Astronomy Basics

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Big Bang Theory

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The idea that the universe began as just a single point, then expanded and stretched to grow as large as it is right now—and it is still stretching.

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Star

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A fixed luminous point in the night sky which is a large, remote incandescent body like the sun.

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Star System

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A small number of stars that orbit each other.

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Galaxies

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A system of millions or billions of stars, together with gas and dust, held together by gravitational attraction.

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Sun

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The star around which the earth orbits.

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Planet

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A celestial body moving in an elliptical orbit around a star.

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Moons

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The natural satellite of the earth, visible by reflected light from the sun.

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Comets

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A celestial object consisting of a nucleus of ice and dust and, when near the sun, a 'tail' of gas and dust particles pointing away from the sun.

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Meteors

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A small body of matter from outer space that enters the earth's atmosphere.

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Lightyear

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A unit of astronomical distance equivalent to the distance that light travels in one year, which is 9.4607.