An Etymological Dictionary of Astronomy and Astrophysics

A → European Space Agency  → astrometry mission launched on 19 December 2013. Gaia’s goal is to create the largest and most precise three-dimensional chart of the → Milky Way galaxy by providing unprecedented positional (position on the sky and distance to the Sun) and annual → proper motion measurements for about one billion stars in our Galaxy and throughout the → Local Group. Moreover, the third component of the velocity, the → radial velocity, will be obtained for all stars down to V = 17 mag. Similarly, multi-color photometry will be carried out on all stars down to V = 20 mag. Gaia will achieve the planned astrometric requirements
by repeatedly measuring the positions of all objects down to V = 20 mag with final accuracies of about 20 microarcsec at 15 mag. It will provide distances accurate to 20% as far as the → Galactic Center. The satellite is expected to be launched in 2012 and be placed in a → Lissajous orbit around the Sun-Earth → Lagrangian point L2. Gaia is a much more advanced version of the → Hipparcos mission.

See also: Initially, GAIA was the short for Global Astrometric Interferometer for Astrophysics. Although subsequently the interferometer option was abandoned, the acronym was maintained in lower case.

1. A measure of the → amplification of an electronic device, usually expressed as the ratio of → output power to → input power.
   
2. → antenna gain.

Etymology (EN): From M.Fr. gain, from O.Fr. gaaigne, from guaaignier “to obtain,” from Germanic *waidanjan “to hunt, plunder,” also “to graze, pasture,” from P.Gmc. *wartho “hunting ground” (cf. Ger. weide “pasture, pasturage”); PIE base *weiə- “to go after something, strive after.”

Etymology (PE): Bahré, from bahr “part, portion, share, lot;” Av. baxəδra- “portion,” from bag- “to attribute, allot,” → division.

1. Of or pertaining to a → galaxy.
   
2. Usually with capital G, pertaining to our galaxy, the → Milky Way.

See also: Adjective of → galaxy.

The point in the → Galactic plane that lies directly opposite the → Galactic center. It lies in the constellation → Auriga at approximately R.A. 05h 46m, Dec. +28° 56'.

See also: → galactic; → anticenter.

An elongated bar-shaped structure composed of stars present in some spiral galaxies. About two-third of such galaxies contain bars that cross their centers. Bars, like → spiral arms, result from a → density wave
in which stars take very elliptical orbits.
They form when the → galactic disk dominates the → galactic bulge, → Ostriker-Peebles criterion. Bars play an extremely important role in a galaxy’s evolution. The gravity from a bar is the mechanism that drives → interstellar gas from the outer parts of a → spiral galaxy inward toward the central regions, and into the galactic nucleus itself. This causes tremendous bursts of star formation. Therefore, a majority of massive stars are born in such starbursts in the nuclei of galaxies. Bars may also channel the material that falls into black holes within active galactic nuclei, releasing enormous power in radiation and particles from tiny regions at the centers of some galaxies. Bars disappear as galactic centers grow more massive (after some 2 to 8 Gyr).

See also: → galactic; → bar.

The central → galaxy bulge of the → Milky Way.

See also: → galactic; → bulge.

1. The rotational center of the → Milky Way galaxy located in the direction of the → Sagittarius constellation at a distance of 7.62 ± 0.32 kpc (2005, ApJ 628, 246). Its equatorial coordinates (J2000 epoch) are: R.A. 17h45m40.04s, Dec. -29° 00’ 28.1’’.
   The Sun orbits around the Galactic center once every 200 million years at a speed of 220 km per second. It is believed that there is a → supermassive black hole at the Galactic center.
   

2. The innermost region of a → spiral galaxy characterized by high number of stars per unit volume. The center may contain a → supermassive black hole.

See also: → galactic; → center.

One of the three massive clusters located toward the → Galactic center: → Quintuplet cluster, → Arches cluster, → Central cluster. Heavily extinguished by the presence of dust clouds and only accessible at infrared (and longer) wavelengths or in X-rays, each of these clusters has a population of more than a hundred → massive stars. The three clusters are similar in most respects, each containing about 10⁴ solar masses in stars. The Arches cluster is younger than the two others.

See also: → galactic; → center; → cluster.

1. Same as → open cluster.
   
2. same as → cluster of galaxies.

See also: → galactic; → cluster.

