Andromeda (M31) is the nearest large spiral galaxy to the Milky Way, the other giant of the Local Group and the farthest thing most people can see unaided.
At a glance
| type | galaxy |
|---|---|
| distance from the Sun | 2.54 million light-years (780 kpc) |
| its light left it | 2.54 million years ago |
| size in the piece | about 220,000 light-years acrossmodel |
Read off the piece’s own catalogue, the same values it draws with. Model marks a property of the piece’s galaxy model rather than a measurement.
About Andromeda (M31)
Andromeda is a spiral galaxy comparable to the Milky Way, seen steeply tilted so that its disc appears as a long ellipse. Under dark skies it is visible to the naked eye as a faint oval smudge in the constellation Andromeda; the Persian astronomer al-Sufi described it as a little cloud. Its light reaching us now set out long before there were humans to see it.
For a long time it was argued whether the spiral nebulae were clouds inside our Galaxy or galaxies in their own right. Edwin Hubble settled the question by finding Cepheid variable stars in Andromeda: their pulsation periods gave their true brightness, and their faintness showed the galaxy lay far beyond the Milky Way.
Andromeda is approaching us. It is accompanied by satellite galaxies, the brightest of them the compact elliptical M32 and the larger, diffuse M110. Its own star formation is concentrated in a great ring rather than in sharp spiral arms, and its nucleus appears double, an effect of stars orbiting its central black hole on elongated paths.
What you see in the piece
It belongs to the pull-back: from inside the Milky Way it stays out of frame, and it fades in as you move out toward the Local Group, tilted at its measured angle, with M32 and M110 beside it.
Measured or modelled
Whether Andromeda and the Milky Way will eventually collide and merge is uncertain: the outcome depends on Andromeda’s sideways motion and the masses of both galaxies, and recent estimates put it close to even odds. The extent drawn here is modelled.
Questions
How far away is the Andromeda Galaxy?
Andromeda is about 2.54 million light-years (780 kpc) from the Sun, so the light you see left it 2.54 million years ago.
Will Andromeda collide with the Milky Way?
The two galaxies are moving toward each other, but whether they will eventually merge is uncertain; it depends on motions and masses that are still being measured.