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Astronomy

Key Terms

AstronomyKey Terms
asteroid
a stony or metallic object orbiting the Sun that is smaller than a planet but that shows no evidence of an atmosphere or of other types of activity associated with comets
comet
a small body of icy and dusty matter that revolves about the Sun; when a comet comes near the Sun, some of its material vaporizes, forming a large head of tenuous gas and often a tail
differentiation
gravitational separation of materials of different density into layers in the interior of a planet or moon
giant planet
any of the planets Jupiter, Saturn, Uranus, and Neptune in our solar system, or planets of roughly that mass and composition in other planetary systems
half-life
time required for half of the radioactive atoms in a sample to disintegrate
meteor
a small piece of solid matter that enters Earth’s atmosphere and burns up, popularly called a shooting star because it is seen as a small flash of light
meteorite
a portion of a meteor that survives passage through an atmosphere and strikes the ground
planetesimals
objects, from tens to hundreds of kilometers in diameter, that formed in the solar nebula as an intermediate step between tiny grains and the larger planetary objects we see today; the comets and some asteroids may be leftover planetesimals
radioactivity
process by which certain kinds of atomic nuclei decay naturally, with the spontaneous emission of subatomic particles and gamma rays
solar nebula
the cloud of gas and dust from which the solar system formed
terrestrial planet
any of the planets Mercury, Venus, Earth, or Mars; sometimes the Moon is included in the list
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