The Andromeda Galaxy, also known as
Messier 31, M31, or NGC 224 and originally the Andromeda
Nebula (see below), 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 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 trillion solar 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, combined with preliminary reports on a 2019 study
estimating a higher mass of the Milky Way. 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. |
