232 gloSSary electron: A negatively charged elementary particle, which is usually in orbit around the nucleus of an atom. fusion: The joining together of light atomic nuclei to form a heavier nucleus, accompanied by the release of energy (and possibly other particles). Fusion powers most stars. galaxy: A large collection of stars, typically hundreds of mil- lions to hundreds of billions, as well as dust and gas held together by gravity. Also see MilkyWay. gas giant planet: A massive planet, such as Jupiter, com- posed primarily of gaseous material, thus lacking a solid sur- face (though it may have a solid core). greenhouse effect: The warming of a planet by certain gases in the atmosphere that permit visible light from the parent star to reach the surface but prevent the planet surface from re-radiating some of the heat back into space at longer, in- frared wavelengths. Carbon dioxide, methane, and water vapor are among the important greenhouse gases. Without the greenhouse effect, the Earth would be too cold for liq- uid water, but a runaway greenhouse effect has turned Venus into an inferno. Habitable zone: Usually defined as the region around a star where the temperatures are in the correct range for water to exist in liquid form. The assumption is that life as we know it requires liquid water. Heavy elements: See Metals. Helium: The second lightest and second most common ele- ment in the universe, with two protons in its nucleus. Stars fuse hydrogen into helium; helium itself fuses into carbon and oxygen. Hot Jupiter: A gas giant planet located very close to its star, taking only a few days to circle it, thus is heated to high temperatures.