A system of astronomical coordinates using → latitude (b^II) measured north and south from the → Galactic equator and → longitude (l^II), measured from the → Galactic Center in the sense of increasing → right ascension from 0 to 360 degrees. In the old system (l^I,b^I), the Galactic center was at l^I = 327°41’. Same as → galactic system.

See also: → galactic; → coordinate.

The flattened component of a → spiral galaxy which is composed of stars and concentrations
of dust and molecules. → Star formation takes place mainly in the disk.

See also: → galactic; → disk.

The study of the → motions of the → stars, → gas, and → dark matter in a → galaxy to explain the main → morphological and → kinematical features of the galaxy.

See also: → galactic; → dynamics.

The great circle in the sky defined by the place of the → Galactic plane or the → Milky Way. At an angle of about 62°, the Galactic equator intersects the celestial equator at two points located in the constellations → Monoceros and → Aquila.

See also: → galactic; → equator.

A region of the Galaxy whose boundaries are set by its calm and safe environment and access to the chemical materials necessary for building terrestrial planets similar to the Earth. → circumstellar habitable zone; → habitable zone.

See also: → galactic, → habitable;
→ zone.

A roughly spherical aggregation of → globular clusters, as well as the oldest stars and unseen mass that surrounds the Galaxy.

See also: → galactic, → halo.

In the → Galactic coordinate system, the angle between the line of sight to an object and the → Galactic equator. Galactic latitude, usually represented by the symbol b^II, ranges from +90 degrees to -90 degrees.

See also: → galactic; → latitude.

In the → Galactic coordinate system, the angle between the → Galactic Center and the projection of the object on the → Galactic plane.
Galactic longitude, usually represented by the symbol l^II, ranges from 0 degrees to 360 degrees.

See also: → galactic; → longitude.

A concentration of stars and gas in the innermost region of a galaxy, sometimes extending over thousands of light-years from the center of the galaxy.

See also: → galactic; → nucleus.

→ galactic-scale outflow.

See also: → galactic; → outflow.

The plane in which the → disk of a → spiral galaxy, such as our → Milky Way, lies.

See also: → galactic; → plane.

The point on the sky, north or south, at which the galaxy’s rotation axis would meet the celestial sphere.

See also: → galactic; → pole.

A diffuse radio signal that originates outside the solar system. It is strongest in the direction of the Galactic plane.

See also: → galactic; → radio;
→ noise.

The revolving of the gaseous and stellar content of a galaxy around its central nucleus. The rotation is not uniform, but differential. One revolution of the Sun within our own Galaxy takes about 220 million years, or one cosmic year.

See also: → galactic; → rotation.

The discrepancy between observed galaxy → rotation curves and the theoretical prediction, assuming a centrally dominated mass associated with the observed luminous material.

See also: → galactic; → rotation; → problem.

The global shape and the arrangement of the various parts or constituents of a galaxy.

See also: → galactic; → structure.

Same as → galactic coordinates.

See also: → galactic; → system.

An outflow of hot gas, analogous to the → solar wind, from a galaxy that has recently undergone a high → burst of star formation or has an → active galactic nucleus. Galactic winds are streams of high speed charged particles
blowing out of galaxies with speeds of 300 to 3,000 km s^-1. In the case of starbursts, galactic winds are powered by → stellar winds driven by → massive stars and → supernova explosions. Galactic winds contain a mixture of extremely hot metal-enriched supernova ejecta and cooler entrained gas and dust.
Outflowing material has been observed at great distances from galaxies (10 to 100 kpc). In some cases they escape the galaxy potential well and pollute the → intergalactic medium with → heavy elements. A prominent example is the → superwind of the starburst galaxy M82.

See also: → galactic; → wind.

The regions near the Galactic plane where there is low absorption of light by interstellar clouds so that some external galaxies may be seen through them.

See also: → galactic; → window.

The time taken for the Sun to revolve once around the center of the Milky Way, amounting to about 220 million years.

See also: → galactic; → year.

The enormous amounts of → mass and → energy released from active galaxies into the → intergalactic medium. → Supermassive black holes,
believed to exist at the centres of active galaxies (→ active galaxy), → accrete matter and liberate huge quantities of energy. The energy output is often observed as → active galactic nuclei (AGN) outflows in a wide variety of forms, e.g. → collimated  → relativistic jets and/or huge overpressured cocoons in → radio, → blueshifted broad → absorption lines in the → ultraviolet and → optical, → warm absorbers and ultrafast outflows in → X-rays, and → molecular gas in → far infrared.

