The Milky Way and Andromeda are heading toward each other, but the collision is far less certain than scientists once thought. For over a decade, the standard answer was a head-on impact in about 4 to 5 billion years. A 2025 study using the latest data from the Hubble and Gaia space telescopes has revised that picture dramatically: there is roughly a 50-50 chance the two galaxies will merge within the next 10 billion years, and only about a 2% chance of a direct collision in the 4-to-5-billion-year window.
What We Thought We Knew
In 2012, astronomers at the Space Telescope Science Institute published a landmark analysis based on years of Hubble observations. They measured Andromeda’s sideways motion across the sky for the first time and found it was remarkably small, just 17 kilometers per second, compared to the 109 kilometers per second at which it’s barreling toward us. That meant Andromeda was on a nearly head-on trajectory. The conclusion seemed firm: a direct collision in no more than 5 billion years, with the two galaxies fully merging into one about 6 billion years from now.
NASA visualizations depicted this timeline in vivid detail. At 3.75 billion years, the galaxies would begin their first close pass. By 4 billion years, tidal forces would warp the Milky Way’s disk and stretch Andromeda into long tails of stars. After swinging past each other once or twice, they’d settle into a single merged galaxy around the 6-billion-year mark.
Why the Odds Have Shifted
The 2025 revision comes from incorporating better measurements and a broader view of what’s happening in our cosmic neighborhood. The Gaia satellite, which maps the positions and motions of stars with extreme precision, provided updated velocity data for Andromeda. But the bigger change was accounting for the gravitational influence of two other massive galaxies: the Triangulum Galaxy (M33), a suspected companion of Andromeda about 3 million light-years from Earth, and the Large Magellanic Cloud, a satellite galaxy orbiting the Milky Way.
These two galaxies are massive enough to tug on the orbits of both the Milky Way and Andromeda in meaningful ways. When researchers ran simulations of the full four-galaxy system using the most accurate position, motion, and mass estimates available, the uncertainties compounded. Small errors in each galaxy’s current speed or weight snowball over billions of years, producing wildly different outcomes. Some simulations still show a merger in 5 billion years. Others show the galaxies missing each other entirely or not merging for well over 10 billion years.
The bottom line: the collision is plausible, even likely on very long timescales, but calling it inevitable is no longer accurate.
What the Collision Would Look Like
If the merger does happen, it would unfold over billions of years, not in a single dramatic crash. Galaxies are mostly empty space. The distance between individual stars is so vast that almost no stars would physically collide with one another. Instead, the event would be gravitational: the two galaxies’ combined pull would reshape each other’s structure.
During the first close pass, tidal forces would fling long streams of stars and gas outward, creating sweeping tails visible across the sky. The Milky Way’s flat spiral disk would buckle and warp. Andromeda, already about twice the Milky Way’s mass, would be stretched and distorted in turn. After one or two passes over roughly a billion years, the galaxies would eventually settle into a single, bloated structure.
The end product would look nothing like either spiral galaxy does today. Simulations consistently show the merger remnant resembling an elliptical galaxy: a large, roughly spherical ball of stars with no defined spiral arms, little organized rotation, and stars moving on random, looping orbits. Researchers have nicknamed this future galaxy “Milkomeda.” Its density profile, stellar orbits, and overall shape match what astronomers see in real elliptical galaxies that likely formed through similar ancient mergers.
What Happens to Our Solar System
The Sun currently orbits in the Milky Way’s relatively calm disk, about 26,000 light-years from the galactic center. During a merger, it would almost certainly not collide with another star. The spaces between stars are simply too enormous. What would change is the Sun’s orbit. Gravitational reshuffling would likely toss the solar system onto a much more elongated path, swinging closer to the new galaxy’s center at some points and farther into its outskirts at others.
There’s also a small statistical chance the solar system could be flung entirely out of the merged galaxy into intergalactic space, riding one of those long tidal tails. But simulations suggest the overwhelming majority of stars in both galaxies end up bound to the final merged structure, just on different orbits than they started with.
None of this would matter for life on Earth. The Sun will exhaust its hydrogen fuel and expand into a red giant in about 5 billion years, rendering Earth uninhabitable long before any galactic merger would be complete.
The Triangulum Galaxy’s Role
The Triangulum Galaxy, the third-largest member of our Local Group, adds an unpredictable element. It currently shows few signs of gravitational interaction with its neighbors, maintaining a tidy spiral structure. But it orbits in Andromeda’s gravitational sphere, and if Andromeda and the Milky Way do collide, M33 would almost certainly be pulled into the event as a third participant. Depending on the timing, it could merge separately with the combined galaxy or be flung onto a distant orbit. Its presence is one of the key factors that makes the long-term outcome so hard to predict.
How the Night Sky Would Change
Andromeda is already the most distant object visible to the naked eye, a faint smudge of light spanning several times the width of the full Moon. As it approaches over the next few billion years, it would grow steadily larger and brighter. In the final billion years before a close pass, it would dominate the night sky, its spiral arms stretching across a huge swath of the view overhead. During the active merger phase, the sky would be filled with unfamiliar patterns of stars, glowing clouds of gas compressed into new star-forming regions, and the two galactic cores shining as bright concentrations of light before eventually combining into one.