Galactic collisions represent the most dramatic and prolonged events in the life cycle of the cosmos, shaping the structure of the universe over billions of years. This process is a slow, gravitational interaction between two or more large stellar systems, not a sudden, catastrophic smash. These encounters unfold over hundreds of millions to a billion years, with the galaxies passing through each other multiple times before finally merging. The gravitational forces involved distort the original shapes, triggering changes that affect everything from stellar orbits to the activity of central supermassive black holes.
The Interaction of Stars and Stellar Systems
The term “collision” is misleading when describing the fate of individual stars, as the space within a galaxy is vast and mostly empty. Although a galaxy contains hundreds of billions of stars, the average distance between them is so great that direct stellar impacts are extremely rare.
Instead of colliding, stars are primarily affected by the changing gravitational field of the merging system. As the galaxies approach and interpenetrate, the combined gravitational pull rapidly changes, a process sometimes called “violent relaxation.” This gravitational disruption pulls stars from their orderly, circular paths and flings them into new, highly elongated orbits. These forces can also stretch out long streamers of stars and gas, known as tidal tails, extending far into intergalactic space. The organized rotation of stars within a spiral disk is transformed into a chaotic, thermalized motion throughout a larger, more spherical volume.
The Fate of Gas, Dust, and Star Formation
The interstellar medium, composed of gas and dust clouds, behaves differently from the widely spaced stars. Unlike stars, these clouds are physically large and can easily collide when the galaxies interpenetrate. These collisions occur at high speeds, compressing the material and creating shockwaves that travel through the gas.
This compression strips gas of its angular momentum, causing it to fall toward the galactic core, where density increases. The resulting dense, cold pockets of gas trigger an increase in the birth rate of new stars, known as a starburst phase. The star formation rate during a major merger can increase by a factor of three or four times the combined rate of the two pre-merger galaxies. This burst of young, bright stars temporarily changes the color and brightness of the merging system, often consuming a large fraction of the available gas fuel.
The Merger of Supermassive Black Holes
Nearly every large galaxy hosts a supermassive black hole (SMBH) at its center, meaning a galactic collision introduces two SMBHs to the same gravitational arena. After the host galaxies merge, the two SMBHs begin a slow, spiraling dance toward each other as a “binary black hole” system. This inward spiral is driven by dynamical friction, where the SMBHs transfer orbital energy to surrounding stars, causing the stars to be flung outward while the black holes sink toward the new galactic center.
In the final stages of the merger, when the black holes are within about a light-year of each other, the process is dominated by the emission of gravitational waves. These ripples in the fabric of spacetime carry away the remaining orbital energy, causing the SMBHs to merge into a single, more massive black hole. The energy release from this final coalescence represents one of the most powerful events in the universe. Additionally, the gas funneled toward the core during the collision can feed the merging black holes, causing them to glow brightly as a temporary Active Galactic Nucleus (AGN) or quasar.
The Resulting Galactic Structure
The final, stable product of a major galactic collision is typically a single, massive elliptical galaxy. The starburst phase consumes or expels much of the cold gas, halting the formation of new stars and leaving a galaxy composed primarily of old, redder stars. The chaotic, randomized orbits, established during the merger’s gravitational mixing, replace the ordered rotation of the original spiral disks, giving the remnant its characteristic smooth, spheroidal shape.
This new structure is often referred to as a “red and dead” galaxy due to its lack of current star formation. For instance, the collision between the Milky Way and the Andromeda galaxy, predicted to begin in about 4.5 billion years, is expected to eventually form a new, giant elliptical galaxy nicknamed Milkomeda. This single, massive galaxy will dominate the local universe.