What is the Andromeda Galaxy?

The Milky Way’s larger spiral “big sister”, about the furthest thing most people can see with the naked eye.

Worth knowing about the Andromeda Galaxy

Its size: 137,600 light-years across, out to where its light fades; our own galaxy, measured the same way, is about 87,000 light-years. Its faint outer disc reaches much further, which is where the 220,000 you often see comes from. You see the Andromeda Galaxy as it was about 2.5 million years ago.

Why it stands out: May merge with the Milky Way in the next ten billion years — the latest measurements put the chance at about even. Its two brightest close satellite galaxies, M32 and M110, orbit it, among dozens of smaller ones.
Its name and its story: Named for the mythological princess Andromeda, chained to a rock and rescued by Perseus — the constellation got the name first, the galaxy borrowed it.