Moreover, the processes of → star formation and → supernova explosions release mass/energy into the surroundings. This → stellar feedback heats up, ionizes and drives gas outward, often generating large-scale outflows/→ winds. Galactic outflows are observed at low redshifts reaching a velocity as large as 1000 km s^-1 and at high-z up to z ~ 5, sometimes extending over distances of 60-130 kpc.

Galactic-scale outflows may be a primary driver of galaxy evolution through the removal of cool gas from star-forming regions to a galaxy’s → halo or beyond.

See also: → galactic; → scale; → outflow.

Of or relative to the center of a galaxy.

See also: From galacto-, combining form of → galaxy

• centric, adj. of rarr; center.

The distance from the center of a galaxy.

See also: → galactocentric; → distance.

1. Generally, a large body of → gas, → dust, and → stars held together by their mutual → gravitational attraction and ranging in mass from about 10⁶ to 10¹³ Msun. If a galaxy also contains
   → dark matter its mass will be much larger.
   Galaxies are grouped into three main categories: → spiral galaxy, → elliptical galaxy, and → irregular galaxy
   (→ Hubble classification).
   

2. With capital G, the galaxy to which our Sun belongs; → Milky Way galaxy.
   

See also:
→ active galaxy, → Andromeda galaxy, → barred spiral galaxy, → biased galaxy formation, → binary galaxy, → blue compact dwarf galaxy, → broad-line radio galaxy, → bulge of a galaxy, → Cartwheel Galaxy, → compact galaxy,

→ core-halo galaxy, → disk galaxy, → dwarf elliptical galaxy, → dwarf galaxy, → dwarf irregular galaxy, → dwarf spheroidal galaxy, → early-type galaxy, → edge-on galaxy,

→ face-on galaxy, → field galaxy, → flocculent spiral galaxy, → galaxy bimodality, → galaxy cluster, → galaxy formation, → galaxy harassment, → galaxy main sequence, → gas-poor galaxy, → gas-rich galaxy, → grand design spiral galaxy, → green pea galaxy, → halo of galaxy, → halo of the Galaxy, → Haro galaxy, → host galaxy, → hypergalaxy, → infrared galaxy, → Irr I galaxy, → Irr II galaxy, → isolated galaxy, → late-type galaxy, → lensing galaxy, → lenticular galaxy, → low surface brightness galaxy, → luminous infrared galaxy, → Lyman break galaxy, → Markarian galaxy, → metagalaxy, → metal-deficient galaxy, → metal-poor galaxy, → parent galaxy, → passive galaxy, → passively evolving galaxy, → peculiar galaxy, → primordial galaxy, → progenitor galaxy, → protogalaxy, → radio galaxy, → receding galaxy, → retired galaxy, → ring galaxy, → Sagittarius Dwarf Elliptical Galaxy, → Sagittarius Dwarf Irregular Galaxy, → satellite galaxy, → Sculptor Dwarf Elliptical Galaxy, → Seyfert galaxy, → shell galaxy, → Sombrero galaxy, → starburst galaxy, → strong arm spiral galaxy, → submillimeter galaxy, → superthin galaxy, → superwind galaxy, → tidal dwarf galaxy, → Triangulum galaxy, → ultraluminous infrared galaxy, → violent galaxy, → weak arm spiral galaxy, → Whirlpool galaxy, → Wolf-Rayet galaxy.

Etymology (EN): From L.L. galaxias “Milky Way,” from Gk. galaxis (adj.), from gala (genitive galaktos) “milk.”

In Gk. mythology, Jupiter, hoping to immortalize his infant son Hercules (who was born to a mortal woman), placed the baby on Hera’s breast. Her milk spilled up, forming the Milky Way. A painting by Italian artist Jacopo Tintoretto (c. 1518-1594), called “The Origin of the Milky Way,” depicts the legend describing how the Milky Way was formed.

