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galactic cannibalism The process by which a larger galaxy strips material from a smaller one. 
galactic cluster  An "open" cluster of stars located in the spiral arms or disk of the Galaxy. 
galaxy A large assemblage of stars; a typical galaxy contains millions to hundreds of billions of stars.
Galaxy The galaxy to which the Sun and our neighboring stars belong; the Milky Way is light from remote stars in the disk of the Galaxy. 
gamma rays  Photons (of electromagnetic radiation) of energy higher than those of x rays; the most energetic form of electromagnetic radiation. 
general relativity theory  Einstein's theory relating acceleration, gravity, and the structure (geometry) of space and time. 
geocentric Centered on the Earth. 
giant (star)  A star of large luminosity and radius. 
giant molecular cloud  Large, cold interstellar clouds, with diameters of dozens of light years and typical masses of 105 solar masses; found in the spiral arms of galaxies, these clouds are where massive stars form. 
globular cluster  One of about 120 large spherical star clusters that form a system of clusters centered on the center of the Galaxy. 
grand unified theories (GUTS)  Physical theories that attempt to describe the four interactions (forces) of nature as different manifestations of a single force. 
granulation  The rice-grain-like structure of the solar photosphere; granulation is produced by upwelling currents of gas that are slightly hotter, and therefore brighter, than the surrounding regions, which are flowing downward into the Sun. 
gravity The mutual attraction of material bodies or particles. 
gravitational energy  Energy that can be released by the gravitational collapse, or partial collapse, of a system; i.e., by particles that fall in toward the center of gravity. 
gravitational lens  A configuration of celestial objects, one of which provides one or more images of the other by gravitationally deflecting its light. 
gravitational redshift  The redshift of electromagnetic radiation caused by a gravitational field. The slowing of clocks in a gravitational field. 
great circle  Circle on the surface of a sphere that is the curve of intersection of the sphere with a plane passing through its center. 
greenhouse effect  The blanketing (absorption) of infrared radiation near the surface of a planet by, for example, carbon dioxide in its atmosphere. 
ground state  The lowest energy state of an atom.
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