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:

The Andromeda Galaxy is moving towards our own Milky Way Galaxy within the Local Group; thus, when observed from earth, its light is undergoing a blue shift.

When observing spiral galaxies, the side spinning towards us will have a slight blue shift (see Tully-Fisher relation).

Also, Blazars are known to propel relativistic jets towards us, emitting synchrotron radiation and Bremsstrahlung that appears blue shifted.

See also

• Doppler effect
• redshift
• theory of relativity

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Blue Shift is also the title of a story by Stephen Baxter.
Half-Life: Blue Shift is a computer game.