Galaxy Collision
A galaxy collision is the gravitational interaction between two or more galaxies that move closer together and eventually merge. Despite the term “collision,” the stars themselves very rarely come into direct contact—the interstellar distances are too great. It is gravitational forces that distort, disrupt, and ultimately merge the galaxies into a single object over hundreds of millions of years.
A Slow and Transformative Process
The collision of two galaxies is an extremely slow process on a human timescale. When two galaxies approach each other, their tidal tails—long streams of stars torn away by gravity—form first. The two galaxies sometimes pass through each other several times before finally merging. The entire process, from initial contact to complete merger, can take a billion years or more.
Triggering of Star Formation
Galaxy collisions are violent events for interstellar gas, even though individual stars do not collide. Gas clouds, which are much more extensive than stars, collide, compress, and trigger powerful waves of star formation. These starburst episodes produce thousands of stars per year— compared to just a few in a normal galaxy—giving rise to particularly luminous galaxies.
The Fate of the Milky Way
The Andromeda Galaxy is approaching the Milky Way at a speed of about 110 kilometers per second. In about 4 to 5 billion years, the two galaxies will collide. The complete merger will take several more billion years, resulting in the formation of a large elliptical galaxy sometimes called Milkomède. This prospect poses no danger to individual stars, but will completely transform the structure of our galaxy.
A concrete example
The Antennae (NGC 4038 and NGC 4039) are a spectacular example of an ongoing galaxy collision, located about 45 million light-years away in the constellation Corvus. The long tidal tails that characterize them have given this system its popular name. Images from the Hubble Space Telescope have revealed thousands of regions of active star formation there.
Frequently Asked Questions
Do stars collide with each other during a galaxy merger?
Virtually never. Stars are so small compared to the distances between them that the probability of a direct collision is negligible, even when two galaxies merge. The Milky Way contains about 200 billion stars spread out over 100,000 light-years— they are immensely far apart from one another.
What happens to the central black hole during a galaxy merger?
When two galaxies merge, their central supermassive black holes gradually draw closer together due to dynamic friction with the surrounding stars. They eventually form a binary black hole system, then merge themselves, emitting a powerful burst of gravitational waves. This type of event is one of the priority targets for the future LISA space detector.
Are galaxy collisions common?
Yes, on a cosmic scale. Interactions between galaxies were much more frequent in the early universe, when galaxies were closer together and more numerous. Today, about 25% of large galaxies show signs of ongoing interaction or merger. The Milky Way has already absorbed several dwarf galaxies throughout its history.