Andromeda Galaxy

The largest galaxy in the Local Group at 2.5 million light-years away, Andromeda is on a collision course with the Milky Way — the two will merge in roughly 4.5 billion years. At over 200,000 light-years across it is twice the diameter of our own galaxy, and on a dark night is visible to the naked eye as a faint smudge in the constellation Andromeda.