Andromeda is about 2.5 million light years away and closing at roughly a hundred kilometers a second. Two hundred and twenty thousand miles an hour, give or take, and it has been doing this since before anyone knew what a galaxy was.
Astronomers noticed the approach in 1912, when Andromeda was still catalogued as a nebula and nobody was certain there was anything outside our own galaxy at all. The measurement came first and the understanding of what had been measured came second, which happens more often in astronomy than people expect.
What was the old answer?
In 2012 a team at the Space Telescope Science Institute did something genuinely hard. They measured Andromeda’s sideways motion.
Anything approaching you is easy to measure, because the light shifts. Motion across your line of sight is not, especially at two and a half million light years. Years of Hubble observation produced a number: about 17 kilometers per second sideways, against 109 straight at us.
That ratio is what settled it. Seventeen against a hundred and nine is very nearly head-on. The conclusion followed cleanly. First close pass at 3.75 billion years. Tidal forces warping the Milky Way’s disk and pulling Andromeda into long streamers of stars by four billion. One or two more passes, then a single merged galaxy at around six billion years. Somebody named the result Milkomeda and it stuck.
NASA produced visualizations. Textbooks carried it. For over a decade this was simply what was going to happen.
Why did the answer change in 2025?
Because the two galaxies are not alone, and the old calculation treated them almost as if they were.
The Local Group contains something like a hundred known galaxies. Two of them are big enough to matter here. M33, the Triangulum galaxy, and the Large Magellanic Cloud, which is close enough to us to have a real gravitational say in where we go.
A team led by Till Sawala at the University of Helsinki, with colleagues at Durham and Toulouse, put those into the model. They used newer data from Hubble and from Gaia, the European star-mapping mission that has measured the positions and motions of well over a billion stars. Then they accounted for twenty-two separate variables including the ones nobody knows precisely, the masses and positions and velocities that earlier work had filled in with best guesses.
And they ran it a hundred thousand times.
The result, published in Nature Astronomy in June 2025: roughly a fifty percent chance of a merger within the next ten billion years. And only about two percent for the head-on collision in the four to five billion year window everyone had learned.
What changed the odds?
Two galaxies pulling in opposite directions, which is a satisfying answer because it is so mechanical.
M33 makes a merger more likely. It travels with Andromeda and adds mass to that side of the problem.
The Large Magellanic Cloud makes it less likely. It pulls the Milky Way off the orbital plane and away from Andromeda. Sawala’s phrasing is careful: it does not mean the LMC saves us, it means it makes the merger somewhat less probable.
The larger point is about error, not physics. Every one of those twenty-two variables carries an uncertainty, and uncertainties compound. Enough of them stacked together turn a confident prediction into a coin flip. Nothing was discovered to be wrong in the 2012 work. It was a correct calculation performed on a simplified system with the unknowns pinned to their likeliest values, and that is a different thing from a prediction.
What happens when galaxies collide?
Almost nothing hits anything, and this is the part most people have backwards.
The nearest star to the Sun is about four light years away. Scale the Sun down to a grain of sand and the next grain is fifty kilometers off. A galaxy is not a crowd of objects, it is a small number of specks in an enormous volume of nothing, and when two galaxies pass through each other the odds of any two stars striking are effectively zero.
What does interact is gravity and gas. Tidal forces pull long streams of stars out into tails hundreds of thousands of light years long. Gas clouds, which are enormous and diffuse, do collide, and the shock triggers a burst of star formation. The two central black holes eventually spiral together and merge, which takes a very long time and releases gravitational waves.
The stars themselves are rearranged, not destroyed. The orderly spiral disk becomes a shapeless elliptical blob. As one astronomer put it, the milky band across the sky becomes a milky blob.
Our own Sun is likely to be flung outward into a wider, more distant orbit. Nothing dramatic happens to it. It simply ends up somewhere else, in a galaxy that no longer looks like anything on any star chart.
How do you measure a galaxy moving sideways?
This deserves its own section because it is the most impressive thing in the story and it never gets explained.
Motion toward or away from you is easy to measure. Light from an approaching object shifts blue, from a receding one shifts red, and the size of the shift gives you the speed. Andromeda’s blueshift has been known since 1912.
