The Andromeda galaxy is drifting on a slow collision course with the Milky Way

About two and a half million light years from Earth, the Andromeda Galaxy is not just a smudge of light in the night sky but an entire spiral galaxy larger than the Milky Way, and it has been falling toward the Milky Way for billions of years. Astronomers have long treated the eventual collision between the two galaxies as one of the most dramatic, if distant, events in the solar system’s future, though recent research has complicated exactly how, and even whether, that merger will unfold as originally predicted.

Two Spiral Galaxies Closing the Gap

Andromeda, also cataloged as M31, is the nearest large spiral galaxy to the Milky Way and the most distant object visible to the naked eye from Earth on a dark night. It contains roughly a trillion stars, considerably more than the few hundred billion estimated within the Milky Way, and it anchors its own system of smaller satellite galaxies much as the Milky Way does. The two galaxies, along with dozens of smaller companions, together make up what astronomers call the Local Group, a loosely bound cluster of galaxies spanning several million light years across otherwise empty intergalactic space.

Measuring a Collision Millions of Years in the Making

For decades astronomers knew Andromeda’s light was blueshifted, meaning the galaxy is moving toward the Milky Way rather than away from it, but they lacked a precise measurement of its sideways motion needed to know whether it would strike the Milky Way head-on or sweep past at a distance. According to Wikipedia’s overview of the Andromeda-Milky Way collision, researchers using the Hubble Space Telescope tracked Andromeda’s tiny apparent shift across the sky over several years and concluded the two galaxies are on a course that would bring them together in roughly four to four and a half billion years. Andromeda is closing the distance at somewhere between about 110 and 300 kilometers per second, a speed that sounds enormous but, across millions of light years, still takes billions of years to close the gap entirely.

Why Stars Won’t Actually Crash Into Each Other

Despite the term collision, the merger will not resemble stars smashing into one another. The distances between individual stars within a galaxy are so vast, measured in light years, that the odds of any two stars actually colliding during the encounter are considered negligible; the Sun and its planets are far more likely to be gravitationally reshuffled into a new orbit within the merged galaxy than to encounter another star directly. What will collide, however, are the vast clouds of gas and dust each galaxy carries, and those interactions are expected to trigger bursts of new star formation as the clouds compress and collapse under increased gravitational pressure, briefly lighting up the merging galaxies with newly formed stars.

The Giant Elliptical Galaxy Left Behind

Simulations of the merger suggest that after an extended period of repeated close passes and gravitational disruption, the two spiral galaxies will eventually settle into a single, larger elliptical galaxy, sometimes informally nicknamed Milkomeda, that would bear little resemblance to either galaxy’s current pinwheel shape. The process is expected to unfold gradually over hundreds of millions of years, rather than as a single violent event, with the galaxies passing through each other more than once before fully merging into one stable structure. Whatever remains of the solar system by then would likely occupy a very different position within that merged galaxy than it does within the Milky Way today, potentially flung outward to its sparser outskirts.

A Timeline Scientists Are Still Revising

Not every astronomer agrees the collision is a certainty. More recent analyses incorporating updated measurements of Andromeda’s motion, along with the gravitational influence of satellite galaxies such as the Large Magellanic Cloud and the Triangulum Galaxy, have found the outcome is less predictable than earlier studies suggested, with some researchers now putting the odds of a direct merger within the next ten billion years at closer to a coin flip rather than a certainty. The uncertainty stems largely from how difficult it remains to measure the small satellite galaxies’ masses precisely, since even modest changes in those figures can shift the simulated outcome from a head-on merger to a distant near-miss.

A Merger Far Beyond Any Human Timescale

Either way, the timescales involved are so vast that the event has no bearing on any planning humanity might realistically undertake, and by the time any merger could occur, the Sun itself is expected to have already evolved into a red giant, reshaping the inner solar system regardless of what has happened to the wider galaxy around it. Astronomers studying the eventual merger treat it less as a threat to worry about than as a natural experiment already unfolding in slow motion, one that offers a preview of how galaxies across the universe have repeatedly combined over cosmic history to build the larger elliptical galaxies seen throughout the observable universe today. Telescopes already capture snapshots of similar mergers happening between other pairs of galaxies at various distances from Earth, effectively showing different stages of a process the Milky Way and Andromeda are only just beginning, giving astronomers a rough visual timeline of what the Local Group’s own distant future may eventually look like.

This article was produced with the assistance of AI and reviewed by Morning Overview editors.

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