The
Andromeda Galaxy, also known as
Messier 31,
M31, or
NGC 224 and originally the
Andromeda Nebula, is a
barred spiral galaxy with diameter of about 220,000 ly approximately 2.5 million
light-years (770
kiloparsecs) from Earth and the nearest large galaxy to the
Milky Way.
[6] The galaxy's name stems from the area of Earth's sky in which it appears, the
constellation of Andromeda, which itself is named after the
Ethiopian (or Phoenician) princess who was the wife of
Perseus in
Greek mythology.The
virial mass of the Andromeda Galaxy is of the same order of magnitude as that of the Milky Way, at 1
trillionsolar masses (2.0×1042
kilograms). The mass of either galaxy is difficult to estimate with any accuracy, but it was long thought that the Andromeda Galaxy is more massive than the Milky Way by a margin of some 25% to 50%. This has been called into question by a 2018 study that cited a lower estimate on the mass of the Andromeda Galaxy,=10.5px combined with preliminary reports on a 2019 study estimating a higher mass of the Milky Way.=10.5px The Andromeda Galaxy has a diameter of about 220,000
ly (67
kpc), making it the largest member of the
Local Group in terms of extension.The Milky Way and Andromeda galaxies are
expected to collide in around 4-5 billion years,=10.5px merging to form a giant
elliptical galaxy=10.5px or a large
lenticular galaxy.=10.5px With an
apparent magnitude of 3.4, the Andromeda Galaxy is among the brightest of the
Messier objects,=10.5px making it visible to the
naked eye from Earth on moonless nights,=10.5px even when viewed from areas with moderate
light pollution.
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