How our Milky Way once underwent a dramatic flip

The research offers new clues about how our galaxy formed and evolved over billions of years.

Using supercomputer simulations of galaxies like the Milky Way, a team of astronomers from our Department of Physics found that galaxies with slowly rotating stellar haloes are more likely to have experienced a major ‘disc flip’, where the galaxy’s disc changed its orientation by more than 90 degrees.

Durham PhD student Kirill Batrakov presented the study at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham, UK.

Milky Way and galaxies

Most of the Milky Way’s stars are found in its flat spiral disc, while a much larger but sparser halo surrounds it. This halo is mainly made up of stars from smaller galaxies that were absorbed by the Milky Way through past mergers.

We already know that the Milky Way’s stellar halo rotates very slowly, but astronomers have not understood why. 

Researchers studied 25 Milky Way-like galaxies using simulations and found that the slowest-rotating haloes had two things in common: a major head-on galaxy merger and a disc flip.

Cosmic back flip

The Milky Way is known to have collided with a dwarf galaxy called Gaia-Sausage-Enceladus around 10 to 11 billion years ago, matching the first criterion of a major head-on merger. Given an extremely slow rotation of the Milky Way stellar halo, researchers also believe the Milky Way’s disc has undergone a flip, changing the paths of many stars, possibly including our Sun.

The Milky Way is the best laboratory for studying how galaxies and dark matter evolve. Because disc flips do not happen in every galaxy, evidence that the Milky Way experienced one could give astronomers valuable clues about how our galaxy, and others like it, formed over time.

Find out more

• Kirill Batrakov is a PhD student in Astrophysics at Durham University.
• The National Astronomy Meeting is the UK’s largest professional astronomy conference bringing together leading astronomers and space scientists from across the UK and around the world.
• Our Department of Physics is ranked joint 67th in the QS World University Rankings by Subject 2026 and fourth in the UK in the Complete University Guide 2027. Visit our Physics webpages for more information on our undergraduate and postgraduate programmes.
• About the image: Artist’s impression of debris of the Gaia-Enceladus galaxy. Yellow arrows represent the positions and motions of stars originating from Gaia-Enceladus in a simulation of a galactic merger with the Milky Way with characteristics similar to those implied by Gaia data. Credit: ESA (artist’s impression and composition); Koppelman, Villalobos and Helmi (simulation) / CC BY-SA 3.0 IGO