The Andromeda galaxy is the closest big galaxy to our Milky Way. It sits about 2.5 million light-years away and contains roughly a trillion stars - about twice as many as ours. On a dark night, you can actually see it with the naked eye, as a faint smudge in the constellation Andromeda.
What you can’t see, but astronomers can measure, is that Andromeda is heading toward us. It’s currently moving at about 250,000 miles per hour, closing the distance between us. In about 4 billion years, the two galaxies will collide and start merging into a single giant galaxy.
It won’t be a violent crash, despite the speed. Galaxies are mostly empty space - actual star-on-star collisions will be incredibly rare. Instead, the two galaxies will slowly pull through each other over hundreds of millions of years, their stars getting flung around by changing gravity, until eventually they settle into one massive new galaxy. Earth probably won’t be around to see it (the Sun will be a red giant by then), but the merged galaxy already has a fan-given nickname: “Milkomeda.”
