The Milky Way galaxy as we know it today was shaped by a colossal cosmic crash 10 billion years ago, which knocked the entire galactic disc on its side.
According to astronomers, a wayward dwarf galaxy known as the Gaia Sausage collided head-on with the Milky Way long before our solar system was even born. While the Milky Way ultimately tore the intruder to shreds and absorbed its stars, the chaotic event triggered a slow but dramatic shift, rotating the entire galactic disc by more than 90 degrees into its current position.
This dramatic "disc flip" likely took a few hundred million years to complete, according to researchers from Durham University. The scientists made the discovery while trying to solve a long-standing mystery about the rotation of stars in the galactic halo—the sparse, spherical cloud of stars surrounding the main disc of the galaxy.
While stars in the Milky Way's main disc orbit the galactic center at a rapid 220 kilometers per second, stars in the halo move far more slowly, at only about 25 kilometers per second. To understand this massive difference, the Durham team used supercomputer simulations to model how Milky Way-like galaxies evolve. The simulations revealed that head-on collisions with dwarf galaxies of a certain size would naturally lead to a disc flip, leaving behind slow-moving stars in the halo.
The ancient collision was first detected in 2018, when astronomers used data from the European Space Agency’s Gaia satellite to map the motion of stars. They noticed a group of stars moving in unusual, sausage-shaped orbits—a clear sign of a past collision with an incoming galaxy, which was subsequently named the Gaia Sausage.
Although classified as a dwarf galaxy, the Gaia Sausage was immense, containing stars, gas, and dark matter with a total mass over 10 billion times that of our sun. Astronomers now view the event as a defining milestone in our galaxy's history, as it reshaped the Milky Way, formed the central bulge, and populated the stellar halo.
Such catastrophic events are rare, and the Milky Way's next major collision is not expected for a few billion years, when it is predicted to merge with the Large Magellanic Cloud. While another merger is currently underway with the much smaller Sagittarius dwarf galaxy, scientists expect it to have a minor impact. The Milky Way has grown significantly since its ancient collision with the Gaia Sausage, making it far too massive to be easily knocked off course again.