An Etymological Dictionary of Astronomy and Astrophysics
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In nuclear physics, a potential barrier near the surface of the nucleus that inhibits the release of alpha particles.

Gamow, after George Gamow (originally Georgiy Antonovich Gamov), the Ukrainian born theoretical physicist and cosmologist, who discovered quantum tunneling; → barrier.

The constraint on the → baryon number density at T ~ 10⁹ K in the early → expanding Universe. Gamow recognized that a key to the element buildup is the reaction n + p ↔ d + γ. Deuterium needs to be produced in sufficient abundance for higher elements to form, but if all → neutrons are immediately locked up into → deuterium, no higher elements can form either. The Gamow condition is expressed by n_b<σv>t ~ 1, where n_b is the baryon number density, σ is the cross section for the reaction at relative → velocity v, and t the expansion time-scale for the → Universe. This means that the time-scale for the above reaction is comparable to the expansion time. From this condition the baryon number density at the start of element buildup is found to be n_b ~ (σvt)^-1 ~ 10¹⁸ cm^-3 at T = 10⁹ K (P. J. E. Peebles, 2013, Discovery of the Hot Big Bang: What happened in 1948, arXiv.1310.2146).

→ Gamow barrier; → condition.

In nuclear fusion, the product of the Maxwell-Boltzmann distribution with the tunnelling probability of the nuclei through their Coulomb barrier. This is the energy region where the reaction is more likely to take place: at higher energies, the number of particles becomes insignificant while at lower energies the tunnelling through the Coulomb barrier makes the reaction improbable.

→ Gamow barrier; → peak.

The seventh and largest of → Jupiter's known satellites. This → Galilean satellite has a diameter of 5270 km, slightly larger than Mercury, a mass about 1.48 × 10²³ kg (about 2 Earth Moons); an → orbital period of 7.155 days, and an → eccentricity of e = 0.0015. It was discovered by Galileo and Marius in 1610. The mean → surface temperature of Ganymede is -160 °C. It is the only moon known to have a → magnetosphere.

In Gk. mythology, Ganymedes, a unusually beautiful prince of Troy who was abducted to Olympus by Zeus and made the cup-bearer of the gods.

An empty space or interval; interruption in continuity; a break or opening, as in a fence, wall. → Encke gap.

Gap, from O.N. gap "chasm," related to gapa "to gape."

Gâf, variant kâf "split, slit," stem of kâftan, kâvidan "to split; to dig," Mid./Mod.Pers. škâf- škâftan "to split, burst," Proto-Iranian *kap-, *kaf- "to split;" cf. Gk. skaptein "to dig;" L. cabere "to scratch, scrape," P.Gmc. *skabanan (Goth. skaban; Ger. schaben; E. shave). PIE base *(s)kep- "to cut, to scrape, to hack."

A variable → red supergiant star of → spectral type M2 Ia in the → constellation → Cepheus. Also called → Mu Cephei. Its → apparent magnitude is usually about 4.5 and varies from 3.6 to 5.1. It is also a → triple star.

Garnet "a deep-red color," from the more or less transparent, usually red, silicate mineral that has a vitreous luster. So named by William Herschel from its unusual deep reddish tint. From O.Fr. grenat "garnet," from M.L. granatum, originally an adj., "of dark red color," probably abstracted from pomegranate, from M.L. pomum granatum "apple with many seeds," from pome "apple, fruit" + grenate "having grains."

Nârsang, from nâr, from anâr "pomegranate," from Mid.Pers. anâr "pomegranate" + sang, → stone.

A substance whose physical state is such that it always occupies the whole of the space in which it is contained.

Gas, from Du. gas, probably from Gk. khaos "empty space," → chaos. The term gas was coined by the Belgian physician Jean-Baptiste van Helmont (1579-1644) to designate aerial spirits.

Gâz, loanword from Fr.

For a given quantity of an → ideal gas, the product of its → pressure and the → volume divided by the → absolute temperature (R = PV/T).

