Nib asked
What happens when two galaxies collide?
Colliding galaxies pass through each other and almost no stars ever hit, because stars are so far apart. What does happen is gravity: it drags out long tails, gas clouds crash and ignite new stars, and the pair eventually settles into one…
7 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1huge gaps
Picture the Sun as an orange, about seven centimeters across. On that scale the nearest other star, Proxima Centauri, sits roughly 2,000 kilometers away, about the distance from London to Athens. Stars are tiny dots in an enormous emptiness.
Stars are so far apart that, shrunk to oranges, neighbors would be 2,000 kilometers apart.
a common mix-upColliding galaxies are not crowds of stars smashing into each other.
2mostly empty
Our galaxy holds a few hundred billion stars, spread over about 100,000 light-years. When two galaxies overlap, each star is passing through a space that is nearly empty. Direct hits between stars are so rare that, in a typical galaxy collision, you would expect almost none.
The galaxies overlap, but the stars slip past each other.
Why do stars almost never hit when galaxies collide?
Show the answer
They are separated by enormous gaps, so the galaxies are mostly empty space.
3gravity's tails
Gravity reaches across the gap. As two galaxies swing past, the pull on the near side of each is stronger than on the far side, which tugs out long streamers of stars and gas called tidal tails. The Antennae Galaxies, about 70 million light-years away, are a famous pair caught in the act.
Gravity stretches colliding galaxies into long tidal tails.
4new stars
Unlike stars, the gas between them is spread through whole galaxies, so clouds of it really do run into each other. The gas is squeezed, collapses, and forms many new stars at once. Galaxies in the middle of such collisions can make stars tens or hundreds of times faster than the Milky Way does.
Gas clouds do collide, and the squeeze makes a burst of new stars.
5our turn?
Andromeda, 2.5 million light-years away, is the nearest big galaxy to the Milky Way and is moving toward us at about 110 kilometers a second. For years the forecast was a collision in about 4.5 billion years. A 2025 study using Hubble and Gaia data found it closer to a coin flip whether they merge within ten billion years, and if they do merge, the most likely time is 7 to 8 billion years from now.
The Milky Way may merge with Andromeda, most likely in 7 to 8 billion years, but astronomers are not sure.
6the merger
In a merger of two big spirals, the discs are thrown into tails, the gas is shocked into new stars, and over a billion years or so the whole thing relaxes into one smooth, oval galaxy, called an elliptical. At the middle, the two supermassive black holes sink together and eventually merge. Some astronomers nickname the Milky Way and Andromeda's future remnant Milkomeda.
Mergers turn two spirals into one oval galaxy with one central black hole.
What does a merger of two spiral galaxies leave behind?
Show the answer
One big oval, elliptical, galaxy with a single central black hole.
7the whole idea
When galaxies collide, the stars almost never hit one another, because the gaps between them are so vast. Gravity drags out long tidal tails, gas clouds crash and light up with new stars, and over hundreds of millions to billions of years the two galaxies merge into one.
Galaxy collisions are gravitational, not star on star.
A galaxy collision is the slowest, quietest crash in the universe, where nothing hits and everything is reshaped.