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apparent brightness
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a measure of the amount of light received by Earth from a star or other object—that is, how bright an object appears in the sky, as contrasted with its luminosity
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brown dwarf
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an object intermediate in size between a planet and a star; the approximate mass range is from about 1/100 of the mass of the Sun up to the lower mass limit for self-sustaining nuclear reactions, which is about 0.075 the mass of the Sun; brown dwarfs are capable of deuterium fusion, but not hydrogen fusion
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color index
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difference between the magnitudes of a star or other object measured in light of two different spectral regions—for example, blue minus visual (B–V) magnitudes
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giant
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a star of exaggerated size with a large, extended photosphere
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luminosity
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the rate at which a star or other object emits electromagnetic energy into space; the total power output of an object
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magnitude
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an older system of measuring the amount of light we receive from a star or other luminous object; the larger the magnitude, the less radiation we receive from the object
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proper motion
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the angular change per year in the direction of a star as seen from the Sun
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radial velocity
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motion toward or away from the observer; the component of relative velocity that lies in the line of sight
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space velocity
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the total (three-dimensional) speed and direction with which an object is moving through space relative to the Sun
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spectral class
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(or spectral type) the classification of stars according to their temperatures using the characteristics of their spectra; the types are O, B, A, F, G, K, and M with L, T, and Y added recently for cooler star-like objects that recent survey have revealed