The Andromeda collision: what happens in 4 billion years
Two galaxies merging, and essentially zero chance of any two stars hitting each other.
4 min read · Published 27 August 2026
The short answer
Andromeda is approaching the Milky Way at roughly 110 km/s and the two are expected to interact in around four billion years, eventually merging into a single elliptical galaxy. Essentially no stars will collide — the spacing between stars is so vast relative to their size that direct hits are negligibly unlikely. Recent analyses have added genuine uncertainty about whether a merger is certain.
Why nothing will hit anything
This is the part that surprises people, and it is a matter of scale.
The Sun is about 1.4 million kilometres across. The nearest star is about 40 trillion kilometres away — roughly 30 million times the Sun's diameter.
Scaled down, if the Sun were a grain of sand, the nearest grain would be tens of kilometres away. Two clouds of sand grains at those separations passing through each other produce essentially no contacts.
So the galaxies will pass through one another with the stars themselves almost entirely unaffected by direct collision. What changes is the orbits, rearranged by the combined gravitational field.
The likely sequence
Over roughly two billion years of interaction
- First approach. Tidal forces begin distorting both discs, drawing out long streams of stars and gas.
- First pass. The galaxies pass through each other. Spiral structure is severely disrupted; gas compression triggers widespread star formation.
- Separation and return. They move apart, slowed by mutual gravity and dynamical friction, then fall back together.
- Further passes. Successive encounters progressively destroy the ordered rotation of both discs.
- Merger. The two settle into a single galaxy with stars on largely random orbits — a giant elliptical, of the kind described in galaxy types.
- The black holes merge. The central supermassive black holes spiral together over a long period and eventually coalesce, releasing enormous gravitational wave energy.
The recent uncertainty
The merger has often been presented as certain. Recent analyses incorporating improved measurements of Andromeda's transverse motion — its sideways velocity, which is far harder to measure than its approach speed — have found more uncertainty than previously assumed.
The approach velocity is straightforward: it comes from a Doppler shift. Sideways motion must be measured as a tiny angular displacement over years, at a distance of 2.5 million light years, and small errors change the outcome substantially.
Depending on the transverse velocity, and on the gravitational influence of other Local Group members, outcomes range from a direct merger to a more distant passage that may or may not lead to a later merger.
So the honest statement is that a merger remains likely and is no longer a certainty. That shift is a good illustration of how a widely repeated fact can rest on a measurement harder than it appears.
What Earth experiences
Nothing dramatic, and it will not matter.
The Sun's orbit within the merged galaxy would change, possibly moving it much further out. Night skies would be spectacular during the interaction — a second galaxy filling a large part of the sky, and intense star formation regions visible.
But the timescale is the point. In about five billion years the Sun becomes a red giant, and Earth becomes uninhabitable well before that — likely within one to two billion years as solar luminosity rises.
So the merger is not a threat to anything. It is an event our planet will not be habitable for, involving a galaxy that will not damage us, on a timescale where the Sun's own evolution matters far more.
Common questions
- Will Andromeda collide with the Milky Way?
- It is approaching at roughly 110 km/s and a merger in around four billion years has long been the expectation. Recent analyses using improved measurements of its sideways motion have introduced genuine uncertainty, so a merger is likely rather than certain.
- Will stars collide when the galaxies merge?
- Essentially none. The spacing between stars is around 30 million times a star's own diameter — scaled down, grains of sand tens of kilometres apart. The galaxies will pass through each other with almost no direct stellar contacts.
- What will the merged galaxy look like?
- A giant elliptical. Successive passes destroy the ordered rotation of both discs, leaving stars on largely random orbits with little remaining gas after the merger-triggered starburst consumes it.
- Will the collision destroy Earth?
- No, and Earth will not be habitable by then regardless. Rising solar luminosity is expected to make the planet uninhabitable within one to two billion years, and the Sun becomes a red giant in about five billion.