers. haft; Av. hapta; cf. Skt. sapta; Gk. hepta, L. septem; PIE *septm.