galactic cannibalism A galaxy merger in which a larger galaxy consumes a smaller one.
galactic center The center of the Milky Way, or any other, galaxy. The point about which the disk of a spiral galaxy rotates.
galactic disk Flattened region of gas and dust that bisects the galactic halo in a spiral galaxy. This is the region of active star formation.
galactic halo Region of a galaxy extending far above and below the galactic disk, where globular clusters and other old stars reside.
galactic nucleus Small central high-density region of a galaxy. Nearly all the radiation from an active galaxy is emitted from the nucleus.
galaxy Gravitationally bound collection of a large number of stars. The Sun is a star in the Milky Way Galaxy.
galaxy cluster See cluster of galaxies.
gamma ray Region of the electromagnetic spectrum, beyond x-rays, corresponding to radiation of very high frequency and very short wavelength.
gamma-ray burst An outburst that radiates tremendous amounts of energy equal to or greater than a supernova, in the form of gamma rays and X-rays, in a few minutes.
Giant Molecular Cloud (GMC) Huge, cool clouds of dust grains, and gas, much of which is in the form of molecules. GMC?s appear to be where most of the stars are formed in galaxies.
giant star A star with a radius between 10 and 100 times that of the sun.
globular cluster Tightly bound, roughly spherical collection of hundreds of thousands, and sometimes millions, of stars spanning about 100 light years. Globular clusters are distributed in the halos around the Milky Way and other galaxies.
Grand Unified Theories Theories which describe the behavior of the single force that results from unification of the strong, weak, and electromagnetic forces in the early universe.
gravitational field Field created by an object with mass, extending out in all directions, which determines the influence of that object on all others. The strength of the gravitational field decreases as the square of the distance.
gravitational instability A condition whereby an object�s (inward-pulling) gravitational potential energy exceeds its (outward-pushing) thermal energy, thus causing the object to collapse.
gravitational lensing Banding of light from a distant object by a massive foreground object.
gravitational red shift A prediction of Einstein's general theory of relativity. Photons lose energy as they escape the gravitational field of a massive object. Because a photon's energy is proportional to its frequency, a photon that loses energy suffers a decrease in frequency, or redshift, in wavelength.
gravity, (gravitational force) The attractive effect that any massive object has on all other massive objects. The greater the mass of the object, the stronger is its gravitational pull.
gravitational wave The gravitational analog of an electromagnetic wave whereby gravitational radiation is emitted at the speed of light from any mass that undergoes rapid acceleration.
ground state The lowest energy state that an electron can have within an atom.