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galactic bulge Thick distribution of warm gas and stars around the galactic center.

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. [More Info: Photo Album]

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. Most galactic nuclei are thought to harbor a supermassive black hole.

galaxy A gravitationally-bound system of stars, gas, dust and dark matter. [More Info: Field Guide]

galaxy cluster Galaxies can swarm together to form groups and clusters of galaxies held together by their mutual gravity. X-ray observations show that these enormous systems of galaxies are filled with colossal clouds of hot gas. These clouds have temperatures as high as a hundred million degrees and contain as much mass as all the stars in the galaxies in the cluster. [More Info: Field Guide]

gamma ray Region of the electromagnetic spectrum, beyond x-rays, corresponding to radiation of very high frequency and very short wavelength. [More Info]

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. At least two classes of gamma-ray bursts have been identified: short-duration bursts lasting less than a few seconds, and more powerful, long-duration gamma-ray bursts that last a few minutes. Long-duration bursts may be produced by black holes formed in the explosion of extremely massive stars, or hypernovas. Short duration bursts may be related to the merger of two neutron stars, or of a neutron star and a black hole. [More Info: Field Guide]

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. [More Info]

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.

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