The Milky Way meets Andromeda

How our galaxy and its giant neighbour may merge into one, over billions of years

Step by step

Two giant neighbours

Andromeda is the most distant thing most people can see with their own eyes: on a dark night it is a faint smudge in the constellation Andromeda. The light we see left it about 2.5 million years ago. It spans six times the width of the full Moon, though only its bright centre shows to the eye.

Both are spiral galaxies: flat disks of stars and gas turning round a bright bulge, wrapped in a far larger halo of dark matter. Counting that dark matter, each weighs about a trillion Suns. The Sun is marked here, some 26,000 light-years from our galaxy's centre.

Measured from Earth

Andromeda's light is shifted towards blue: it is coming towards us at 301 km/s. Most of that is the Sun's own motion round our galaxy; towards our galaxy's centre, Andromeda closes at about 115 km/s.

Speed along our line of sight is easy to measure from the shift in a galaxy's light. Speed across the sky is not: at Andromeda's distance, even 100 km/s sideways moves it less than a millionth of a degree in a century. Hubble, and then the Gaia space telescope, measured that drift by following its stars against far more distant galaxies for years.

That sideways speed decides everything. Were it zero, the two galaxies would fall straight into each other. It is small but not zero, and not yet known precisely, so no one can say for certain how closely they will pass. This is Andromeda as we see it, its disk steeply tilted to us.

A coin toss

Here are dozens of futures the measurements allow, each Andromeda's path over the next 10 billion years. In about half, the two galaxies merge; in the rest they pass at a distance and never meet again.

In 2025 astronomers followed the Local Group's next 10 billion years in thousands of simulations, each starting from different values within the measurements' errors: the galaxies' masses, distances and motions. The Milky Way and Andromeda merged within that time in only about half of them, most often around 7.6 billion years from now.

The two biggest companions matter too. The Triangulum Galaxy, falling into Andromeda, makes a merger more likely; the Large Magellanic Cloud, whose orbit crosses theirs at right angles, makes it less likely. The story follows one future in which they do merge. Better measurements, above all of Andromeda's sideways motion, will settle which it is.

A small galaxy falls in

The Large Magellanic Cloud is 160,000 light-years away and easy to see with the eye from the southern hemisphere. With its dark matter it has over a tenth of the Milky Way's mass. Moving through our galaxy's halo, it gathers a wake of dark matter behind it that pulls it back: dynamical friction, the same drag that will bring Andromeda in.

Tides tear it apart as it falls, and its stars end up spread through the Milky Way, as those of many smaller galaxies swallowed before it already are. How it is torn apart is drawn here: only simulations that follow the pull of every star on every other can show it in detail.

Andromeda's companion

The Triangulum Galaxy is the third largest of the Local Group, a spiral a little over half the Milky Way's width, 2.8 million light-years from us. Like Andromeda, it can just be seen with the eye under a very dark sky. It seems to be falling towards Andromeda for the first time.

Its pull matters for our own future. Falling into Andromeda, it tugs Andromeda sideways, and in the study's simulations that made a merger with the Milky Way more likely. Here too, how its stars are torn apart and spread through Andromeda is drawn.

Closing in

Time sped up: over billions of years the two galaxies fall together, faster and faster, pulled by each other's gravity. Andromeda grows in our night sky.

Most of what pulls them together is dark matter: each galaxy's halo of it weighs several times its stars and reaches hundreds of thousands of light-years out. Once the haloes overlap, they drag on each other, and that is what slows the galaxies enough to merge.

From Earth, Andromeda would grow larger and brighter until it filled much of the sky, its disk tilted as we see it today. By then, though, the Sun will have changed: about five billion years from now it begins to swell into a red giant.

The first pass

Each galaxy pulls harder on the near side of the other than on its far side, stretching it. How much a disk is torn depends on how it turns: Andromeda's turns the same way as their orbit, so its outer stars are pulled along and flung out, while the Milky Way's, turning against it, holds together better.

The galaxies pass through each other's haloes of dark matter, which drag on them and rob them of speed. Their stars almost never hit each other: they are so far apart that even as two whole galaxies overlap, a collision between two stars is extremely rare.

Apart, and back again

Without the drag of their dark matter they could swing far apart again. With it, each pass takes away more of their orbit's energy, so the next comes sooner and closer. In the study's simulations, most mergers came on the second pass.

Meanwhile the stirred-up disks settle a little, but the stars flung out in the first pass keep drifting outwards, forming faint streams and shells round both galaxies, like those astronomers see round many galaxies that have met another.

The merger

As the galaxies merge, the pull on every star changes quickly, throwing their orderly circling into loops in every direction. Clouds of gas crash together and may set off a burst of new stars, and gas falling to the centre can feed the black holes, which in time sink together and merge too.

The Milky Way's black hole, Sagittarius A*, weighs about four million Suns and Andromeda's about a hundred million. How the merger's end unfolds is drawn here after what full simulations show, for the stars shown here do not pull on each other.

One galaxy

A few hundred million years on, the two have become one round galaxy of old stars without spiral arms: an elliptical galaxy, sometimes called Milkomeda.

Elliptical galaxies are common among the largest galaxies we see, and many are thought to have formed in mergers like this. With little gas left, few new stars are born, and over billions of years the galaxy's light grows redder as its bright blue stars die out.

The Large Magellanic Cloud and the Triangulum Galaxy are part of it, and many of the Local Group's smaller galaxies may follow. Galaxies beyond the group will by then have drawn farther away, carried off by the expansion of the universe.

Where the Sun ends up

The Sun survives the merger, as nearly every star does. In this future it ends up farther from the new galaxy's centre than it is from ours today.

Simulations of the merger agree that the Sun is most likely carried farther out, though exactly where is impossible to predict. Nearby stars would not come close enough to disturb the planets: even as the galaxies merge, stars stay trillions of kilometres apart.

By then the Sun will no longer be the star it is now. About 7.6 billion years from now it reaches its largest as a red giant, then sheds its outer layers and becomes a white dwarf, a dim ember about the size of Earth, still circling in the new galaxy.