What does extragalactic mean?
Extragalactic is an adjective describing anything that lies outside, originates from, or pertains to regions beyond the Milky Way — the galaxy that contains our solar system. It is chiefly a term of astronomy and astrophysics, appearing in phrases such as 'extragalactic astronomy,' 'extragalactic objects,' and 'extragalactic background light.' The word gained prominence after Edwin Hubble demonstrated in the 1920s that certain spiral nebulae were actually distant galaxies rather than features of the Milky Way, opening up an entire field devoted to what lay beyond. Because it refers to the largest scales studied by science, extragalactic often connotes immensity, remoteness, and the frontiers of human knowledge. It is formal and technical in register, with no idiomatic or figurative use, and it is spelled and pronounced identically in British and American English.
adjectiveRelating to or situated outside the Milky Way galaxy; originating or existing beyond our own galaxy.
- Relating to, located, or occurring outside the Milky Way galaxy.
"The Hubble Space Telescope has revealed countless extragalactic objects scattered across the deep sky."
"Edwin Hubble's discovery of Cepheid variables in Andromeda proved that some nebulae were in fact extragalactic."
"The survey catalogued thousands of faint extragalactic sources at the edge of the observable universe."
When astronomers say 'extragalactic,' they mean everything beyond the Milky Way — a realm so vast it contains roughly two trillion other galaxies.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of extragalactic
Extragalactic derives from the Latin prefix extra-, meaning 'outside' or 'beyond,' combined with galactic, from Greek gala (genitive galaktos), meaning 'milk.' The underlying root reflects the ancient observation that the Milky Way resembles a band of spilled milk across the night sky. The term emerged in astronomical usage in the early twentieth century, as observations established that many nebulae lay far beyond the boundaries of our own galaxy.
How extragalactic is actually used
A technical term used primarily in astronomy, astrophysics, and popular science writing. It carries no figurative or informal usage; it is neutral in connotation and identical in British and American English.
Easily confused with extragalactic
Intergalactic means situated between galaxies, while extragalactic means outside or beyond the Milky Way specifically.