Etymology (PE): Kahkešân, short for (râh-e) kahkešân literally “the (path of the) chaff-draggers” or “trail of chaff,” from kah, kâh “chaff, straw, hay” (Mid.Pers. kâh “chaff, straw;” cf. Pali kattha- “a piece of wood;” Skt. kastha- “stick;” Gk. klados “twig;”
O.Ir. caill “wood;” Ger. Holz “wood;” E. holt; PIE *kldo-)

• kešân pr.p. of kešidan/kašidan “to carry, draw, protract, trail, drag” (Mid.Pers. kešidan “to draw, pull;” Av. karš- “to draw; to plow,” karša- “furrow;” cf. Skt. kars-, kársati “to pull, drag, plow;”
  Gk. pelo, pelomai “to move, to bustle;” PIE base k^wels- “to plow”). The term (râh-e) kahkešân may be a popular corruption of Mid. Pers. (râh-i) Kâwôsân “the path of Kâwos” referring to the Persian mythological king Kay Kâwôs, who built an eagle-propelled throne to fly to China, as recounted in the Dênkard and the Shâhnâmé.

The division of galaxies into a “red sequence” and a “blue sequence” in the → color-magnitude diagrams of galaxies involving large statistical surveys. In both sequences, redder galaxies tend to be brighter. The blue sequence is truncated at the red magnitude ~ -22, while the red sequence extends to brighter magnitudes. The division between the two classes of galaxies is associated with a critical stellar mass ~ 3 × 10¹⁰ Msun. Galaxies below the critical mass are typically blue, star forming spirals and reside in the field. Galaxies above the critical mass are dominated by red spheroids of old stars and live in dense environments (Kauffmann et al, 2003, MNRAS 341, 33 & 54).

See also: → galaxy; → bimodality.

A → spheroidal region at the center of a → spiral galaxy which mostly contains → old stars.
Galactic bulges are generally classified into two types: → classical bulges and → pseudo-bulges.

See also: → galaxy; → bulge.

An aggregation of galaxies, made up of a few to a few thousand members, which may or may not be held together by its own gravity. Same as → cluster of galaxies.

See also: → galaxy; → cluster.

The study dealing with the processes that gave rise to galaxies in a remarkably → early Universe. See also
→ structure formation, → protogalaxy

See also: → galaxy; → formation.

Frequent, high speed galaxy → encounters within → galaxy clusters. Harassment can disturb the morphologies of the galaxies involved, often inducing a new
→ burst of star formation. Asymmetrical galaxies,
→ warps, → bars, and → tidal tails can all be produced through galaxy harassment.

See also: → galaxy; → harassment.

The dominant member of the → Virgo cluster of galaxies, which contains some 2,000 galaxies. Also known as NGC 4486, it has an → apparent visual magnitude 9.6. Discovered in 1781 by Charles Messier, this → elliptical galaxy is located 55 million → light-years away from Earth in the constellation → Virgo.

M87 is the home of several thousand billion stars, a → supermassive black hole (SMBH) and a family of roughly 15,000 → globular clusters. For comparison, our → Milky Way galaxy contains only a few hundred billion stars and about 150 globular clusters.

M87 is characterized by a prominent kiloparsec scale → relativistic jet emitted by the central SMBH. As gaseous material from the center of the galaxy → accretes onto the black hole, the energy released produces a stream of subatomic particles that are accelerated to velocities near the → speed of light.

See also: → galaxy; → Messier catalog.

A scaling relation between the → star formation rate (SFR) in galaxies and the total stellar mass (M) of the galaxies. This relation, colloquially called the “galaxy main sequence,” extends over several orders of magnitudes in M and out to → high redshifts, with a modest scatter of ~ 0.3 dex which includes both intrinsic scatter and measurement uncertainties. The existence of such tight scatter at all observed epochs suggests
that most galaxies assembled their stellar mass fairly steadily rather than predominantly in → starburst episodes, implying that → mergers have a sub-dominant contribution to the global star formation history (Wuyts et al., 2011 ApJ 742, 96).

See also: → galaxy; → main; → sequence.

An unusually strong wind.

Etymology (EN): Gale, from gaile “wind,” origin uncertain, perhaps from O.N. gol “breeze,” or O.Dan. gal “bad, furious.”

Etymology (PE): Tondbâb “gale,” from tond “swift, rapid, brisk; fierce, severe,” Mid.Pers. tund “sharp, violent;” Sogdian tund “violent;” cf. Skt. tod- “to thrust, give a push,” tudáti “he thrusts;” L. tundere “to thrust, to hit” (Fr. percer, E. pierce, ultimately from L. pertusus, from p.p. of pertundere “to thrust or bore through,” from per- + tundere, as explained); PIE base *(s)teud- “to thrust, to beat” + bâd, → wind.

