Blue shift is the opposite of redshift, the latter being much more noted due to its importance to modern astronomy.
Blue shift is the phenomenon that the frequency of an electromagnetic wave (such as light) emitted by a source moving towards the observer is shifted towards the blue end of the spectrum (that is, its wavelength is decreased).
The phenomenon of shifting wavelengths in frames of reference moving relatively to each other is commonly known as Doppler shift or Doppler effect.
While the general redshift of starlight is seen as proof for an expanding universe, there are few examples of blue shift in astronomy:
