An Etymological Dictionary of Astronomy and Astrophysics
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A rule base in a → fuzzy logic system constructed to control the → output variable. A fuzzy rule is a simple if-then rule with a condition and a conclusion.

→ fuzzy; → rule; → base.

A set of → truth values in → fuzzy logic that does not have sharp boundaries. Instead, set members have degrees of membership. If the value of 1 is assigned to objects entirely within the set and a 0 is assigned to objects outside of the set, then any object partially in the set will have a value between 0 and 1. This contrast with → crisp sets in → classical logic where members assume a precise value of 1 or 0. Fuzzy sets were first introduced by Lotfi A. Zadeh (1965) and defined as follows. Let X be a space of points, with a generic element of X denoted by x. Thus X = {x}. A fuzzy set A in X is characterized by a → membership function  f_A(x) which associates with each point in X a real number in the interval [0,1], with the values of f_A(x) at x representing the "grade of membership" of x in A. Thus, the nearer the value of f_A(x) to unity, the higher the grade of membership of x in A. Generally, the intersection operations of fuzzy sets are the expansion of that operation on → nonfuzzy sets. In other words, operations on nonfuzzy sets are a particular case of operations on fuzzy sets.

→ fuzzy; → set.

A conspicuous band of molecular → CH (methylidine) at 4300 Å, which is present in the spectra of late-type G-K stars.

G refers to → G type stars in the spectra of which this feature is strong. → band.

Waves trapped inside stars, whose restoring force is the → buoyancy. Same as → gravity mode. See also: → oscillation modes; → p mode; → f mode.

g referring to gravity; → mode.

The → Saturn's ring, with a width of 8,000 km, lying before the → F ring, at 164,000-172,000 km from the center of Saturn.

→ ring.

A member of a class of stars to which the Sun belongs. The G-type stars on the → main sequence have → surface temperatures of 5,300-6,000 K and therefore appear yellow in color. G type → giant stars (such as → Capella) are almost 100-500 K colder than the corresponding main sequence stars. G type → supergiants have temperatures of 4,500-5,500 K. The spectrum of early type G stars, such as the Sun (G2), is dominated by ionized lines of calcium (→ H and K lines, mainly) and neutral metals. In later type G stars the molecular bands of → CH molecules and → CN molecules become visible. The main sequence and giant stars have masses of ~ 1 solar mass, while the supergiants are of ~ 10 solar masses. The luminosities of G-type giants are almost 30-60 times greater than that of the Sun, whereas the supergiants are 10,000-30,000 times more luminous.

G, from the → Harvard classification; → star.

A yellowish star whose surface temperature is about 6000 K and its spectrum is dominated by H and K lines of ionized calcium (Ca II 3968 Å and 3934 Å).

G from the alphabetical sequence of spectral types; → type; → star.

A relatively uncommon → carbonaceous carbonaceous asteroid whose spectrum contains a strong → ultraviolet  → absorption feature below 0.5 μm (→ Tholen classification). In the → SMASS classification it corresponds to the Cg and Chg types , depending on the presence or absence (respectively) of the absorption feature at 0.7 μm. The most remarkable "asteroid" in this type is → Ceres (now classified as a → dwarf planet).

→ type; → asteroid.

Same as → G star.

→ G star; → type.

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.

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.

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."

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.

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'.

→ 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).

→ galactic; → bar.

The central → galaxy bulge of the → Milky Way.

→ 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.

→ 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.

→ galactic; → center; → cluster.

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

→ 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.

→ 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.

→ galactic; → disk.