The Milky Way and Andromeda are on a collision course, closing the gap between them at roughly 120 kilometers per second. But the timeline is far less certain than scientists once thought. A 2012 analysis of Hubble data predicted a direct impact within 5 billion years. Updated simulations incorporating data from both Hubble and the Gaia spacecraft now put the odds of a collision within the next 10 billion years at about 50-50, with only a 2% chance of a head-on collision in the next 4 to 5 billion years.
Either way, the two largest galaxies in our neighborhood are gravitationally bound to each other. They’re currently separated by about 2.5 million light-years and approaching steadily. What happens when they finally meet won’t be a violent crash so much as a slow, spectacular dance lasting billions of years.
How the Collision Would Unfold
Galaxy collisions don’t happen the way car crashes do. Stars within galaxies are separated by such enormous distances that direct star-on-star impacts are vanishingly unlikely. Instead, the two galaxies would pass through and around each other, their shapes distorted by gravitational pull.
Based on simulations run by the European Space Agency, the most likely scenario involves the Milky Way and Andromeda making their first close pass about 4.5 billion years from now, skimming within roughly 400,000 light-years of each other. After that initial flyby, the galaxies would swing outward again, pulled back by their mutual gravity, then return for another pass. This back-and-forth would repeat, with each pass drawing them closer, pulling long streamers of stars into sweeping tidal tails. Eventually, after billions of additional years, the two would settle into a single merged galaxy.
In about half the simulations ESA researchers ran, the galaxies fly past each other separated by half a million light-years or less, then drift apart before eventually merging in the far future. In many of the remaining cases, they don’t come close enough to lose orbital energy effectively, and the two spirals continue circling each other for an extremely long time.
What the Night Sky Would Look Like
If Earth still exists at that point (the Sun will be nearing the end of its life), the view would be extraordinary. NASA has produced visualizations based on Hubble data showing how the sky would change at each stage.
Around 3.75 billion years from now, Andromeda would fill a huge portion of the night sky, visibly distorting the band of the Milky Way through tidal pull. By 3.85 billion years, the sky would be lit up with brilliant blue-white clusters of newly forming stars. At the 4-billion-year mark, Andromeda would appear tidally stretched across the sky while the Milky Way itself becomes visibly warped. By 5.1 billion years, the cores of both galaxies would be visible as a pair of bright lobes. And by 7 billion years from now, the merger would be complete: a massive elliptical galaxy with a single blazing core dominating the night.
A Burst of New Stars
When galaxies collide, the stars themselves almost never touch. But the vast clouds of gas and dust within each galaxy do interact, and that’s where things get interesting. As the two galaxies pass through each other, their gas clouds compress and fragment into massive, dense clumps. Star formation in those clumps can become up to 10 times more efficient than normal.
That said, the starburst won’t be as dramatic as mergers between gas-rich galaxies earlier in the universe’s history. Both the Milky Way and Andromeda are steadily using up their gas reserves right now, just making stars at their normal pace. By the time the merger is complete, most of the available gas will already have been consumed. The result is a modest bump in star formation, not the spectacular fireworks of more gas-rich collisions.
The Solar System Gets Relocated
The Sun and its planets sit about 25,000 light-years from the center of the Milky Way today. Simulations by astrophysicists T.J. Cox and Avi Loeb at the Harvard-Smithsonian Center for Astrophysics show that the merger would likely fling the solar system outward to roughly 100,000 light-years from the center of the new galaxy, four times its current distance.
This wouldn’t be dangerous. The solar system would remain intact, its planets still orbiting the Sun in their usual paths. Gravitational disruption of individual star systems during galaxy mergers is extremely rare because of the vast distances between stars. You’d simply find yourself in a much lonelier corner of a much larger galaxy, with fewer nearby stars and a more distant view of the galactic core.
Two Black Holes Become One
Both galaxies harbor supermassive black holes at their centers. The Milky Way’s central black hole has a mass of about 4 million Suns. Andromeda’s is estimated to be significantly larger. After the galaxies merge, these two black holes would spiral inward toward the center of the combined galaxy, gradually losing energy through gravitational interactions with surrounding stars.
This inspiral takes time, but not as much as you might expect. Simulations published in the journal Astronomy & Astrophysics estimate that the two black holes would coalesce within about 17 million years after the galaxies fully merge. In cosmic terms, that’s almost instantaneous. Their final plunge together would release a powerful burst of gravitational waves, the kind of ripple through spacetime that next-generation detectors could pick up from similar mergers happening elsewhere in the universe.
The Final Result: An Elliptical Galaxy
The Milky Way is a spiral galaxy with distinct arms of stars, gas, and dust. So is Andromeda. But the merger would destroy both spirals. The gravitational chaos of repeated close passes scrambles the orderly rotation that gives spiral galaxies their shape, scattering stars onto random orbits. The end product would be a large elliptical galaxy, a smooth, rounded collection of mostly older stars without the dramatic arm structure we see today.
Astronomers have nicknamed this future galaxy “Milkomeda.” It would be significantly larger than either parent galaxy but not fundamentally different from the many elliptical galaxies we already observe throughout the universe. Most large elliptical galaxies are thought to have formed through exactly this kind of merger.
The Triangulum Galaxy Joins In
There’s a third galaxy involved. The Triangulum galaxy (M33), the third-largest member of our Local Group, is gravitationally linked to Andromeda and would likely participate in the merger. ESA’s Gaia spacecraft has tracked its motion, and projections show Triangulum’s orbit intertwining with the paths of the Milky Way and Andromeda over the coming billions of years. Its much smaller mass means it would be absorbed into the larger merged galaxy rather than significantly altering the outcome, but it adds one more ingredient to the eventual Milkomeda.