What is the Andromeda Galaxy?
Quick answer
The Andromeda Galaxy (M31) is the nearest large galaxy to our Milky Way, about 2.5 million light-years away. It’s a spiral galaxy similar to ours but somewhat larger, with perhaps a trillion stars. On a dark night, it’s the most distant object most people can see with the naked eye.
Andromeda vs the Milky Way
| Milky Way | Andromeda | |
|---|---|---|
| Type | Barred spiral | Spiral |
| Diameter | ~100,000 light-years | ~150,000–220,000 light-years (estimates vary) |
| Stars | ~100–400 billion | Up to ~1 trillion |
| Distance from us | We’re inside it | ~2.5 million light-years |
Recent studies suggest the two galaxies may be closer in total mass than once thought, because of the invisible dark matter surrounding each.
Our cosmic neighborhood
The Milky Way, Andromeda and the smaller Triangulum Galaxy are the three big spirals in the Local Group, a family of more than 80 galaxies. The closest galaxies of all are small dwarf galaxies orbiting the Milky Way, such as the Sagittarius Dwarf and the Large and Small Magellanic Clouds.
Will Andromeda hit us?
Andromeda is moving toward us at about 110 km per second. Astronomers long predicted a merger in about 4.5 billion years, forming one giant elliptical galaxy sometimes nicknamed “Milkomeda.” Newer studies using more precise data suggest the collision is less certain, roughly a coin flip over the next 10 billion years. Either way, stars are so far apart that they would rarely, if ever, collide directly.
How to see it
- Look in the constellation Andromeda, near the “W” shape of Cassiopeia.
- Best seen in autumn from the Northern Hemisphere, away from city lights.
- To the eye it’s a faint, fuzzy oval; binoculars show more.
The light you see left Andromeda about 2.5 million years ago, before modern humans existed.
A history-making galaxy
In the 1920s, Edwin Hubble measured stars in Andromeda and proved it lies far outside the Milky Way, showing that the universe contains many galaxies. The space telescope named after him is explained in the first space telescopes.
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