Definition
A galaxy collision is a giant astronomical event in which two or more galaxies drifting through space are pulled together by gravity, tangle up, and merge. Despite the name, stars rarely smash into each other. Instead, interstellar gas mixes, gives birth to many new stars, and completely changes the shape of the galaxies.
Why don't stars crash into each other?
Hear that galaxies collide in the night sky, and you might picture huge explosions and doom. But a galaxy collision is nothing like the disaster scenes in sci-fi movies. The hundreds of billions of stars inside a galaxy almost never hit each other during the collision.
In the vastness of space, stars are tiny compared with the distances between them. If you shrank the Sun to the size of a ping-pong ball, the nearest star would be another ping-pong ball more than 1,000 kilometers away. Even when two galaxies meet head-on, the stars glide through the empty space like ghosts and gently pass each other by.
So the heart of a galaxy collision is not stars physically hitting each other. It is a giant gravitational tug-of-war as the two galaxies pull on one another.
The fireworks when gas and dust meet
Stars slip past each other, but the gas and dust clouds spread widely through a galaxy are a different story. The huge interstellar gas clouds that fill a galaxy crash head-on, create strong shock waves, and start to pile up in one place.
When gas packs tightly together, pressure and temperature rise, and baby stars are born in a burst. Astronomers call this a starburst. So a galaxy collision is not really destruction. It works more like a cradle where many new stars are born.
The supermassive black holes at the centers of the two galaxies also slowly spiral toward each other. In the process they release enormous energy and shake up the whole galaxy.
To be more precise: the future of our Milky Way
To be more precise, a galaxy does not slam into another in an instant, like a car crash. Two galaxies bound by each other's gravity circle one another several times over hundreds of millions of years, tangling up as if dancing. Along the way, the spiral arms can be stretched out into long tails.
For a long time, our Milky Way was predicted to collide with the neighboring Andromeda galaxy in about 4.5 billion years. But a 2025 study recalculated the odds that the two galaxies merge within 10 billion years at about one in two, so it is still uncertain. If they do merge completely, they are expected to become one giant elliptical galaxy instead of today's spiral shape.
The chance that Earth and the solar system hit another star is close to zero. Our orbit could be pushed out toward the edge of the new galaxy, though. A galaxy collision is not the end of the universe. It is one scene in a natural evolution, as galaxies grow bigger and renew themselves.
Common misconceptions
When galaxies collide, countless stars smash head-on and shatter into pieces.
Compared with the size of a star, the distance between stars is far greater than you would imagine, so stars almost never hit each other directly. Instead, giant gas clouds collide, and new stars are born in large numbers.
Where you meet it
The Antennae Galaxies in the constellation Corvus are a famous example: two spiral galaxies are colliding right now and throwing out long tails of gas.
If the Milky Way and Andromeda merge in the far future, the giant elliptical galaxy they form is nicknamed "Milkomeda."
In one sentence
A galaxy collision is not stars smashing and destroying each other. It is a cosmic process in which gravity gathers gas into new stars and merges galaxies into one bigger galaxy.