The Milky Way once flipped upside down. Astronomers have found that our galaxy’s disc inverted about 10 billion years ago after a violent collision with a smaller dwarf galaxy. The discovery reshapes how scientists understand the early life of the galaxy humans call home.
A cosmic smash that turned the galaxy on its head
Researchers from the University of Hertfordshire and the Chinese Academy of Sciences studied data from the European Space Agency’s Gaia space observatory. They tracked the movements and ages of stars in the Milky Way’s disc. What they found surprised them. The stars told a story of a massive collision that literally turned the galaxy’s disc over.
The dwarf galaxy that hit the Milky Way was called Gaia Enceladus. It crashed into our galaxy billions of years ago. The impact was so powerful that it caused the entire disc of the Milky Way to flip. The disc is the flat plane where most of the galaxy’s stars, including our Sun, reside.
Why local astronomers care about a 10 billion year old crash
For astronomers in the United Kingdom and around the world, this finding solves a long standing puzzle. The Milky Way’s disc has two distinct populations of stars. One group is older and thicker. The other is younger and thinner. Scientists could not explain why these two groups existed. The collision and flip theory now offers an answer.
The older stars were born before the crash. The younger stars formed after the disc settled into its new orientation. The Gaia satellite’s precise measurements allowed the team to map stellar positions and ages across the galaxy. This gave them the evidence they needed to confirm the flip.
The study was published in the journal Nature Astronomy. It involved researchers from multiple institutions. The lead author said the data showed clear signs of the ancient upheaval.
A new view of our galaxy’s violent past
The discovery does more than explain the disc’s structure. It reveals that the Milky Way was not always the calm, orderly spiral it appears to be today. In its youth, it was a chaotic place shaped by massive collisions. The flip happened when the galaxy was only about 3 billion years old. That means the Milky Way’s early history was far more turbulent than previously thought.
Gaia Enceladus is no longer a separate galaxy. It was torn apart by the collision and its stars are now scattered throughout the Milky Way. The event that flipped the disc also helped build the galaxy’s halo, the spherical region that surrounds the disc.
For anyone who looks up at the night sky, the finding adds a new layer to the story of the stars. The galaxy that contains Earth was once turned inside out by a crash with another galaxy. The evidence is written in the motions of stars that are still moving from that ancient impact.
