The shift of spectral lines toward shorter wavelengths, indicating motion toward the observer.Blueshift is the decrease in wavelength (and increase in frequency) of electromagnetic radiation emitted by an object moving toward the observer. It is the opposite of redshift, which occurs when the source moves away. Blueshift is a direct consequence of the Doppler effect, applied to light and other waves. In astronomy, it reveals the motion of stars, galaxies, and other celestial bodies along the line of sight.
The Doppler effect was proposed by Christian Doppler in 1842 for sound waves, and later applied to light by Hippolyte Fizeau in 1848, who suggested it could be used to measure stellar velocities.
When a source of waves moves toward an observer, each successive wave crest is emitted from a position closer to the observer, so the waves are compressed. This compression shortens the wavelength and raises the frequency. For light, a shorter wavelength corresponds to a shift toward the blue end of the spectrum, hence the name. The effect is quantified by the formula Δλ/λ = v/c for non-relativistic speeds, where v is the relative velocity and c is the speed of light.
Blueshift is used in astronomy to measure the velocities of nearby galaxies, stars, and exoplanets, and in radar and lidar systems to detect the speed of approaching objects.
The Andromeda Galaxy is blueshifted, meaning it is moving toward the Milky Way, and will eventually collide with it in about 4.5 billion years.