Of or pertaining to Galileo Galilei (1564-1642), Italian physicist and astronomer.

Same as → Galilean relativity.

See also: → Galilean; → invariance.

Same as → Galilean satellites.

See also: → Galilean; → moon.

Same as → inertial reference frame.

See also: → Galilean; → reference; → frame.

The principle according to which the fundamental laws of physics are the same in all frames of reference moving with constant velocity with respect to one another (→ inertial reference frames). Same as → Galilean invariance and → Newtonian relativity.

See also: → Galilean transformation, → Einsteinian relativity.

See also: → Galilean; → relativity.

The four largest and brightest satellites of → Jupiter, that is: → Io (Jupiter I), → Europa, → Ganymede, and → Callisto.

See also: Galileo, who had discovered them, called them Sidera Medicæa “Medicean Stars” in honor of the Medici family. → Galilean Moons; → satellite.

The method of relating a measurement in one → reference frame to another moving with a constant velocity with respect to the first within the → Newtonian mechanics. The Galilean transformation between the coordinate systems (x,y,z,t) and (x’,y’,z’,t’) is expressed by the relations: x’ = x - vt, y’ = y, z’ = z. Galilean transformations break down at high velocities and for electromagnetic phenomena and is superseded by the → Lorentz transformations.

See also: → Galilean; → transformation.

A space mission whose main goal was to explore → Jupiter and its moons and rings.
The spacecraft was launched on October 19, 1989, arrived at Jupiter in December 1995.
It disappeared on September 21, 2003, after eight years orbiting Jupiter, when mission controllers crashed it into → Jupiter’s atmosphere.

On December 7, 1995, Galileo’s probe dived into Jupiter’s atmosphere, and measured atmospheric pressure, density, and composition, and explored the planet’s → radiation belts. Galileo had two parts: an orbiter and a descent probe that parachuted into Jupiter’s atmosphere. The orbiter sent back hundreds of pictures of the four large → Galilean satellites of Jupiter (→ Io, → Europa, → Ganymede, and → Callisto).

It made many discoveries during its eight years looping around Jupiter. It found evidence for layers of salt water below the surface on Europa, Ganymede, and Callisto, and measured high levels of volcanic activity on Io. When → Shoemaker-Levy slammed into Jupiter in 1994, Galileo had the only direct view of the → comet striking Jupiter’s atmosphere. Galileo determined that → Jupiter’s rings are formed from dust hurled up by → meteorite impacts on planet’s inner moons. Measurements by the orbiter’s → magnetometer revealed that Io, Europa, and Ganymede have metallic cores, while Callisto does not. Also, Galileo discovered that Ganymede possesses its own → magnetic field; it is the first moon known to do so. The orbiter also found that the Galilean satellites all have thin atmospheres.

During it’s trip from Earth to Jupiter, Galileo passed by and studied two asteroids: → Gaspra in 1991 and → Ida in 1993, around which it discovered → Dactyl, the first moon orbiting an asteroid (windows2universe.org).

See also: → Galileo; → mission.

In the absence of air resistance, any two bodies that are dropped from rest at the same moment will reach the ground at the same time regardless of their mass.

See also: Galileo (1564-1642) was the first to determine, at the start of the seventeenth century, the law of constant acceleration of free-falling bodies. → law; → fall;
→ body.

An electrolytic cell capable of producing electric energy by electrochemical reaction.

See also: → galvanism; → cell.

A pair of dissimilar conductors, commonly metals, in electrical contact.

See also: → galvanism; → couple.

The direct electric current that flows between metals or conductive nonmetals in a → galvanic couple.

See also: → galvanism; → current.

1. The production of electricity from a chemical reaction.
   

2. The therapeutic application of electricity to the human body.

See also: From Fr. galvanisme, after Luigi Galvani (1737-1798), the Italian physiologist, who demonstrated (1790) muscular action due to contact with dissimilar metals.

The coating of steel or iron with → zinc, either by immersion in a bath of molten zinc or by electrolytic deposition from a solution of zinc sulfate, to give protection against corrosion.

See also: Verbal noun of → galvanize.

1. To coat a metal with → zinc by dipping into molten zinc or by electrolytic deposition.
   

2. To stimulate by application of an electric current.

Etymology (EN): From Fr. galvaniser, from galvanisme, → galvanism.

