The distance to the Andromeda Galaxy is 2.54 million light-years, or 778 kiloparsecs.
The Andromeda Galaxy can be seen with the unaided eye, so skywatchers have been observing it for thousands of years. Charles Messier cataloged it as M31 in his 1764 list. Back then, astronomers thought that Andromeda was a nebula, and based on its size, Messier guessed that it was only about 2,000 times further than the star Sirius.
Astronomers discovered variable star called novae in Andromeda in 1917, and quickly realized that they were 10 times less bright than similar objects in the Milky Way. Astronomers Heber Curtis proposed that Andromeda was a separate “island universe”, located about 500,000 light-years away. Edwin Hubble ended the controversy once and for all in 1925 when he identified Cepheid variable stars in Andromeda, and calculated that the galaxy was actually 1.5 million light-years away.
Modern astronomers are continuing to calculate the distance to Andromeda. In 2003, astronomers calculated that Andromeda is 2.57 million light-years away. And in 2004, astronomers redid Hubble’s Cepheid variable calculations, and determined that Andromeda was 2.51 million light-years. Another group used a different technique in 2005 to calculate that Andromeda was 2.52 million light-years away. And yet another technique in 2005 put it at 2.56 million light-years away. And so, the agreed distance of 2.54 million light-years is an average of the distances measured so far.
There are several galaxies closer to Earth than Andromeda. The Large Magellanic Cloud is only 160,000 light years away, and the Canis Major Dwarf Galaxy is a mere 25,000 light-years from Earth. But Andromeda is the largest grand spiral galaxy to us.
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