Our Milky Way galaxy appears to have flipped 90 degrees long ago. But why?

The Milky Way's spiral disk may have flipped over by 90 degrees, in turn altering the solar system's orbit around the center of the galaxy.

Spiral galaxies such as the Milky Way have two main, visible, structural components: a disk that encapsulates most of its stars, gas and the central black hole, and a more diffuse halo of older stars that encapsulates the disk.

However, the Milky Way's stellar halo has always been a bit of an oddity. The European Space Agency's Gaia mission measured the motion of the halo's stars to find they rotate slowly compared to other material within the spiral disk. Now, supercomputer models led by Kirill Batrakov of the University of Durham have found what triggers this: galactic mergers and spiral disks becoming flipped over.