Motion across your line of sight gives you no shift at all. You have to watch the object physically change position against more distant background objects, and at 2.5 million light years the apparent movement of an entire galaxy is smaller than almost anything else astronomers try to measure.
The 2012 team did it by imaging the same star fields in Andromeda years apart with Hubble and measuring how they had shifted against faraway galaxies behind them. The answer, 17 kilometers per second, came with real uncertainty attached, and that uncertainty is one of the twenty-two variables the 2025 study refused to pin down.
Gaia changed the picture again. It has measured positions and motions for well over a billion stars, which gives a far better handle on the Milky Way’s own mass and movement, and our own galaxy turns out to be one of the harder things to measure precisely because we are inside it.
Has this happened to other galaxies?
Constantly, and we can watch it.
The Antennae Galaxies are two spirals partway through a merger, with the tidal tails that give them their name stretched hundreds of thousands of light years out. NGC 2207 and IC 2163 are in an earlier stage. There are enough examples at enough points in the process to assemble the sequence the way a biologist assembles a life cycle from specimens.
Large galaxies are believed to be built this way. The Milky Way has absorbed smaller galaxies repeatedly, and the streams of stars left over from those events are still detectable, moving through the halo on their own orbits. Andromeda would not be the first galaxy we have eaten. It would be the first one big enough to change what we are.
Does any of this matter for Earth?
No, and the reason is bleak in a way that puts the whole question in perspective.
The Sun brightens as it ages. In roughly a billion years the increase is enough to end the conditions life needs here, which is three to four billion years before the earliest collision scenario. In about five billion years the Sun becomes a red giant and the inner solar system stops being a place.
Whatever the galaxies do, nobody on this planet is watching. The collision is a question about the sky, not about survival.
Which is precisely why it is such good material. It is one of the very few catastrophic events we can describe in detail and be entirely safe from. The stakes are pure curiosity.
Why write it as fiction?
Collision With Andromeda began with one question. What would the Milky Way do if it understood it was going to collide with Andromeda and lose its consciousness?
Not a human talking to a galaxy. The galaxy itself, aware, and afraid of its own ending.
I wrote a few chapters eight years ago, enough to know the shape, and then it went into a pile of half-finished books and sat there. It got finished in 2026 as part of clearing that pile, and of everything in it, this turned into the most ambitious thing I have written.
What it grew into is not a story about two galaxies drifting together. It is about a lonely galaxy that murders the sister it loved, a humanity used as the murder weapon, and the difference between knowing what you have done and understanding it.
The method that got it there is one I think of as dropping inkblots. I do not take a single concept and expand it. I drop several separate concepts on the page and let each one grow arms until the arms touch. Galaxies are alive and conscious. Humans have to interact with that. The galaxy uses them. The humans find out they helped murder an entire galaxy. Other galaxies find out what was done. The book is what forms where the blots finally meet, which is why the threads only connect late.
The real physics matters to a book like that because of scale. Invent the numbers and the reader has nothing to stand on. Keep them real and the strangeness does the work by itself.
What the revision teaches a writer
This is the second Book Background article where the science moved after the book was written, and the lesson repeats.
The 2012 prediction was in every popular account for thirteen years. Any novel that built its plot on a certain collision in four billion years would now be arguing for something the field has stepped back from.
What has not moved: the distance, the approach velocity, the fact that stars do not hit each other, the behavior of gas and tidal tails, the shape of the merged result, and the Sun’s own timeline. Those are measurements and mechanisms. What moved was a probability derived from them.
The rule is the same one Toba teaches. Build on the mechanism and the physical experience, and a revised probability leaves the work standing. Build on the headline number and the work has an expiry date printed on it that you cannot see while you are writing.
The honest current answer
If someone asks whether the Milky Way and Andromeda will collide, the accurate response in 2026 is that nobody knows, and that this is a recent development.
Fifty percent within ten billion years. Two percent for the version everyone learned. The Finnish team’s own summary was that reports of our galaxy’s impending demise appear greatly exaggerated, and their published line is blunter: the fate of our galaxy is still completely open.
That is a better sentence than the certainty it replaced. A galaxy whose future is genuinely undecided is more interesting than one on rails, and it is the honest state of the question.
The novel is Collision With Andromeda. More of the research behind the fiction is in the writing hub.
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