ASTR 103 - Astronomy

Glossary - G

g

(see also acceleration of gravity).

G

see gravitational constant.

Galactic cluster

Open star cluster in the Galaxy.

Galactic equator

Great circle in the sky passing through the central plane of the Milky Way.

galactic halo

Outer, nonflattened stellar portions of a galaxy. In our Galaxy, an approximately spheroidal distribution of stars.

Galactic latitude

Number of degrees of an object above or below the Galactic equator.

Galactic longitude

number of degrees of an object measured along the Galactic equator northward from the direction of the Galactic center.

Galactic year (Gy)

 

galaxy

a large system of stars, dust, and gas held together by the mutual gravitational attraction of its members.

Galaxy

 

Galilean satellite

four natural satellites of Jupiter discovered by Galileo: lo, Europa, Ganymede, Callisto.

gamma rays

highest-energy photons of radiation possessing the shortest wavelengths (less than about 0.1 angstrom) and energies greater than 100,000 electron volts.

gamma-ray astronomy

 

gas

that state of matter in which the constituent particles maintain no permanent relationship to each other.

gaseous nebula

a diffuse collection of gaseous material emitting radiation and lying in between the stars of a galaxy.

gauss (G)

scientific unit of magnetic field strength.

gene

unit in the chromosome that determines a unique set of inheritable characteristics.

general relativity

Einstein's theory of the nature of the gravitational force in the space-time geometry of four dimensions.

giant star

large star of higher than average luminosity.

gibbous

phase of the Moon between first quarter and full, or between full and last quarter.

globular cluster

 

globule

compact spheroidal assemblage of tens of thousands of stars found in the halo portion of a galaxy.rains: see interstellar dust.

granule (solar)

small, roundish patch of dark nebulosity that may be the precursor of a protostar.

grating

optical surface (transmissive or reflective) upon which is ruled a large number of finely spaced grooves. A beam of light impinging upon it is broken into several spectral orders on each side of the central image.

gravitational constant (G)

 

gravitational collapse

rapid contraction of a body whose gravitational force greatly exceeds its normal outward balancing, gas-pressure force.

gravitational deflection of light

slight bending in the light path experienced by a light ray skimming past the limb of a massive body.

gravitational force

the field of force exerted by all particles of matter.

gravitational potential

stored energy that is convertible to heat energy when a gaseous body such as a star contracts gravitationally.

gravitational radiation

weak oscillations, traveling with the speed of light, emitted by a highly accelerated body, as predicted by the theory of relativity. Also known as gravity waves.

gravitational redshift

redshift in wavelength experienced by a photon leaving the surface of a massive object, as predicted by the theory of relativity.

gravity

force due to the mutual attraction between two masses directed along the line of their centers and having a magnitude inversely proportional to the square of the distance between their centers.

greatest elongation

 

greenhouse effect

the heating of an atmosphere by absorption of infrared radiation trying to escape out through the atmosphere.

ground state

the lowest energy orbit of the electron in the atom.

G stars

Stars of spectral type G are yellowish stars with surface temperature of about 5000 to 6000 K in whose spectra the H and K lines of Ca II are dominant.