red shift or redshift, in astronomy, the systematic displacement of individual lines in the spectrum of a celestial object toward the red, or longer wavelength, end of the visible spectrum. The effect was discovered by V. M. Slipher of Lowell Observatory. Some red shifts are the result of the Doppler effect, i.e., of the relative motion of the earth and the object away from each other. The amount of displacement is a function of the object's recessional velocity relative to the observer. All distant galaxies show a red shift proportional to their distance from the earth as a result of the general expansion of space-time (see Hubble's law). Known as the cosmological red shift, this results when the wavelength of light is stretched as it moves through the expanding universe. Red shifts are also produced by gravitation (the gravitational red shift) in accordance with the general theory of relativity. Because of the strong gravitational field, the frequency of the light emitted by atoms in a dense, compact star will be lower and the wavelengths consequently longer; such effects have been observed in white dwarfs. Not all celestial bodies have spectra displaced toward the red end of the spectrum. Of the billions of known galaxies, about 100?for example, the Andromeda Galaxy?are blue-shifted, indicating that they are approaching earth rather than receding from it. Most of these are dwarf galaxies in Milky Way's Local Group and in orbit about one another. See also blue shift; cosmology.
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