Galaxies are constantly moving through space, driven by gravitational forces. Our Milky Way is locked in a gravitational embrace with its nearest major neighbor, the Andromeda Galaxy. This relationship is not static; the two galaxies are currently rushing toward one another, setting the stage for an inevitable and spectacular cosmic event. This impending merger represents the ultimate fate of our galaxy.
The Two Cosmic Neighbors
The Milky Way and Andromeda (Messier 31 or M31) are the two most massive members of the Local Group of galaxies. They are currently separated by approximately 2.5 million light-years. Andromeda is a large spiral galaxy, and recent research suggests its total mass is roughly comparable to the Milky Way’s.
Evidence of their future meeting comes from Andromeda’s observed movement. The galaxy exhibits a blueshift in its light spectrum, indicating it is approaching us at a high radial velocity. This approach speed is measured to be around 110 kilometers per second, or about 250,000 miles per hour. This motion, driven by mutual gravitational attraction, confirms the pair is on a direct collision course.
The Collision Timeline
Precise astronomical measurements have addressed when this galactic collision will occur. The current scientific estimate places the initial contact between the two galaxies at approximately 4 billion to 4.5 billion years from now. This prediction became certain when astronomers used the Hubble Space Telescope to measure Andromeda’s lateral motion.
Before 2012, the exact trajectory was unknown, and Andromeda might have grazed or missed the Milky Way entirely. Hubble data confirmed that Andromeda’s tangential speed is small compared to its speed of approach, confirming a near head-on trajectory. The first close pass is predicted around 3.9 billion years in the future, with the galaxies merging fully around 5.4 billion years from now.
For context, the Sun will be nearing the end of its main sequence lifetime. It is expected to begin exhausting its hydrogen fuel and evolving into a red giant star just after this initial galactic encounter.
The Galactic Merger Process
Despite the term “collision,” the galactic merger will be a long, drawn-out process governed by gravity, not a sudden crash. Stars within galaxies are separated by immense distances, meaning that direct stellar collisions are highly improbable. The distance between stars is so vast that the galaxies will pass through each other without stars hitting.
Instead of stars hitting, the gravitational influence of the two galaxies will distort and reshape one another through powerful tidal forces. The initial pass will stretch and warp the galaxies, flinging stars into new orbits before they separate and swing back for subsequent passes. This process of multiple interactions will span hundreds of millions of years, with the systems gradually losing orbital energy through dynamical friction.
A noticeable effect of the merger will be a dramatic increase in star formation, known as a starburst. As gas and dust clouds from each galaxy interact and compress, they will trigger the rapid birth of new stars. The central supermassive black holes in both the Milky Way and Andromeda will also spiral toward each other, eventually merging and potentially creating a temporary active galactic nucleus.
The Birth of a New Galaxy
The ultimate result of this multi-billion-year process will be the formation of a single, larger galaxy, informally called “Milkdromeda.” Simulations suggest the final shape will be a massive elliptical galaxy, rather than the spiral shapes of the parent galaxies. This new galaxy will have a spheroidal structure with stars orbiting in a disorganized fashion, unlike a thin, rotating spiral disk.
The stars of both original galaxies will settle into new, stable orbits within this unified system. The fate of our solar system is predicted to be relatively benign, though its location will change significantly. Models indicate a 50 percent chance that the Sun will be displaced to an orbit three times farther from the new galactic core than its current position.
There is also a smaller, though non-zero, chance—estimated at about 12 percent—that the gravitational chaos could eject the solar system entirely, flinging it out into the emptiness of intergalactic space. Regardless of the final position, the Earth and the Sun will remain intact as a system, though the night sky, viewed from a surviving planet, would be dramatically transformed by the brilliant new galaxy.