When galaxies collide

Galaxies do not stay apart for ever. Their gravity pulls them towards their neighbours, and sometimes two of them meet.

It is not a crash like two cars. A galaxy is mostly empty space, and its stars are so far apart that the two galaxies pass through each other with hardly any stars touching. What does change is the shape. Each galaxy pulls on the other, and neat spirals are bent and stretched, with long streamers of stars thrown out to the sides.

The clouds of gas inside the galaxies are a different matter. They are big enough to run into each other, and when they are squeezed, huge numbers of new stars are born.

In the end the two can settle into one bigger galaxy. Our own galaxy grew this way. About 10 billion years ago the Milky Way swallowed a smaller galaxy, and stars from it are still circling among ours today.

Why galaxies meet at all. Space as a whole is expanding, and distant galaxies are carried away from us. Inside a group or a cluster, though, gravity is stronger than the expansion, and the members fall towards one another. Over billions of years that makes meetings common, and signs of them are seen all across the universe. If two galaxies meet too slowly to carry on past each other, they join into one larger galaxy. This is one of the ways galaxies grow.

What happens when two meet. The stars pass between one another. Stars are tiny compared with the gaps between them, so two galaxies can go right through each other and very few stars will ever touch. Three other things do happen.

The shapes are pulled about. The near side of each galaxy feels the other’s gravity more strongly than the far side does. This is the same uneven pull that raises tides on the Earth, on a far larger scale. It draws stars out into long curved streamers called tidal tails.

Gas clouds collide. Unlike stars, the clouds of gas and dust are big enough to hit each other. They are squeezed, and squeezed gas makes stars. A galaxy going through this is called a starburst.

The orbits are scrambled. The neat circling of a spiral’s disc is stirred into orbits pointing every way. When two large spirals merge, what settles out in the end is expected to be a smooth elliptical.

Material can also pour towards the middle, where most large galaxies keep a giant black hole. When the galaxies merge, their two black holes are expected to end up together and in time become one.

A collision under way. The Antennae are two galaxies in the constellation Corvus, about 65 million light years away. They were once ordinary spirals like the Milky Way and have spent the last few hundred million years passing through each other. Two long tails of stars curve away from the pair and give it its name. The Hubble telescope has found more than a thousand bright young star clusters in them, all set off by the collision. They are expected to finish as one large elliptical.

Not every meeting is a merger.

What happens	Result	Example
Two large spirals merge	Both lose their discs, and a single elliptical is expected to be left	The Antennae, part way through
A large galaxy swallows a small one	The large one keeps its shape, and the small one is pulled apart into streams of stars	The Milky Way and the galaxy it swallowed about 10 billion years ago
A large galaxy merges with a smaller one	Ripples throw gas, dust and stars outwards, and can leave a ring at the edge where new stars form	The ring galaxy AM 0644-741, with a ring 150,000 light years across
A large galaxy pulls at a small one that is still in orbit	A stream of gas is drawn off the small one	The stream of hydrogen trailing from the Magellanic Clouds

The galaxy inside our own. Stars keep a memory of where they came from in the way they move. In 2018 a team using the Gaia space telescope picked out about 30,000 stars, from seven million measured, that travel on long stretched paths the opposite way to most of the Milky Way. They are the remains of another galaxy, named Gaia-Enceladus, which merged with ours about 10 billion years ago. It was about the size of one of the Magellanic Clouds. The Milky Way was much smaller then than it is now, so the two were matched at about four to one. Thirteen globular clusters follow the same paths and must have come with it. Its stars now make up most of the inner part of the halo, the thin scatter of old stars round the disc, and the shake it gave the young Milky Way may have helped to puff up the thick layer of the disc.

Andromeda. The nearest large galaxy is coming towards us at about 110 km/s, which has been known from its light since 1912. What its light cannot show is how fast it is moving sideways, and that decides whether it hits or misses.

Year	What was measured	Conclusion
2012	Hubble’s first measurement of the sideways movement, which came out very small	A direct collision in about 4 billion years, and one merged elliptical galaxy about 2 billion years after that
2025	Later Hubble and Gaia measurements, with every uncertainty carried through and the pulls of Triangulum and the Large Magellanic Cloud included	About an even chance of a collision in the next 10 billion years, and only about 2 in 100 of a head-on collision as soon as 4 to 5 billion years from now

In about half of the 2025 runs the two galaxies pass within half a million light years, lose speed to the drag between their haloes of dark matter, turn back and merge much later. In most of the rest they never come near enough for that drag to work, and they go on circling each other for a very long time. Triangulum’s pull makes a merger a little more likely. The Large Magellanic Cloud tugs the Milky Way out of line and makes it a little less likely.

A sum of our own shows how slow all this is. At a steady 110 km/s, crossing the 2.55 million light years to Andromeda would take about 7 billion years. The 2012 figure was shorter because two galaxies falling together speed up as they close.

If the merger does happen, the Sun and its planets are in no danger of being hit. Simulations suggest the Sun would probably be thrown onto a new orbit much further from the centre.

What the 3D view shows. The view opens outside the Milky Way with Andromeda marked, 2.55 million light years away. Triangulum is beside it at 2.64 million, and the two Magellanic Clouds sit close to our own galaxy at 166,000 and 209,000 light years. These are the four galaxies whose pulls were counted in the 2025 study. Everything is drawn where it is today and nothing moves, because nobody knows which way the meeting will go, and in a human lifetime the gap closes by far too little to see. The stream of gas from the Magellanic Clouds, the stars of the swallowed galaxy and the Antennae are not drawn.