Blueshift

Blueshift is the shift of spectral lines toward shorter wavelengths, toward the blue end of the spectrum, caused when an astronomical source moves toward the observer along the line of sight. This effect arises from the Doppler effect and can be observed in stars, planets, and galaxies whose motion brings their light closer to us.

The primary cause of blueshift is the radial motion of the source toward the observer. A gravitational blueshift can also occur when light travels into a stronger gravitational field, gaining energy in the process. Blueshift is the opposite of redshift, which shifts wavelengths toward the red end of the spectrum. In cosmology, most distant galaxies exhibit redshift due to the expansion of the universe, while blueshift is seen in objects that are approaching the Milky Way or other observers.

Measurement of blueshift provides the radial velocity of an object. In non-relativistic cases, the velocity is approximated by v ≈ c (Δλ/λ), where Δλ is the change in wavelength and λ is the rest wavelength. If Δλ is negative (shorter observed wavelengths), the velocity is negative, indicating approach. For higher speeds, the relativistic Doppler formula is used to relate velocity and wavelength shift.

Examples include the Andromeda Galaxy, which shows a blueshift relative to the Milky Way, reflecting its approach with a radial velocity on the order of a few hundred kilometers per second. Blueshift remains a key diagnostic for kinematics in nearby celestial systems and tests of gravitational and Doppler effects.