Etymology (PE): Gâlvânidan, from Gâlvâni, → galvanism,

• -idan, → -ize.

A prefix denoting galvanic or galvanism in compound words, such as → galvanometer, → galvanoplasty.

See also: Galvano-, from → galvanism.

An instrument for measuring or detecting small → direct currents, usually by the mechanical reaction between the magnetic field of the current and that of a magnet.

See also: → galvano- + → -meter.

A process used for covering an object with a thin layer of metal by electrochemical means.

Etymology (EN): → galvano- + -plasty a suffix meaning “molding, formation, surgical repair, plastic surgery,” from Gk. -plastia, from plastos “molded, formed,” from plassein “to mold.”

Etymology (PE): Gâlvânopuši, from gâlvâno-, → galvano-, + puši “covering, coating,” from pušidan “to cover; to put on” (Mid.Pers. pôšidan, pôš- “to cover; to wear;” cf. Mid.Pers. pôst; Mod.Pers. pust “skin, hide;” O.Pers. pavastā- “thin clay envelope used to protect unbaked clay tablets;” Skt. pavásta- “cover,” Proto-Indo-Iranian *pauastā- “cloth”).

1. An amusement or pastime.
   

2. The material or equipment used in playing certain games.
   

3. A competitive activity involving skill, chance, or endurance on the part of two or more persons who play according to a set of rules, usually for their own amusement or for that of spectators (Dictionary.com).

Etymology (EN): M.E. gamen. O.E. gaman “game, joy, fun, amusement;” cf. O.Fris. game “joy, glee,” O.N. gaman, O.H.G. gaman “sport, merriment,” D. gamen, Sw. gamman.

Etymology (PE): Bâzi, from Mid.Pers. wâzig “game, play,” related to bâzidan “to play,” bâxtan/bâz- “to loose (in game);” Proto-Ir. *uāz- “to play, contend;” cf. Skt. vāja- “contest, war, gain, reward” (Cheung 2007).

1. The third letter of the Greek alphabet (γ, Γ).
   

2. Symbol used to denote the ratio of the principal specific heats C_P/C_V of a gas, where C_P is the specific heat at constant pressure and C_V that measured at constant volume.
   

3. Unit of magnetic field intensity, equal to 10^-5 gauss.
   

4. The distance of the Moon’s shadow axis from Earth’s center in units of equatorial Earth radii. It is defined at the instant of → greatest eclipse when its absolute value is at a minimum (F. Espenak, NASA).

See also: The third letter of the Gk. alphabet, from Gk. gamma, from Phoenician gimel.

A bright, third → magnitude (3.22) → giant star of → spectral type K1, also called → Errai, HR 8974, HIP 116727, and HD 222404. γCephei has a → surface temperature of 4920 K a mass of 1.40 Msun, a → luminosity 10.6 solar, and a radius 4.8 solar. Its distance is estimated to be 45 → light-years. γ Cephei will become the → Pole Star in about 2,000 years. γ Cephei has a low mass → companion (B), a main → main sequence star of spectral type M4 V with a mass of 0.4 Msun. It orbits the → primary star every 67.5 years. An → extrasolar planet. (γ Cephe b) has been discovered orbiting the main star.

See also: Gamma, as in → Bayer designation; → Cepheus.

The star → Sadr.

See also: Gk. letter → gamma; Cygni, genitive of → Cygnus.

A type of → radioactivity in which some unstable atomic nuclei dissipate excess energy by a spontaneous electromagnetic process, usually accompanied by → alpha decay or → beta decay.

See also: → gamma; → decay.

A process which reinforces the → kappa mechanism in a → partial ionization zone. Because the temperature in the partial ionization zone is lower than in the adjacent stellar layers, heat tends to flow into the zone during compression, prompting further ionization.

See also: γ, after the smaller ratio of → specific heats caused by the increased values of C_p and C_v; → mechanism.

An → electromagnetic wave with a typical → wavelength less than 10^-2Å (10^-12 m), corresponding to frequencies above 10¹⁹ Hz and photon energies above 100 → keV.

See also: → gamma; → ray.

The study of → gamma rays from → extraterrestrial → sources, especially → gamma-ray bursts.

See also: → gamma ray; → astronomy.

An intense discharge of → gamma rays,
which range in duration from tenth of a second to tens of seconds and occur from sources widely distributed over the sky. The radio wave → afterglow from the → burst can last more than a year, making long-term observations of the sources possible.

