Galaxies are massive structures constantly in motion, bound together and influenced by the relentless force of gravity. Our own home, the Milky Way, is a member of a small cluster called the Local Group, which contains over fifty galaxies. Within this cosmic neighborhood, the two largest members are engaged in an accelerating gravitational dance that will fundamentally reshape our galaxy’s future. Astronomers have mapped the trajectories of our neighbors to predict the ultimate fate of the Milky Way.
The Andromeda Galaxy: Our Inevitable Partner
The galaxy destined to collide with the Milky Way is the Andromeda Galaxy (M31). This massive spiral galaxy is the closest major galactic neighbor to us, currently residing approximately 2.5 million light-years away from Earth. Andromeda is significantly larger than the Milky Way, containing an estimated one trillion stars compared to the Milky Way’s roughly 300 billion stars.
Andromeda is currently hurtling toward us at an immense speed of about 110 kilometers per second. Until recently, astronomers were uncertain if Andromeda’s side-to-side motion, known as tangential velocity, would cause it to merely graze our galaxy or directly collide. Researchers used the Hubble Space Telescope to track the precise positions of stars in Andromeda. These observations allowed scientists to confirm Andromeda’s trajectory and conclude with certainty that a head-on collision is inevitable.
The Physics of Galactic Mergers
When the term “galactic collision” is used, it often conjures an image of stars violently smashing into one another, but the reality is much less dramatic for individual stars. The space between stars, even in the dense galactic core, is so immense that direct stellar collisions are statistically negligible. The probability of any two stars colliding during the merger is less than one in a billion.
The primary interaction during a galactic merger is gravitational, not physical. The galaxies will initially pass through one another, with the immense gravitational forces from both galaxies drastically altering the orbits of stars, gas, and dust. The orderly, disk-like rotation of the spiral galaxies is scrambled into a more random, three-dimensional motion. Dark matter halos surrounding each galaxy also play a major role in the gravitational dynamics, guiding the two stellar systems toward a final, unified structure.
The interstellar medium, composed of gas and dust clouds, is far more susceptible to the collision’s effects than the stars themselves. As these clouds are compressed and violently stirred by the passing gravitational forces, they heat up and collapse. This compression triggers intense periods of star formation, causing a massive “starburst” that will light up the merging system. The two galaxies will pass through each other multiple times over billions of years, gradually losing momentum and merging into a single, cohesive structure.
Precursor Collisions: The Magellanic Clouds
While the Andromeda collision is the most dramatic event on the horizon, the Milky Way is already engaged in a less spectacular, but ongoing, series of mergers. The Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) are two small dwarf galaxies orbiting the Milky Way. They are considered satellite galaxies.
The LMC and SMC are not only interacting with the Milky Way but also with each other, which has distorted their shapes and created a massive stream of gas trailing behind them. The LMC is predicted to fully merge with the Milky Way in approximately 2.4 billion years.
This minor merger will occur much sooner than the Andromeda event and will be a precursor to the main collision. The LMC’s mass is significant enough that its final plunge into the Milky Way’s central region will cause a powerful gravitational disruption. This event will likely trigger a burst of star formation in the Milky Way’s disk, similar to the process that will occur during the later, much larger Andromeda merger.
The Long-Term Outcome: The Birth of Milkomeda
The final, decisive merger between the Milky Way and Andromeda is estimated to begin in about 4 to 4.5 billion years. The process will take hundreds of millions of years to complete, involving multiple close passes before the two galaxies settle into a single, unified structure. The resulting galaxy has been given the nickname Milkomeda.
Milkomeda will not retain the beautiful, flat spiral structure of its parent galaxies. Simulations suggest the merged galaxy will likely be a giant elliptical galaxy, characterized by a smooth, featureless shape and a dense central core. This new morphology is the typical end product of major mergers between two large spiral galaxies. The merger will use up much of the remaining interstellar gas, meaning star formation will slow down considerably.
The ultimate fate of our Solar System is a change in scenery rather than destruction. While the Sun will still be shining, its orbit will be dramatically altered by the new gravitational landscape. It is highly probable that the Sun will be flung far out from its current position, potentially relocating to the distant outskirts of the Milkomeda galaxy. The Sun itself will survive the merger intact, but the familiar constellations in the night sky will be unrecognizable to any beings alive at that time.