An Etymological Dictionary of Astronomy and Astrophysics
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The exponent appearing in the power law that describes the → effective temperature of a → rotating star as a function of the → effective gravity, as deduced from the → von Zeipel theorem or law. Generalizing this law, the effective temperature is usually expressed as T_eff∝ g_eff^β, where β is the gravity darkening exponent with a value of 0.25. It has, however, been shown that the relation between the effective temperature and gravity is not exactly a power law. Moreover, the value of β = 0.25 is appropriate only in the limit of slow rotators and is smaller for fast rotating stars (Espinosa Lara & Rieutord, 2011, A&A 533, A43).

→ gravity; → darkening; → exponent.

A special type of occultation that occurs when the star appears to pass tangentially on the → edge of the → Moon.

→ grazing; → occultation.

The Greatest → elongation of an inferior planet occurring after sunset.

Superlative of → great; → eastern; → elongation.

The largest → elongation of an inferior planet from the Sun. It may be → greatest eastern elongation or → greatest western elongation. The greatest elongation of Mercury is about 28°, and thus Mercury can only be observed 112 minutes after sunset or before sunrise. For Venus, it is about 47°, making it visible at most about 3 hours after sunset or before sunrise.

Superlative of → great; → eastern; → elongation.

The Greatest → elongation of an inferior planet occurring before sunrise.

Superlative of → great; → western; → elongation.

An astronomical observation carried out using a telescope on Earth, as opposed to that from an orbiting satellite.

→ ground; based, adj. of base, from O.Fr. bas, from L. basis "foundation," from Gk. basis "step, pedestal," from bainein "to step;" → observation.

Nepâheš, → observation; az "from," → ex-; zamin, → ground.

The continuum trough observed in the spectra of high redshift quasars (z> 6) at the blue wing of their Lyman-alpha emission line (1216 Å). It is explained by the scattering of the radiation of the quasar by intergalactic neutral hydrogen on the line of sight. Because of the cosmological expansion, the quasar line is redshifted with respect to the continuum trough. The Gunn-Peterson opacity increases rapidly with redshift. It is interpreted as a strong evidence for the reionization of the Universe around z = 6.

After James E. Gunn and Bruce A. Peterson who predicted the effect in 1965; → effect.

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.

→ H I; → region

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.

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.

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.

1) In a cathode-ray tube, the glow surrounding a bright spot that appears on the fluorescent screen as the result of the screen's light being reflected by the front and rear surfaces of the tube's face.
2) The effect in which a halo appears around the image of a bright object recorded on a photographic film or plate. It is produced by the fan-like pattern of light reflected through the emulsion by the medium's backing material.

Halation, from hal(o), → halo + -ation a combination of -ate and -ion, used to form nouns from stems in -ate.

Hâlegiri, from hâlé, → halo + giri, verbal noun of gereftan "to take, seize" (Mid.Pers. griftan, Av./O.Pers. grab- "to take, seize," cf. 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;" PIE base *ghrebh- "to seize").

The moon when, at either quadrature, half its disk is illuminated.

→ half; → moon.

The → probability distribution of the → number of galaxies that a host → dark matter halo of a given mass contains. HOD is a powerful theoretical frame to populate dark matter halos with luminous galaxies. More specifically, it describes the bias between galaxies and dark matter by specifying (a) the probability P(N|M) that a halo of → virial mass M contains N galaxies of a particular class and (b) the relative spatial and velocity distributions of galaxies and dark matter within halos.

→ halo; → occupation; → distribution.

Old stars with very low metallicities (→ metallicity) found in the → halo of the Galaxy. Also called → population II star.

→ halo; → population.

One of a set of equations that describe the motion of a → dynamical system in terms of the → Hamiltonian function and the → generalized coordinates. For a → holonomic system with n degrees of freedom, Hamilton's equations are expressed by: q^._i = ∂H/∂p_i and p^._i = - ∂H/∂q_i, i = 1, ..., n.

→ Hamiltonian function; → equation.