Andromeda Galaxy, also called
Andromeda Nebula, (catalog numbers NGC 224 and M31), great spiral
galaxy in the
constellationAndromeda, the nearest large galaxy. The Andromeda Galaxy is one of the few visible to the unaided eye, appearing as a milky blur. It is located about 2,480,000
light-years from
Earth; its
diameter is approximately 200,000 light-years; and it shares various characteristics with the Milky Way system. It was mentioned as early as 965 CE, in the
Book of the Fixed Stars by the Islamic astronomer
al-Ṣūfī, and rediscovered in 1612, shortly after the invention of the
telescope, by the German astronomer
Simon Marius, who said it resembled the light of a candle seen through a horn. For centuries astronomers regarded the Andromeda Galaxy as a component of the
Milky Way Galaxy—i.e., as a so-called spiral
nebula much like other glowing masses of gas within the local galactic system (hence the misnomer Andromeda Nebula). Only in the 1920s did the American astronomer
Edwin Powell Hubble determine conclusively that the Andromeda was in fact a separate galaxy beyond the Milky Way.
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