The Andromeda Galaxy is the largest galaxy in our Local Group and is currently hurtling toward our own Milky Way galaxy. This event is a scientific certainty, confirmed by precise measurements of Andromeda’s motion, and it will fundamentally reshape our cosmic neighborhood. The gravitational encounter between these two spiral galaxies will result in a single, massive celestial structure, which astronomers nickname Milkomeda or Milkdromeda. This galactic merger is one of the most transformative events predicted in the deep future of our universe.
The Cosmic Timeline and Approach
The gravitational attraction between the Milky Way and Andromeda is currently causing them to rush toward each other at a speed of approximately 110 kilometers per second, or about 250,000 miles per hour. Astronomers confirmed this trajectory by using the Hubble Space Telescope to measure Andromeda’s lateral motion across the sky. These observations demonstrated that Andromeda’s tangential velocity is too small for it to miss us completely.
The first close pass between the two galaxies is projected to occur in about 4.5 billion years. While some earlier estimates suggested a slightly sooner date, the latest data from the European Space Agency’s Gaia spacecraft refined the timing to this 4.5-billion-year mark. This first major encounter will begin a process that unfolds over billions of years, driven by the immense mutual gravitational pull.
Mechanics of a Galactic Merger
The event is more accurately described as a gravitational interaction and merger rather than a simple, destructive collision. Stars within a galaxy are separated by such vast distances that the probability of any two stars physically impacting each other is extremely low.
The galaxies will pass through one another, with their immense gravitational fields reshaping the structures. This initial pass will dramatically distort the spiral shapes of both the Milky Way and Andromeda due to tidal forces. After the first close encounter, gravity will pull the galaxies back toward each other, leading to a series of subsequent passes over hundreds of millions of years.
The combined dark matter halos, which envelop both galaxies, play a significant role in the merger process. These massive, invisible components interact gravitationally, slowing the galaxies down through dynamical friction. This loss of orbital energy causes the two galaxies to spiral inward until they finally coalesce into a single, unified body.
Fate of the Solar System and Earth
For the Solar System, the likelihood of the Sun physically colliding with another star is negligible. The primary effect on our system will be a dramatic change in its location and orbit within the newly forming galaxy. Simulations suggest a roughly 50% chance that the Solar System will be scattered outward to an orbit three times farther from the galactic core than its current position.
There is also a smaller, but significant, chance—around 12%—that the Solar System could be ejected entirely from the new galaxy and flung into intergalactic space. In either scenario, the environment immediately surrounding the Sun and its planets is predicted to remain stable. Gravitational perturbations could slightly alter the orbits of planets, but the Sun’s grip on Earth and the other planets would likely remain intact.
By the time this galactic merger begins in 4.5 billion years, the Sun will already be nearing the end of its life as a main-sequence star. Its luminosity will have increased by 35–40%, initiating a runaway greenhouse effect that will have boiled away Earth’s oceans and ended all terrestrial life long before the galaxies touch. The merger itself will therefore be an astronomical spectacle rather than an existential threat to life, which will have already ceased due to stellar evolution.
The Aftermath
The final structure, Milkomeda, will be a single, massive galaxy of more than one trillion stars. Repeated passes and gravitational settling will destroy the distinct spiral arms of the Milky Way and Andromeda. The resulting structure is predicted to be a giant elliptical galaxy, a smooth, football-shaped system with stars orbiting in random directions rather than in a flat disk.
The merger will cause massive clouds of gas and dust from both galaxies to compress, triggering a final burst of star formation. This starburst phase will temporarily light up the new galaxy as stars are born, though it will be relatively weaker than in other mergers because both the Milky Way and Andromeda will have consumed much of their gas by then. Following this burst, the star-forming material will be largely exhausted, leading to a period of quiescence as the elliptical galaxy ages.
The supermassive black holes at the centers of both original galaxies will eventually spiral inward and merge into a single, larger supermassive black hole at the core of Milkomeda. This process could take millions of years and may briefly create an active galactic nucleus, or quasar, as infalling gas is violently heated before the final coalescence.