The Milky Way, our home galaxy, is on an inevitable collision course with its largest neighbor, the Andromeda galaxy (M31). This cosmic event is not a sudden, explosive crash but a slow, gravitational embrace that will unfold over billions of years. The process will fundamentally reshape our galactic environment, transforming two majestic spiral structures into a single, colossal new entity.
The Cosmic Setup and Timeline
The Andromeda galaxy is currently the most massive member of the Local Group and is located about 2.5 million light-years away from the Milky Way. Observations confirm that the two spiral galaxies are approaching each other at a high closing speed. This movement is measured at approximately 110 kilometers per second, or about 245,000 miles per hour, due to their mutual gravitational attraction.
Astronomers were uncertain if the galaxies would merely pass by each other or collide head-on, as the sideways motion was difficult to measure. However, Hubble Space Telescope measurements in 2012 confirmed that Andromeda’s tangential velocity is much smaller than its speed of approach, making a direct collision the most likely scenario. The first significant gravitational interaction is predicted to begin in approximately 4.5 billion years, culminating in a complete merger.
The Physics of Galactic Mergers
A galactic merger is dramatically different from a collision between two solid objects because a galaxy is mostly empty space. The vast distance separating individual stars means that direct star-on-star collisions are extremely unlikely. The average distance between stars is so great that they will almost certainly pass by one another without ever touching.
The collision is primarily a gravitational event involving the total mass of the two systems, especially the massive, invisible dark matter halos that surround each galaxy. These halos dominate the gravitational forces and initiate the interaction, pulling the galaxies toward each other. As the galaxies interpenetrate, their immense gravitational fields distort and stretch the stellar disks, creating long, spectacular tidal tails of stars and gas flung into deep space.
The two galaxies will pass through each other multiple times over the course of several hundred million years, with the gravitational forces progressively slowing their relative motion. This gravitational interaction also compresses the interstellar gas and dust clouds within the galaxies. This compression can trigger a massive wave of new star formation, known as a starburst event, as the gas reaches the necessary density.
The supermassive black holes at the center of the Milky Way and Andromeda will begin a long, slow dance toward each other. Dynamical friction—the transfer of orbital energy to surrounding stars—will cause the black holes to spiral inward toward the center of the new combined galaxy. Once they are within about a light-year of each other, they will lose orbital energy through gravitational waves, eventually merging to form a single, even more massive supermassive black hole.
Immediate Effects on the Solar System
For the Solar System, the most significant change will not be a physical collision but a profound alteration of its galactic address. The intense and rapidly changing gravitational environment during the merger will dramatically disrupt the orbits of billions of stars. Simulations indicate that our Sun has a high probability of being flung into a new orbit, possibly one that is much farther from the galactic core than its current position.
There is a predicted 50% chance that the Solar System will be swept out to a distance three times greater from the galactic center than it is now. Furthermore, there is an estimated 12% chance that the Sun could be completely ejected from the newly forming galaxy and cast adrift into intergalactic space. Within the Solar System itself, the planets will likely remain gravitationally bound to the Sun, as the gravitational influence of distant stars is too weak to pull them away.
By the time the first major galactic pass occurs in approximately 4.5 billion years, conditions on Earth will already be uninhabitable due to the Sun’s natural evolution. As the Sun ages, its luminosity will gradually increase, causing a runaway greenhouse effect that will boil away the oceans and end all terrestrial life long before the merger begins. The Sun will be nearing its transition into a Red Giant, rendering the collision’s effects moot for any life on Earth.
The Final Product A New Galaxy
The ultimate result of the Milky Way and Andromeda merger will be the formation of a single, larger, and gravitationally stable galaxy. This future galactic structure has been informally nicknamed “Milkomeda” or “Milkdromeda” by astronomers. The process of merging two immense spiral galaxies destroys the original disk-like structures.
The final product will not be a spiral galaxy like its predecessors but a giant elliptical galaxy. Elliptical galaxies are characterized by a smooth, featureless, football-like shape and a much more randomized distribution of stellar orbits compared to the orderly rotation of a spiral disk. The merger process converts the organized rotational energy of the two spirals into the more chaotic, three-dimensional motion seen in elliptical galaxies.