The favored hypothesis is that they are produced by a relativistic jet created by the merger of two → compact objects (specifically two → neutron stars or a neutron star and a → black hole). Mergers of this kind are also expected to create significant quantities of neutron-rich radioactive species, whose decay should result in a faint → transient, known as a → kilonova, in the days following the burst. Indeed, it is speculated that this mechanism may be the predominant source of stable → r-process elements in the Universe. Recent calculations suggest that much of the kilonova energy should appear in the → near-infrared spectral range, because of the high optical opacity created by these heavy r-process elements (Tanvir et al., 2017, Nature 500, 547).

See also: → gamma rays; → burst.

The → object or → phenomenon at the origin of a → gamma-ray burst.

See also: → gamma ray; → burster.

1. An astronomical object that emits → gamma rays.
   
2. A radioactive material that emits gamma rays in a form that can be used in medical imaging.

See also: → gamma ray; → source.

The closest → Wolf-Rayet star, located at 336 → parsecs. Also known as HR 3207, HD 68273, and WR 111. γ² Velorum is composed of a → WC8 component in a → close binary system with an → O star in a 78.5 day orbit (see, e.g., Lamberts et al., 2017, arXiv: 1701.01124).

See also: Gamma, as in → Bayer designation; Velorum, genitive of → Vela.

In nuclear physics, a potential barrier near the surface of the nucleus that inhibits the release of alpha particles.

See also: 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).

See also: → 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.

See also: → 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.

See also: 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.

Etymology (EN): Gap, from O.N. gap “chasm,” related to gapa “to gape.”

Etymology (PE): 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.

Etymology (EN): 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.”

Etymology (PE): 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.

Etymology (EN): 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.

Etymology (PE): 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).

See also: → 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.

See also: → 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.

See also: → 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.

See also: → 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.

See also: → 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.

See also: → 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.

See also: → 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.

See also: → gas; → poor; → galaxy.

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

See also: → 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.

See also: → gas; → dust; → ratio.

1. Existing in the → state of a gas.
   

2. Pertaining to or having the characteristics of gas.

See also: → 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).

See also: → gaseous; → diffusion.

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

See also: → gaseous; → nebula.

1. (n.) A standard of measure or measurement, size, or quantity.
   

2. Any of a wide variety of devices or instruments used for measuring a parameter or characteristic of an object, such as its dimension, quantity, or mechanical accuracy.
   

3. Physics: One of the family of choices for a constant in the expression of → potential energy in a central force field. Force is connected with potential energy by the relation F = - ∂U/∂r, U = ∫F.dr. The upper limit in the integral can be chosen arbitrarily. The potential energy is usually considered zero at infinity.
   

4. (v.) To determine the exact dimensions, capacity, quantity, or force of; measure.

Etymology (EN): From Fr. jauge “gauging rod,” perhaps from Frankish galga “rod, pole for measuring;” cf. O.N. gelgja “pole, perch;” O.H.G. galgo; Lith. zalga “pole, perch;” Arm. dzalk “pole;” E. gallows; see below.

Etymology (PE): Gaz “a yard for measuring cloth; a length of 24 finger-breadths, or six hands; the tamarisk-tree,” from Mid.Pers. gaz “tamarisk,” may be of the same origin as gauge. In verbal form with kardan “to do, to make” (Mid.Pers. kardan; O.Pers./Av. kar- “to do, make, build;” Av. kərənaoiti “he makes;” cf. Skt. kr- “to do, to make,” krnoti “he makes, he does,” karoti “he makes, he does,” karma “act, deed;” PIE base k^wer- “to do, to make”).

A class of elementary particles that includes the gluon, photon, W⁺, W^-, and Z⁰ particles, each having an integral spin.

See also: → gauge; → boson.

The mathematical group associated with a particular set of gauge transformations.

See also: → gauge; → group.

The invariance of any field theory under gauge transformation.

See also: → gauge; → invariance.

A principle underlying the quantum-mechanical description of the three non-gravitational forces. It allows a system to behave in the same way even
though it has undergone various transformations. The earliest physical theory which had a gauge symmetry was Maxwell’s electrodynamics.

See also: → gauge; → symmetry.

A field theory in which it is possible to perform a transformation without altering any measurable physical quantity.

See also: → gauge; → theory.

A change of the fields of a gauge theory that does not change the value of any measurable quantity.

