What is the Andromeda Galaxy?
A spiral galaxy like ours, only bigger, and the nearest big one there is.
How this site works. Every fact is in our own words and linked to its source, beside it. Where good sources disagree, we show each version in a “Sources differ” box.
Our nearest big neighbour
The Andromeda Galaxy, also called M31, is the nearest major galaxy to our own Milky Way. NASA, M31
It is a spiral galaxy in the constellation Andromeda. NASA, M31 Britannica lists its other names: the Andromeda Nebula, M31 and NGC 224. Britannica
The Milky Way, Andromeda and more than 50 other galaxies make up the Local Group. NASA, galaxies
Like the Milky Way, from outside
It shares various characteristics with the Milky Way. Britannica
Astronomers use it as a stand-in for spiral galaxies in general: we would know much less about our own spiral if we did not have Andromeda to study. NASA, M31
NASA says studying the Milky Way from inside is like trying to understand the layout of New York City while standing in the middle of Central Park. NASA, M31
We see Andromeda almost edge on, tilted by 77 degrees. NASA, M31
Bigger than us?
How much bigger? Sources differ
- NASA says Andromeda is as much as twice as massive as the Milky Way. NASA, 2025
- The Royal Observatory says it has more than twice as many stars as the Milky Way. Royal Observatory
One figure is about mass, the other about stars. Both say Andromeda is the bigger of the two.
Looking back in time
So the light we see left Andromeda 2.5 million years ago: we see it as it was then. EarthSky
Our view: we can never see our own galaxy from outside, so Andromeda is the next best thing: a mirror 2.5 million light-years away.
Next: how big and how far →