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what happens when two galaxies collide
When galaxies collide, stars do not crash. Shrink the Sun to an orange, and the nearest star is about two thousand kilometers away.
A galaxy is mostly empty space. Two galaxies can drift through each other and, with stars this far apart, almost none of them ever touch.
What does act is gravity. Each galaxy's pull stretches the other into long tails of stars and gas, as in the Antennae Galaxies here.
The gas clouds inside do collide. They squeeze and ignite, so a collision can set off a burst of brand-new stars.
Andromeda approaches at a hundred ten kilometers a second. A merger is a coin flip, and if it comes, likely in seven or eight billion years.
If it happens, two spirals swirl together, fling out tails, and settle into one big oval galaxy, their central black holes merging.
So galaxies collide without stars colliding: gravity stretches tails, gas crashes into new stars, and the pair settles into one galaxy over billions of years.
A galaxy collision is the slowest, quietest crash in the universe, where nothing hits and everything is reshaped.