Andromeda Galaxy (M31)
Galaxy · Spiral
The Andromeda Galaxy is the nearest large galaxy to the Milky Way – a giant spiral with about a trillion stars. It is the most distant thing most people can see with their own eyes.
- 2.5 million light-years awayEven light – the fastest thing there is – needs 2.5 million years to get here.
- ~1 trillionStars
- ~150,000+ light-yearsWidth
Take a closer look
A Galaxy With Followers
Andromeda is surrounded by a swarm of smaller satellite galaxies, including M32 and M110, which a small telescope shows close to its bright centre. Long streams of stars loop around it too – the leftover wreckage of smaller galaxies that Andromeda has pulled apart and swallowed in the past.
Bigger Than It Looks
If your eyes could pick up faint light better, Andromeda would stretch across the sky about six times wider than the full Moon. We only notice its bright middle, which is why it looks like a small fuzzy smudge rather than a giant glowing disc.
Our Mirror Galaxy
Because we live inside the Milky Way, nobody can step outside to photograph it. Andromeda is the next best thing: a big spiral close enough to study star by star. It shows astronomers what a galaxy like ours looks like from the outside.
How we found out
- 964
Al-Ṣūfī describes it as a “little cloud” in his Book of Fixed Stars.
- 1923
Edwin Hubble finds a Cepheid variable star in Andromeda. He crosses out “N” (nova) on his photographic plate and writes “VAR!” – proof that Andromeda is a separate galaxy far beyond our own.
- 2015
Hubble’s PHAT survey maps over 100 million of its stars.
- 2025
A new study finds only about a 50:50 chance that Andromeda and the Milky Way will merge in the next 10 billion years.
What happens when two galaxies collide?
Stars almost never hit each other – the space between them is enormous. But gas clouds crash together and can spark bursts of new stars.
See it yourself
From a dark site, a faint fuzzy oval near Cassiopeia. Binoculars make it easy.
From the encyclopedia
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