The Andromeda Galaxy, also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy approximately 2.5 million light-years (770 kiloparsecs) from Earth and the nearest large galaxy to the Milky Way.

The number of stars contained in the Andromeda Galaxy is estimated at one trillion (1×1012), or roughly twice the number estimated for the Milky Way.

The Milky Way and Andromeda galaxies are expected to collide in around 4-5 billion years, merging to form a giant elliptical galaxy or a large lenticular galaxy. Under most viewing conditions, the Andromeda Galaxy is one of the most distant objects that can be seen with the naked eye.

The galaxy is commonly located in the sky in reference to the constellations Cassiopeia and Pegasus. Andromeda is best seen during autumn nights in the Northern Hemisphere when it passes high overhead, reaching its highest point around midnight in October, and two hours earlier each successive month. In early evening, it rises in the east in September and sets in the west in February. From the Southern Hemisphere the Andromeda Galaxy is visible between October and December, best viewed from as far north as possible.

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