airglow - The natural glow of the night sky due to reactions that take place in the Earth's upper atmosphere, atom - The smallest particle of any element, binary star - A star which is actually made up of two stars orbiting each other., cepheid - A variable star that scientists can use to determine how distant a galaxy or star cluster is, HI region - Cloud of neutral hydrogen, inferior planets - Mercury and Venus which lie closer to the Sun than the Earth are called inferior planets, nova - A star which suddenly flares up to many times its original brightness before fading again,

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