Site MapHelpFeedback

absolute magnitude  The apparent magnitude a star would have if it were at a distance of 10 parsecs.
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.
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.
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.
apparent brightness  The observed brightness of a celestial body.
apparent magnitude  The observed magnitude of a celestial body.
Balmer series  A series of absorption or emission lines of hydrogen seen in the visible part of the spectrum.
brown dwarf  A star with too low a mass for nuclear fusion to begin in its core.
continuous spectrum  A spectrum containing neither emission nor absorption lines.
dwarf  A main sequence star.
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.
energy level  Any of the many energy states that an atom may have.
giant  A star larger and more luminous than a main sequence star (dwarf) of the same temperature and spectral type.
ground state  The lowest energy level of an atom.
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.
ionization  The removal of one or more electrons from an atom.
Kirchhoff’s laws  Three "laws" that describe how continuous, bright line, and dark line spectra are produced.
light year  The distance that light travels in a year.
luminosity  The rate of total radiant energy output of a body.
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).
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.
lunar eclipse  The darkening of the Moon that occurs when the Moon enters the Earth’s shadow.
Lyman series  A series of absorption or emission lines of hydrogen lying in the ultraviolet part of the spectrum.
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.
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.
mass-luminosity relation  The relationship between luminosity and mass for stars. More massive stars have greater luminosities.
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.
proper motion  The rate at which a star appears to move across the celestial sphere with respect to very distant objects.
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.
solar motion  The motion of the Sun with respect to the nearby stars.
supergiant  An extremely luminous star of large size and mass.
white dwarf  A small, dense star that is supported against gravity by the degenerate pressure of its electrons.
wide pair  A binary star system in which the components are so distant from one another that they evolve independently.

AstronomyOnline Learning Center

Home > Chapter 16 > Glossary