Doppler Blueshift
Doppler blueshift is caused by movement of a source toward the observer. The term applies to any decrease in wavelength (increase in frequency) caused by relative motion, even outside the visible spectrum. Only objects moving at near-relativistic speeds toward the observer are noticeably bluer to the naked eye, but 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; thus, when observed from earth, its light is undergoing a blueshift.
- When observing spiral galaxies, the side spinning toward us will have a slight blueshift relative to the side spinning away from us (see Tully–Fisher relation).
- Blazars are known to propel relativistic jets toward us, emitting synchrotron radiation and bremsstrahlung that appears 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 subtracted from the much larger cosmological redshift to determine relative motion in the expanding universe.
Read more about this topic: Blue Shift