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| Thousands of small planetlike bodies, ranging in size from a few hundred kilometers to less than a kilometer, whose orbits lie mainly between those of mars and Jupiter. |
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| A region roughly midway between the orbits of Mars and Jupiter where most asteroids lie. |
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| The fuzzy, gaseous component of a comet's head. |
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| A small body that generally revolves about the Sun in an elongated orbit. |
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| The eruption of magma's derived from the partial melting of ice rather than silicate rocks. |
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| Celestial bodies that orbit around the Sun, are essentially round due to their self-gravity, but are not the only objects to occupy their area of space. |
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| The intial velocity an object needs to escape from the surface of a celestrial body. |
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| Areas that are elevated several kilometers above the maria. |
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| Produced by the impact of rapidly moving debris (meteroids, asteroids, and comets), a phenomenon that was considerably more common in the early history of the solar system than it is today. |
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| Inner Planet (Terrestrial Planets) |
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| Any of the Earth-like planets, including Mercury, Venus, Mars & Earth. |
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| The Jupiter-like planets: Jupiter, Saturn, Uranus, and Neptune. These planets have relatively low densities. |
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| A region outside the orbit of Neptune where most short-period comets are thought to originate. |
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| A thin, gray layer in the surface of the moon, consisting of loosely compacted, fragmented material believed to have been formed by reported meteoritic impacts. |
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| The Latin name for the smooth areas of the moon formely thought to be seas. |
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| The luminous phenomenon observed when a meteroid enters Earth's atmosphere and burns up; popularly called a "shooting star." |
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| Any portion of a meteorid that survives its traverse through Earth's atmosphere and strikes Earth's surface. |
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| Small solid particles that have orbits in the solar system. |
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| Many meteors appearing in the sky caused when Earth intercepts a swarm of meteoritic particles. |
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| A spherical shell composed of comets that orbit the Sun at distances generally greater than 10,000 times the Earth-Sun distance. |
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| Outer Planet (Jovian Planets) |
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| The Jupiter-like planets: Jupiter, Saturn, Uranus, and Neptune. These planets have relatively low densities. |
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| Asteroid-sized objects from matter gradually formed clumps of material that collided, struck together and grew. |
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| Grew through repeated collisions and accretion (sticking together) from the clumps of matter that the planetesimals formed. |
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| A large rotating cloud of interstellar dust & gas from which the Sun and Planets formed at the same time from this. |
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| The extensively cratered highlands areas of the Moon. |
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| Any of the Earth-like planets, including Mercury, Venus, Mars, and Earth. |
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| The apparent brightness of a star if it were viewed from a distance of 10 parsecs (32.6 light-years). Used to compare the true brightness of stars. |
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| The brightness of a star when viewed from Earth. |
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| A galaxy having straight arms extending from its nucleus. |
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| The theory that proposes that the universe originated as a single mass, which subsequently exploded. |
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| A massive star that has collapsed to such a small volume that its gravity prevents the escape of all radiation. |
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| A cloud of glowing gas excited by unltraviolet rediation from hot stars. |
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| A cloud of interstellar dust that obscures the light of more distant stars and appears as an opaque curtain. |
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| Material in the state where electrons are displaced inward from their regular orbits around an atom's nucleus. |
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| A gelaxy that is round or elliptical in outline. It contains little gas and dust, no disk or spiral arms, and few hot, bright stars. |
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| A gaseous nebula that derives its visible light from the flourescence of ultraviolet light from a star in or near the nebula. |
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| A star that varies in brightness. |
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| A system of galaxies containing from several to thousands of member galaxies. |
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| Hertzsprung-Russell Diagram (H-R Diagram) |
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| A plot of stars according to their absolute magnitudes & spectral types. |
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| Relate's the distance to a galaxy & its velocity. |
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| The conversion of hydrogen through fusion to form helium. |
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| Relatively dense clouds of large particles; composes reflection nebulae. |
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| A galaxy that lacks symmetry. |
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| The distance light travels in a year; about 6 million miles. |
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| The cluster of 20 or so galaxies to which our galaxy belongs. |
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| A number given to a celestial object to express its relative brightness. |
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| A sequence of stars on the Hertzsprung-Russell diagram, containing the majority of stars, that runs diagonally from the upper left to the lower right. |
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| A cloud of interstellar gas and/or dust. |
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| A star of extremely high density composed entirely of neutrons. |
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| A star that explosively increases in brightness. |
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| A shell of incandescent gas expanding from a star. |
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| A collapsing cloud of gas and dust destined to become a star. |
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| A variable ratio source of small size that emits radio pulses in very regular periods. |
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| A variable star that pulsates in size & luminosity. |
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| A large, cool star of high luminosity; a star occuping the upper-right portion of the Hertzsprung-Russell diagram. |
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| A relatively dense dust cloud in interstellar space that is illuminated by starlight. |
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| A flattened, rotating galaxy with pinwheel-like arms of interstellar material and young stars winding out from its nucleus. |
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| A meausure of stellar distance. |
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| A very large star of high luminosity. |
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| An exploding star that increases in brightness many thousands of times. |
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| A star that has exhusted most or all of its nuclear fuel and has collapsed to a very small size; believed to be near its final stage. |
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