→ gas; → constant.

An equation that links the pressure and volume of a quantity of gas with the absolute temperature. For a gram-molecule of a perfect gas, PV = RT, where P = pressure, V = volume, T = absolute temperature, and R = the gas constant.

→ gas; → equation.

A → giant planet composed mainly of → hydrogen and → helium with → traces of → water, → methane, → ammonia, and other hydrogen compounds. Gas giants have a small rocky or metallic core. The core would be at high temperatures (as high as 20,000 K) and extreme pressures. There are four gas giants in our solar system: → Jupiter, → Saturn, → Uranus, and → Neptune. Another category of gas giants is → ice giants. Ice giants are also composed of small amounts of hydrogen and helium. However, they have high levels of what are called "ices." These ices include methane, water, and ammonia.

→ gas; → giant.

A kind of laser where the lasing medium is a gas or a mixture of gases that can be excited with an electric discharge. The first gas laser to operate successfully was built by A. Javan and William R. Bennette at the Bell Telephone Laboratories. This laser used a mixture of helium and neon as the active medium and produced a continuous beam rather than a series of pulses. This laser operated in the infrared region of the spectrum at 1.15 micrometres. A few years later Kumar Patel developed the CO₂ laser.

→ gas; → laser.

The metallicity derived from observations of the gas component of a galaxy. It is mainly measured from optical → emission lines using primarily oxygen abundances. The gas → metallicity is one of the most important tools to investigate the evolutionary history of galaxies. The reason is that the gas metallicity of galaxies is basically determined by their star-formation history. Recent observational studies has allowed the investigation of the gas metallicity even in → high redshift beyond z = 1, such as → Lyman break galaxies, submillimeter-selected high-z galaxies, and so on. Such observational insights on the metallicity evolution of galaxies provide constraints on the theoretical understandings of the formation and the evolution of galaxies.

→ gas; → metallicity.

An aggregate of several different kinds of gases which do not react chemically under the conditions being considered. A gas mixture constitutes a homogeneous thermodynamical system.

→ gas; → mixture.

The → ionized component of a → comet's → tail, driven nearly straight away from the → Sun Sun by the → solar wind. solar wind. Also called → ion tail, → plasma tail, and → Type I tail.

→ gas; → tail.

A galaxy which has a relatively low gas content. More specifically, a galaxy whose → baryonic matter is chiefly in the form of stars and has very little → interstellar matter.

→ gas; → poor; → galaxy.

A galaxy, usually young, which has a relatively important gas content.

→ gas; → rich; → galaxy.

The mass ratio of gas to dust. It amounts to approximately 100 in the → interstellar medium, but may vary in → molecular clouds and → circumstellar disks due to dust → grain evaporation, → dust settling, → condensation of gas, etc. The gas-to-dust ratio depends on the → metallicity. It is larger in galaxies with lower metallicity.

→ gas; → dust; → ratio.

1) Existing in the → state of a gas.
2) Pertaining to or having the characteristics of gas.

→ gas; → -eous.

An → isotope separation process using the different diffusion speeds of → atoms or → molecules for separation. This process is used to divide → uranium hexafluoride (UF₆) into two separate streams of U-235 and U-238. Before processing by gaseous diffusion, uranium is first converted from → uranium oxide (U₃O₈) to UF₆. The UF₆ is heated and converted from a solid to a gas. The gas is then forced through a series of compressors and converters that contain porous barriers. Because uranium-235 has a slightly lighter isotopic mass than uranium-238, UF₆ molecules made with uranium-235 diffuse through the barriers at a slightly higher rate than the molecules containing uranium-238. At the end of the process, there are two UF₆ streams, with one stream having a higher concentration of uranium-235 than the other (EVS, a Division of Argonne National Laboratory).

→ gaseous; → diffusion.

An → H II region, a → planetary nebula, or a → supernova remnant.

→ gaseous; → nebula.