Glossary -- G

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galactic cannibalism

the swallowing up whole of a small galaxy by a large galaxy (usually a large elliptical galaxy at the center of a galaxy cluster).

galaxy

a very large cluster of stars (tens of millions to trillions of stars) gravitationally bound together.

General Relativity

a theory invented by Albert Einstein to describe gravity. It says that gravity is a warping or distortion of space-time around a massive object. Although it applies everywhere in the universe, General Relativity must be used instead of Newton's law of gravity in regions of strong gravity.

geocentric

(universe): model of the universe with the Earth at the center and all other objects moving around it.

giant molecular cloud

large, dense gas cloud (with some dust) that is cold enough for molecules to form. A typical giant molecular cloud has a few hundred thousand to a few million solar masses of material. Stars form in them.

globular cluster

spherical cluster of hundreds of thousands to millions of very old stars. The orbits of most globular clusters are very elliptical and oriented in random directions.

granulation

bright spots of convection on the Sun's surface 700 to 1000 kilometers across forming a honeycomb pattern. Formed from hot, bright gas rising from below in the center of a granule and cooler, dimmer gas falling back down at the edge of a granule.

gravitational lens

the focussing of light from a distant object by the warped space-time around a massive body (such as a galaxy) between you and the distant object as predicted by General Relativity.

gravitational redshift

the lengthening of the wavelength of electromagnetic radiation as it moves away from a region of intense gravity.

gravity

a fundamental force of nature between two objects that is proportional to the product of their masses and inversely proportional to the square of the distance between their respective centers. It depends on nothing else.

greenhouse effect

the trapping of heat energy close to a planet's surface by certain types of gases in the atmosphere (e.g., water, methane, and carbon dioxide). These gases allow visible light from the Sun to reach the surface but prevent the infrared light from the heated surface to radiate back to space.

ground state

the lowest energy state of an atom---all of the electrons are as close to the nucleus as possible.

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last update: 06 August 1999