A manifestation of the Doppler effect in which an object appears bluer when it is moving towards the observer or the observer is moving towards it: The blue shift is a reduction in the wavelength of radiation given off by an oncoming celestial body.

Blue light has a higher frequency than other colors in the spectrum.

The blue shift is the opposite of the red shift.

The observed shift of the characteristic spectral lines of a galaxy, for example, toward the red, longer-wavelength, end of the spectrum as a result of the galaxy's recession from the earth: The red shift is the lengthening of the wavelengths of light from an object as a result of the object's motion away from the Earth.

It is an example of the Doppler effect. The red shift in light from galaxies is evidence that the universe is expanding.