Andromeda is also called Messier 31, or M31 for short, after French astronomer Charles Messier, who cataloged it in the 18th century. M31 is considerably larger than our own galaxy, with its stellar disk extending more than 200,000 light-years across compared with about 100,000 light-years for the Milky Way.
Let us now explore more interesting facts about the Andromeda Galaxy, our nearest large galactic neighbor, but not the closest galaxy to Earth.
M31 is the largest galaxy in the Local Group
The Andromeda Galaxy is the largest major galaxy in the Local Group, the collection of galaxies that includes the Milky Way, the Triangulum Galaxy and dozens of smaller galaxies.
How far away is the Andromeda Galaxy? M31 is located about 2.5 million light-years from Earth, making it one of the most distant objects visible to the unaided eye under dark skies. The Triangulum Galaxy (M33), another major member of the Local Group, lies around 2.7 million light-years away.
Quick M31 facts
- Constellation: Andromeda
- Type: Spiral galaxy
- Diameter: More than 200,000 light-years
- Distance: About 2.5 million light-years
- Stars: Roughly 1 trillion
- Group: Local Group
- Apparent magnitude: +3.44
- Designation: M31, NGC 224
M31 is best seen in autumn and winter
In the Northern Hemisphere, M31 can be seen for much of the year, although it is particularly well placed during the autumn months when Andromeda climbs high in the evening sky. From much of the Southern Hemisphere it appears lower above the northern horizon and becomes increasingly difficult to see at far southern latitudes.
Find M31 using Cassiopeia or Pegasus
The constellations of Pegasus and Cassiopeia are useful landmarks for locating the Andromeda Galaxy.
– Pegasus contains the prominent Great Square of Pegasus. Starting from the northeastern corner of the Great Square, follow the chain of stars extending into Andromeda toward Mirach and then move northward toward the galaxy.
– Cassiopeia is easily recognized by its distinctive W shape. Its stars can also provide a useful general pointer toward the part of the sky occupied by Andromeda.
Why was a galaxy named after Andromeda?
The Andromeda Galaxy is named after the area of the sky in which it appears, namely the constellation of Andromeda. Its brightest star is the hot blue subgiant Alpheratz, located about 97 light-years away and shining at magnitude +2.06.
In Greek mythology, Andromeda was a princess whose parents, King Cepheus and Queen Cassiopeia, chained her to a rock as a sacrifice to a sea monster after Cassiopeia angered the sea god Poseidon. Andromeda was rescued by the hero Perseus.
The characters are commemorated in the neighboring constellations Cepheus, Cassiopeia, Andromeda, Perseus and Pegasus.
M31 was once believed to be a nebula
Before astronomers established the true scale of the Universe, M31 was widely known as the Andromeda Nebula. Astronomers debated whether spiral nebulae such as Andromeda were objects within the Milky Way or enormous independent systems far beyond it.
That question became one of the central astronomical debates of the early 20th century.
M31 appears about 6 times the width of the Moon
The bright central region visible to the naked eye represents only a small part of the Andromeda Galaxy. Long-exposure images reveal a much larger disk extending roughly 3 degrees across the sky — about six times the apparent width of the full Moon.
Binoculars reveal more of its elongated shape, while telescopes under dark skies can show features such as its prominent dust lanes. Its enormous apparent size is easy to underestimate because the galaxy’s outer regions are extremely faint.
Andromeda Galaxy contains around 1 trillion stars
The Andromeda Galaxy is estimated to contain roughly one trillion stars, considerably more than the estimated number in the Milky Way.
Despite containing more stars and having a larger stellar disk, current estimates suggest that Andromeda and the Milky Way have broadly comparable total masses. Much of a galaxy’s mass lies not in its visible stars but in its extended halo of dark matter.
M31 contains a supermassive black hole
Like the Milky Way and other large galaxies, Andromeda contains a supermassive black hole at its center. Known as M31*, it has a mass of roughly 100 million times that of the Sun.
Astronomers have also identified numerous stellar-mass black-hole candidates in M31, making our neighboring galaxy an important laboratory for studying black holes beyond the Milky Way.
Young blue stars surround Andromeda’s central black hole
Near Andromeda’s center is a compact disk of hot blue stars surrounding its supermassive black hole. These are genuinely young stars, making their presence so close to the black hole particularly interesting because the intense gravitational environment makes ordinary star formation difficult to explain.
Farther out, the central region is dominated by much older stellar populations.
The Andromeda Galaxy appears to have a double nucleus
Images of Andromeda’s center reveal two brightness concentrations known as P1 and P2. They were once interpreted as evidence for two separate galactic nuclei.
The leading explanation is different. The apparent double nucleus is produced by stars moving in an eccentric disk around the central supermassive black hole. Their uneven distribution makes the nucleus appear double even though the two bright concentrations are part of the same nuclear structure.
M31 has dozens of satellite galaxies
The Andromeda Galaxy is surrounded by a large population of smaller satellite galaxies. A major Hubble survey examined 36 known dwarf galaxies associated with M31, showing how extensive its satellite system has become as increasingly faint companions have been discovered.
The two best-known companions are M32 and M110, both of which can be observed through amateur telescopes. Andromeda also possesses hundreds of globular clusters, including the exceptionally massive Mayall II, also known as G1.
Will Andromeda collide with the Milky Way?
The Milky Way and Andromeda are moving toward one another, but a future collision is no longer considered inevitable. Recent measurements using the Hubble Space Telescope and ESA’s Gaia mission suggest there is about a 50–50 chance that the two galaxies will collide within the next 10 billion years. Their eventual paths depend on their precise motions and on the gravitational influence of other galaxies in the Local Group.
If the Milky Way and Andromeda do eventually collide, individual stars would be very unlikely to crash into one another. The space between the stars is so vast that the two stellar populations could largely pass through each other. Gravity would instead distort the galaxies, alter stellar orbits and pull material into new structures, while interactions between clouds of gas could trigger new star formation.
M31 helped inspire the island universe debate
In the early 20th century, astronomer Heber Curtis argued that spiral nebulae such as Andromeda were enormous independent stellar systems located far beyond the Milky Way.
Curtis used observations of novae in Andromeda as part of his argument. His position was debated with astronomer Harlow Shapley during the famous Great Debate of 1920. Although the debate itself did not settle the issue, it highlighted one of astronomy’s most important unanswered questions: did the Milky Way constitute almost the entire Universe, or was it simply one galaxy among many?
M31 helped prove the existence of other galaxies
The decisive evidence came from Edwin Hubble’s observations of Cepheid variable stars in Andromeda. Cepheids have a relationship between their pulsation periods and intrinsic luminosities, allowing astronomers to estimate their true brightness and therefore their distance.
Hubble’s measurements placed Andromeda far beyond the known dimensions of the Milky Way. His original distance estimate was much smaller than today’s value of about 2.5 million light-years, but it was sufficient to demonstrate that Andromeda was a separate galaxy.
The discovery transformed our picture of the Universe. The Milky Way was no longer the entire known cosmos, but simply one of many galaxies.