blue shift

A blue shift is a general displacement of spectral lines to shorter wavelengths that happens when a source of electromagnetic radiation moves toward the observer (see Doppler effect): the greater the velocity of approach along the line of sight, the greater the blueshift. Individual stars within the Milky Way Galaxy frequently show blueshifts. However, with the exception of a few galaxies within the Local Group, including the Andromeda Galaxy, most extragalactic objects exhibit redshifts, due to the overall expansion of the universe.