A combining form meaning "face" used in the names of geometrical solid figures having the form or number of faces specified by the initial element.
N.L., from Gk -edron, from hedra "seat, base, chair, face of a geometric solid," from PIE root *sed- "to → sit."
Dimé, from dim, → face.
H and K lines
xatthâ-ye H o K
Fr.: raies H et K
Two prominent → absorption lines, at 3968.5 Å and 3933.7 Å respectively, in the spectra of stars like the → Sun and cooler due to → singly ionized → calcium (Ca II). The strength of H and K lines can be an indication of considerable magnetic activity in the → chromosphere of these stars. The Ca II H and K lines are also common in some kinds of → eruptive variable stars. These lines are not seen in → hot stars, and start to become visible in → A-type stars.
H and K, letters of alphabet, conventionally chosen; → line.
Fr.: H I
Atomic or → neutral hydrogen.
From H, abbreviation of hydrogen + I "one" in Roman number system, nomenclature convention representing neutral atoms.
H I region
nâhiye-ye H I
Fr.: région H I
A region of neutral (atomic) hydrogen in interstellar space. At least 95 percent of interstellar hydrogen is H I. It emits radio waves that are 21 cm long.
Fr.: H II
Ionized → hydrogen, that is a proton nucleus that has lost its unique electron.
From H, abbreviation of hydrogen + II "two" in Roman number system, nomenclature convention representing singly ionized atoms.
H II galaxy
kahkešân-e H II
Fr.: galaxie H II
A low-mass and → metal-poor galaxy
(1/30-1/3 Zsun), experiencing strong
episodes of → star formation,
characterized by the presence of bright → emission lines
on a faint → blue continuum.
The fact that H II galaxies are metal poor and very blue objects seems
to suggest that they are young. Nevertheless, several studies
show the existence of an → old stellar population
present → star burst
in most of these galaxies. This fact indicates that these
objects are not young systems forming their first generation of
stars. Same as → blue compact dwarf galaxy.
H II region
nâhiye-ye H II
Fr.: région H II
A type of → emission nebulae composed of very hot gas (about 104 K), mainly ionized hydrogen, created by the ultraviolet radiation of → massive stars. H II regions originate when O or early-type stars, born in → giant molecular clouds, start heating up the cold gas, causing it to become → ionized and "glow". The effective temperatures of the → exciting stars are in the range 3 x 104 to 5 x 104 K, and throughout the nebula hydrogen is ionized. Helium is → singly ionized, and other elements are mostly singly or → doubly ionized. Typical densities in the H II region are of the order 10 to 102 cm-3, ranging as high as 104 cm-3. Internal motions occur in the gas with velocities of order 10 km s-1. The spectra of H II regions are mainly composed of strong → H I→ recombination lines and → forbidden lines such as [O III], [O II], [N II]. See also → ionization-bounded H II region; → density-bounded H II region; → compact H II region; → ultracompact H II region.
H II region luminosity
tâbandegi-ye nâhiye-ye H II
Fr.: luminosité de région H II
The total number of → Lyman continuum photons emitted by an → H II region. It is usually derived using → radio continuum observations which are less affected by → interstellar extinction. The measured value is often a lower limit because of photon leakage from the H II region and absorption. See also → density-bounded H II region.
Fr.: H-alpha (Hα)
The → Balmer series spectral line of hydrogen which results from → atomic transition between the → energy levels 2 and 3. It has a wavelength of 656.4 nm and falls in the red region of the visible spectrum.
H, symbol of → hydrogen; alpha (α), the first letter of Gk. alphabet.
Fr.: H-beta (Hβ)
The → Balmer series spectral line of hydrogen which results from → atomic transition between the → energy levels 2 and 4. It has a wavelength of 486.1 nm and falls in the → blue region of the → visible spectrum.
H, symbol of → hydrogen; beta (β), the second letter of Gk. alphabet.
Fr.: diagramme H-R
Same as → Hertzsprung-Russell diagram.
Short for → Hertzsprung-Russell diagram.
Fr.: champ de Habing
A unit used to express the strength of average → far ultraviolet (FUV) intensity in the → interstellar radiation field. It is equal to 1.2 × 10-4 erg cm-2 s-1 sr-1 = 1.6 × 10-3 cm-2 s-1 = 108 photons cm-2 s-1.
Named after Harm Habing, a pioneer in this field (Habing, H. J., 1968, Bull. Astr. Netherlands 19, 421).
M.E., from O.Fr. habitation, from L. habitare "to live, dwell," frequentative of habere "to have, to hold, possess," from PIE base *ghrebh- "to seize, take, hold, have, give, receive" (cf. Mod.Pers. gereftan "to take, seize;" Mid.Pers. griftan; O.Pers./Av. grab- "to take, seize;" Skt. grah-, grabh- "to seize, take," graha "seizing, holding, perceiving;" M.L.G. grabben "to grab," from P.Gmc. *grab, E. grab "to take or grasp suddenly"); → zone.
habitable zone (HZ)
Fr.: zone habitable
A zone around a → star where the → temperature would be in the range 0-100 °C to sustain → liquid water on the surface of rocky planets (or sufficiently large moons). Water is thought to be a necessary component to the → formation and evolution of Earth-type life. This zone depends on the parent star's luminosity and distance; it will be farther from hotter stars. A more accurate definition of HZ needs to include other factors, such as orbital → eccentricity, heat sources other than stellar irradiation, and atmospheric properties. Same as → circumstellar habitable zone; → ecosphere.
A type of intense dust storm that blows in the deserts of North Africa and Arabia, particularly severe in areas of drought.
Haboob, from Ar. habub (
Hadar (Beta Centauri)
Fr.: Hadar (β Centauri)
A blue-white → giant star of → spectral type B1 III with a visual magnitude of V = 0.61 lying in the constellation → Centaurus. It lies at a distance of 350 → light-years and is the eleventh brightest star of the night sky. Also called → Agena
Hadar, from Ar. haZâr (
Any elementary particle which experiences the strong nuclear force. There are two sorts of hadrons: mesons, which have zero spin, and baryons, which have spin 1/2 or 3/2.
Hadron, from Gk. hadr(os) "thick, bulky" + -on a suffix used in the names of subatomic particles (gluon, meson, neutron), quanta (photon, graviton), and other minimal entities or components (magneton).
Fr.: ère hadronique
The interval lasting until some 10-5 seconds after the Big Bang when the Universe was dominated by radiation and its temperature was around 1015 kelvins. It is preceded by → Planck era and followed by → lepton era.
Of or related to → hadrons.
mâde-ye hâdroni (#)
Fr.: matière hadronique
Ordinary matter composed of → hadrons.