| 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. |