§ DICTIONARY · PHENOMENON
Blueshift
The compression of a wave's wavelength when source and observer move together.
§ 01
Definition
The mirror image of redshift. When a light source approaches the observer, the observed wavelength is shorter than the emitted wavelength. Spectral lines shift toward the blue end of the spectrum.
In astronomy, blueshift is rare: the Andromeda galaxy is one of the few that is approaching us, at about 300 km/s. Most of the universe is redshifted. Within our own galaxy, stars orbiting the Milky Way's core show blueshifts and redshifts depending on which side of their orbit they are on.