What does blueshift mean?
A blueshift is the shift of light or other electromagnetic radiation toward shorter wavelengths — that is, toward the blue end of the spectrum — that occurs when the source of the radiation moves toward the observer. The effect arises from the Doppler principle: as an approaching source emits successive waves, each crest is sent from slightly closer than the last, compressing the waves and raising their observed frequency. Astronomers detect blueshifts by measuring how familiar spectral lines appear displaced toward the blue end compared with laboratory values. The term also covers analogous frequency increases produced by gravity, such as light gaining energy as it falls into a gravitational well. Blueshift is best known as the counterpart of redshift, the lengthening of wavelengths from receding sources. Because most galaxies are carried away from us by cosmic expansion, a genuinely cosmological blueshift is rare — which makes nearby approaching objects like the Andromeda Galaxy especially notable.
nounA shift of the light or other radiation emitted by an object toward shorter (bluer) wavelengths, caused by the object moving toward the observer; the opposite of a redshift.
- A displacement of spectral lines toward shorter wavelengths in the light from an approaching astronomical object, resulting from the Doppler effect.
- A general increase in the frequency of any wave (including sound) received by an observer moving toward the source, or by a photon climbing out of a gravitational field.
"The galaxy's spectrum showed a distinct blueshift, indicating it was moving toward us."
"Measurements revealed a slight blueshift in the star's absorption lines, confirming its motion toward Earth."
"Because Andromeda is approaching the Milky Way, its spectrum displays a small but measurable blueshift."
Standard pluralization; commonly used when comparing measurements across multiple stars or galaxies.
"The survey catalogued dozens of blueshifts among the nearby white dwarfs."
Andromeda is one of the few galaxies headed our way — its light arrives blueshifted because it's barreling toward the Milky Way at about 110 kilometers per second.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of blueshift
The word is a modern compound formed within English from the adjective 'blue' and the noun 'shift', coined alongside 'redshift' as astronomers developed spectroscopy in the late nineteenth and early twentieth centuries to measure stellar and galactic motions. It derives ultimately from the Doppler effect, first described for sound waves by Christian Doppler in 1842 and later applied to light. Unlike many scientific terms, it has no classical root; its components are ordinary Germanic-origin English words combined to describe a newly measured phenomenon.
Related word forms
How blueshift is actually used
A technical term used almost exclusively in astronomy and physics; rare outside scientific contexts. Often written with a space ('blue shift') in older texts, though 'blueshift' is now standard. The verb use is uncommon and generally avoided in formal writing.
Easily confused with blueshift
Redshift is the stretching of light to longer, redder wavelengths as a source recedes, while blueshift is the compression of light to shorter, bluer wavelengths as a source approaches.