See also: → gauge; → transformation.

A technique in which the thickness, density, or quantity of a material is determined by the amount of radiation it absorbs.

Etymology (EN): Gauging, from → gauge + → -ing, suffix of nouns formed from verbs, expressing the action of the verb or its result.

Etymology (PE): Gazkard, from gaz, → gauge, + kard past stem of kardan “to do, make,” → gauge.

In the atomic theory of spectral line formation, a quantum mechanical correction factor applied to the absorption coefficient in the transition of an electron from a bound or free state to a free state.

See also: Gaunt, after John Arthur Gaunt (1904-1944), English physicist born in China, who significantly contributed to the calculation of continuous absorption using quantum mechanics;
→ factor

The c.s.g. unit of magnetic flux density (or magnetic induction), equal to
1 maxwell per square centimeter, or 10^-4 tesla.

See also: Named after the German mathematician and physicist Carl Friedrich Gauss (1777-1855).

The total electric flux ψ out of an arbitrary closed surface in free space is equal to the net charge within the surface divided by the → permittivity. In differential form: ∇ . E = ρ/ε₀, where ρ is the → charge density and ε₀ the permittivity. The integral form of the law: ∫E . dS = Q/ε₀ (closed surface integral). This is one of the four → Maxwell’s equations.

See also: → gauss; → law; → electricity.

The → magnetic flux through an arbitrary closed surface equals zero. Mathematically, in differential form: ∇ . B = 0 and in integral form: Φ_B = ∫B.dS = 0 (closed surface integral). This is one of the four → Maxwell’s equations. This law expresses the fact that there are no free magnetic poles (→ monopoles) in nature and that all the lines of force of a magnetic field are closed curves.

See also: → gauss; → law;
→ magnetism.

If a → polynomial with → integer coefficients can be → factorized into polynomials with → rational number coefficients, it can be factorized using only integers.

See also: → Gaussian; → lemma.

The total normal induction over any closed surface drawn in an electric field is equal to 4π times the total charge of electricity inside the closed surface. Gauss’s theorem applies also to other vector fields such as magnetic, gravitational, and fluid velocity fields. The theorem can more generally be stated as: the total flux of a vector field through a closed surface is equal to the volume → integral of the vector taken over the enclosed volume. Also known as → divergence theorem, Ostrogradsky’s theorem,
and Gauss-Ostrogradsky theorem.

See also: → gauss; → theorem.

Of or relating to Carl Friedrich Gauss or his mathematical theories of magnetism, electricity, astronomy, or probability. → Gaussian distribution; → Gaussian profile.

See also: → gauss.

A theoretical frequency distribution for a set of variable data, usually represented by a bell-shaped curve with a
mean at the center of the curve and tail widths proportional to the standard deviation of the data about the mean.

See also: → Gaussian; → distribution.

A method of solving a matrix equation of the form A x = b, where A is a matrix and x and b are vectors. The process consists of two steps, first reducing the elements below the diagonal to 0 and second, back substituting to find the solutions.

See also: → Gaussian; → elimination.

The function e^-x2, whose integral in the interval -∞ to +∞ gives the → square root of the → number pi: ∫e^-x2dx = √π. It is the function that describes the → normal distribution.

See also: → Gaussian; → function.

The constant, denoted k, defining the astronomical system of units of length (→ astronomical unit), mass (→ solar mass), and time (→ day), by means of → Kepler’s third law. The dimensions of k² are those of Newton’s constant of gravitation: L ³M ^-1T ^-2. Its value is: k = 0.01720209895.

See also: → Gaussian; → gravitational; → constant.

A → complex number whose → real and → imaginary parts are both integers.

See also: → Gaussian; → integer.

The shape of a curve representing a normal distribution.

See also: → Gaussian; → profile.

Math.: The condition of having → Gaussian distribution. The extent to which something is Gaussian.

See also: → Gaussian + → -ity.

1. Law of combining volumes. The volumes of gases used and produced in a chemical reaction, are in the ratio of small whole numbers when measured at constant temperature and pressure.
   

2. For a gas held at constant volume, there is a direct correlation between temperature and pressure: P₁/T₁ = P₂/T₂. Gay-Lussac’s law, → Boyle-Mariotte law, and → Charles’ law were later unified into the → ideal gas law.

See also: Named after Joseph Louis Gay-Lussac (1778-1850), a French chemist and physicist; → law.