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).
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 creation 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.
Habitable, 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.
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.
Fr.: expérience de Hafele-Keating
An experiment performed in 1971 using four atomic → cesium clocks transported in jet airplanes eastward and westward around the Earth to verify the → time dilation predicted by the theory of → special relativity.
J.C. Hafele and R. E. Keating, 1972, Science 177, 166; → experiment.