The Andromeda Galaxy is roughly 2.5 million light-years away from the Milky Way, but that won't always be the case, because both are on a collision course that will ultimately alter the structure of each galaxy forever. The galaxies are rushing closer to one another at about 70 miles per second, which is 112 kilometers per second, and it is estimated that the collision will begin in around 4.5 billion years from now. While this collision might seem like it could only end in destruction, galaxy mergers often lead to extreme bursts of star formation, leaving the night sky a wash with bright, new stars. Although, unfortunately, humans probably won't be around long enough to see this majestic cosmic show play out.