Doppler redshift and blueshift

Blue shift is one example of the Doppler effect. It is the opposite effect to redshift.

Doppler blueshift is caused by movement of a source towards the observer. The term applies to any decrease in wavelength caused by relative motion, even outside the visible spectrum.

The wavelength of any reflected or emitted photon or other particle is shortened in the direction of travel.

Doppler blueshift is used in astronomy to determine relative motion:

• The Andromeda Galaxy is moving toward our own Milky Way galaxy within the Local Group. When observed from earth, its light shows a blueshift.
• Components of a binary star system will be blueshifted when moving towards Earth
• When observing spiral galaxies, the side spinning toward us will have a slight blueshift relative to the side spinning away from us.
• Blazars may emit relativistic (near the speed of light) jets toward us that appear blueshifted.
• Nearby stars such as Barnard's Star are moving toward us, resulting in a very small blueshift.
• Doppler blueshift of distant objects (high z) can be got from the much larger cosmological redshift. This shows relative motion in the expanding universe.