A model universe with Earth at the center, such as the Ptolemaic universe.
giant star
Large, cool, highly luminous star in the upper right of the H-R diagramm. Typically 10-100 times the diameter of the Sun.
globular star cluster
A star cluster containing 100,000 - 1 million stars in a sphere about 75 light years in diameter. Generally old, metal-poor, and found in the spherical component of the galaxy.
grating
A piece of material in which numerous microscopic parallel lines are scribed. Light encountering a grating is dispersed to form a spectrum.
gravitational lens
The focusing of light from a distant object by an intervening massive object to change the apparent brightness or produce multiple images of the distant object.