| absolute magnitude | The apparent magnitude a star would have if it were at a distance of 10 parsecs.
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| absorption line | A dark line superimposed on a continuous spectrum when a gas absorbs light from a continuous source that is hotter than the absorbing gas.
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| antapex | The direction in the sky away from which the Sun is moving. Because of the Sun’s motion, nearby stars appear to converge toward the antapex.
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| apex | The direction in the sky toward which the Sun is moving. Because of the Sun’s motion, nearby stars appear to diverge from the apex.
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| apparent brightness | The observed brightness of a celestial body.
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| apparent magnitude | The observed magnitude of a celestial body.
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| Balmer series | A series of absorption or emission lines of hydrogen seen in the visible part of the spectrum.
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| brown dwarf | A star with too low a mass for nuclear fusion to begin in its core.
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| continuous spectrum | A spectrum containing neither emission nor absorption lines.
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| dwarf | A main sequence star.
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| emission line | A narrow, bright region of the spectrum. Emission lines are produced when electrons in atoms jump from one energy level to a lower energy level.
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| energy level | Any of the many energy states that an atom may have.
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| giant | A star larger and more luminous than a main sequence star (dwarf) of the same temperature and spectral type.
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| ground state | The lowest energy level of an atom.
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| Hertzsprung-Russell diagram (H-R diagram) | A plot of luminosities of stars against their temperatures. Magnitude may be used in place of luminosity and spectral type in place of temperature.
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| ionization | The removal of one or more electrons from an atom.
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| Kirchhoff’s laws | Three "laws" that describe how continuous, bright line, and dark line spectra are produced.
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| light year | The distance that light travels in a year.
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| luminosity | The rate of total radiant energy output of a body.
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| luminosity class | The classification of a star’s spectrum according to luminosity for a given spectral type. Luminosity class ranges from I for a supergiant to V for a dwarf (main sequence star).
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| luminosity function | The distribution of stars or galaxies according to their luminosities. A luminosity function is often expressed as the number of objects per unit volume of space that are brighter than a given absolute magnitude or luminosity.
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| lunar eclipse | The darkening of the Moon that occurs when the Moon enters the Earth’s shadow.
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| Lyman series | A series of absorption or emission lines of hydrogen lying in the ultraviolet part of the spectrum.
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| magnitude | A number, based on a logarithmic scale, used to describe the brightness of a star or other luminous body. Apparent magnitude describes the brightness of a star as we see it. Absolute magnitude describes the intrinsic brightness of a star.
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| main sequence | The region in an H-R diagram occupied by stars that are fusing hydrogen into helium in their cores. The main sequence runs from hot, luminous stars to cool, dim stars.
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| mass-luminosity relation | The relationship between luminosity and mass for stars. More massive stars have greater luminosities.
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| parsec (pc) | The distance at which a star has a parallax of 1 second of arc. At a distance of 1 parsec, an AU fills an angle of 1 second of arc.
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| proper motion | The rate at which a star appears to move across the celestial sphere with respect to very distant objects.
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| radial velocity | The part of the velocity of a body that is directed toward or away from an observer. The radial velocity of a body can be determined by the Doppler shift of its spectral lines.
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| solar motion | The motion of the Sun with respect to the nearby stars.
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| supergiant | An extremely luminous star of large size and mass.
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| white dwarf | A small, dense star that is supported against gravity by the degenerate pressure of its electrons.
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| wide pair | A binary star system in which the components are so distant from one another that they evolve independently.
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