Andromeda (astronomy), in astronomy, large constellation of the northern hemisphere situated just south of the constellation Cassiopeia and west of the constellation Perseus. Andromeda contains no stars of the first magnitude but is noted as the area of sky containing the Andromeda Galaxy, a member of the local group to which our own Milky Way belongs. At a distance of 2.2 million light-years, the Andromeda Galaxy is both the nearest spiral galaxy and the most distant object that can be seen with the naked eye. Before its nature was determined by means of powerful telescopes, it was erroneously believed to be a nebula, or cloud of interstellar matter. Through telescopes it is seen to have two small companion galaxies of elliptical form.
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