Andromeda galaxy

Astron.

a spiral galaxy, appearing to the naked eye as a fuzzy oval patch in the constellation Andromeda; it is a close neighbor to our own galaxy.

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or M31

Great spiral galaxy in the constellation Andromeda.

It is the nearest spiral galaxy outside the Milky Way Galaxy and one of the few visible to the unaided eye, appearing as a milky blur. About 2 million light-years from Earth, it has a diameter of about 200,000 light-years, which makes it the largest galaxy in the Local Group. For centuries astronomers considered it part of the Milky Way; only in the 1920s did Edwin Hubble determine conclusively that it was a separate galaxy.

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also called  Andromeda Nebula  

  (catalog numbers NGC 224 and M31), great spiral galaxy in the constellation Andromeda, the nearest external galaxy (except for the Magellanic Clouds, which are companions of the Milky Way Galaxy, in which the Earth is located). The Andromeda Galaxy is one of the few visible to the unaided eye, appearing as a milky blur. It is located about 2,000,000 light-years from the 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 AD 965, in the Book of the Fixed Stars, by the Islāmic astronomer ạs-Ṣūfī, and rediscovered in 1612, shortly after the invention of the telescope, by the German astronomer Simon Marius (Marius, Simon), who said it resembled the light of a candle seen through a horn. For centuries astronomers regarded the Andromeda 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 (Hubble, Edwin Powell) determine conclusively that the Andromeda was in fact a separate galaxy beyond the Milky Way.

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