An Etymological Dictionary of Astronomy and Astrophysics
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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.

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 underlying the 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.
Spectroscopically, H II galaxies are essentially identical to the → giant H II regions found in nearby → irregular and → late-type galaxies. The correlation among structural parameters (→ H-beta → luminosity, → velocity dispersion, → linewidths) and between these parameters and the → chemical composition favors the interpretation of H II galaxies as giant H II regions in distant → dwarf irregular galaxies similar to the ones found nearby. Some examples of H II galaxies are: I Zw 18, SBS 0335-052, II Zw 33, UM 408.

→ H II; → galaxy.

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

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.

→ H II; → region; → luminosity.

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.

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.

Same as → Hertzsprung-Russell diagram.

Short for → Hertzsprung-Russell diagram.

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).

In → exobiology, having a → temperature range within which → liquid water can exist on the surface of a → planet.

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.

Zistpazir, from zist, → life, + pazir "admitting, accepting, having," → -able.

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.

→ habitable; → 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 (هبوب) "a wind which blows hard and raises the dust."

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 (حضار) "white camel."

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).

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.

→ hadron; → era.

Of or related to → hadrons.

→ hadron; → -ic.

Ordinary matter composed of → hadrons.

→ hadronic; → matter.

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.

A transition metal found in zirconium ores. This silvery, ductile metal is used in control rods for nuclear reactors and in tungsten filaments and electrodes. Symbol: Hf; Atomic mass: 178.49; Atomic number: 72; melting point 2230°C; boiling point 4602°C.

Hafnium, from N.L. Hafn(ia) "Copenhagen" + -ium. Hafnium was first observed by the French chemist Georges Urbain in 1911 in rare earth samples. Subsequently, the Danish physicist Nils Bohr predicted hafnium's properties using his theory of electronic configuration of the elements.

A dimensionless number characterizing the importance of → viscous force in a → forced flow.

named after the German hydraulic engineer Gotthilf H. L. Hagen (1797-1884); → number.

The interference fringes seen with thick plates near normal incidence.

W. K. von Haidinger (1798-1871), Austrian mineralogist and geologist; → fringe.