2.5 million light-years
Andromeda is a spiral galaxy close to our Milky Way, and it is the largest galaxy visible to the unaided eye. By using ultraviolet light, astronomers can highlight different structures: blue colours represent light given off by brilliant young stars in the spiral arms, while orange tones are from the older, cooler stars in the galaxy's core. Billions of years from now, the Milky Way and Andromeda will merge.
Credit: Galaxy Evolution Explorer Team for NASA / JPL-CalTech.
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