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Glossary of Terms for Earth Science Chapter 1 Activity 1 (Book 2)

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astronomical unit
a unit of measurement equal to the average distance between the Sun and Earth
light-year
a unit of measurement equal to the distance light travels in one year
parsec
a unit used in astronomy to describe large distance, such as the widths of galaxies
nebula
a cloud of gas and dust that was probably cast off from other stars that once were in our region of the galaxy
Nebular Theory
the idea that the solar system evolved from such a swirling cloud of dust (nebula)
constellation
a grouping of stars in the night sky that fall into a recognizable pattern
planetesimal
a small body formed from the solar nebula and eventually grew into protoplanetary bodies
protoplanetary body
a clump of material fromed from planetetesimals of the early stages of solar system formation that were the forerunner of today's planets
nuclear fusion
a nuclear proccess that releases energy when lightweight nuclei combine to form heavier nuclei during which hydrogen atoms fused to create helium atoms
terrestrial planets
[Mercury, Venus, Earth, and Mars] planets formed in the inner part of the solar system, consisting of mainly rocky materials
gas giant planets
[Jupiter, Saturn, Uranus, and Neptune] planets formed in the outer part of the solar system, composed mostly of hydrogen, helium, and methane, and having a density less than 2 gm/cm^2
comet
a chunk of frozen gases, ice, and rocky debris that orbits the sun found mainly in the outer solar system
asteroid
a small planetary body in orbit around the Sun that is larger than a meteoroid but smaller than a planet that are commonly found between the orbits of Mars and Jupiter
Milky Way
our gallaxy: gallaxies are classified according to their shape; elliptical, spiral, or irregular
AGES
Solar System - 4.5 billion years
Milky Way Galaxy - 10 billion years
Universe - 12 to 14 billion years

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