blue shift or blueshift, in astronomy, the systematic displacement of individual lines in the spectrum of a celestial object toward the blue, or shorter wavelength, end of the visible spectrum. The amount of displacement is a function of the object's relative velocity toward the observer. Most observed blue shifts are the result of the Doppler effect. The blue shift is the opposite of the red shift. Blue shifted celestial bodies are quite rare. Of the billions of known galaxies, only about 100, including the Andromeda galaxy, are blue shifted.