The Andromeda Galaxy Is on a Collision Course With the Milky Way

Almost every galaxy in the universe is racing away from us as space expands. There’s one glaring, enormous exception — and it’s headed straight for us.

The Andromeda Galaxy, our nearest large galactic neighbor at 2.5 million light-years, is falling toward the Milky Way at about 400,000 kilometers per hour. Gravity has locked the two galaxies together, and they are on a slow-motion collision course. Hubble measurements confirmed it: in roughly 4.5 billion years, the two galaxies will begin to merge.

The event even has a name astronomers use: “Milkomeda,” the single giant elliptical galaxy the collision will eventually create.

Now for the counterintuitive part — the part that surprises almost everyone. When two galaxies collide, the stars don’t smash into each other. Not even close.

Galaxies are overwhelmingly empty space. The stars within them are separated by such vast distances that when Andromeda and the Milky Way pass through each other, the odds of any two stars actually colliding are essentially zero. The galaxies will glide through one another like two swarms of gnats crossing paths, their gravity churning everything into new shapes over hundreds of millions of years, but their stars sailing past untouched.

What will change:

• The night sky will transform — over hundreds of millions of years, Andromeda will grow from a faint smudge into a vast structure filling the heavens
• Gas clouds will collide, triggering bursts of new star formation
• The two central black holes will eventually spiral together and merge

Will Earth survive? Possibly — but it’s academic. By the time Milkomeda forms, the Sun will be nearing the end of its life, swelling toward its red giant phase and rendering Earth uninhabitable anyway.

Look at Andromeda on a dark night (you can, with the naked eye) and you’re seeing the galaxy that will one day become part of our own.