Andromeda Is Headed Straight for the Milky Way — What Happens When Two Galaxies Collide

On a clear dark night far from city lights, the Andromeda Galaxy is visible to the naked eye as a faint smudge in the constellation Andromeda — the most distant object humans can see without optical aid. At 2.537 million light-years away, the light entering your eye left Andromeda when our ancestors were making stone tools. And that galaxy is moving toward us at approximately 110 kilometers per second.

NASA scientists, using data from the Hubble Space Telescope and various ground-based observatories, have calculated that Andromeda will begin its collision with the Milky Way in approximately 4.5 billion years, with the two galaxies completing their merger into a single larger elliptical galaxy roughly 6 billion years from now. The precision of this prediction reflects how well astronomers understand the gravitational dynamics of the local group of galaxies — and how clearly the trajectory of Andromeda has been established through decades of careful measurement.

What a Galaxy Collision Actually Looks Like

The phrase "galactic collision" conjures images of catastrophic destruction — billions of stars slamming into each other, solar systems annihilated, the fabric of space torn by the impact. The reality is simultaneously more spectacular and more anticlimactic. Galaxies are mostly empty space.

The distance between stars in a typical galaxy is immense. Our nearest stellar neighbor, Proxima Centauri, is 4.24 light-years away — approximately 40 trillion kilometers. Even in the densest regions of a galaxy, the probability that any given star will physically collide with another star during a galactic merger is very small. Simulations of the Milky Way-Andromeda collision suggest that actual stellar collisions will be rare, and that most stars will simply follow complex trajectories determined by the combined gravitational fields of both merging systems.

What will happen instead is gravitational disruption. Stars will be flung into new orbits by tidal forces as the two galaxies pass through each other — first one pass, then a second, then gradually settling into a merged configuration over hundreds of millions of years. The spiral structure of both galaxies will be destroyed. New star formation will be triggered in regions where gas clouds collide and compress, producing bursts of stellar birth that will light up the merged galaxy in ways neither the Milky Way nor Andromeda currently exhibit.

The Fate of Our Solar System

For the Sun and Earth specifically, the Andromeda collision will most likely not be directly destructive. The probability of the solar system experiencing a close encounter with another star system during the merger — close enough to significantly perturb planetary orbits — is estimated at a few percent. Most likely, the solar system will simply find itself in a new orbital configuration within the merged galaxy, its planets intact and their orbits largely unchanged by the galactic drama occurring over billions of years around them.

There is, however, a more existential concern unrelated to Andromeda: in 5 to 6 billion years, the Sun itself will expand into a red giant, swallowing Mercury, Venus, and possibly Earth. This event is likely to overlap roughly with the later stages of the Milky Way-Andromeda merger. Any civilization concerned about the merger is facing a more pressing local deadline.

A Preview in the Sky

The best preview of what the Milky Way-Andromeda merger will eventually look like is visible in telescopic images of other merging galaxy pairs throughout the universe. Systems like the Antennae Galaxies and the Mice Galaxies show the characteristic tidal tails, bridges of stars pulled between merging systems, and intense star-forming regions that the models predict for our own future merger. These ongoing collisions play out over timescales of hundreds of millions of years — far too slow to watch in real time, but providing a complete snapshot gallery of different merger stages for astronomers to study and compare against their simulations. The future of our galaxy is already on display, in other parts of the universe, in various stages of completion.