The Milky Way has grown over time as other galaxies have collided with and been consumed by our galaxy. Each collision triggered wrinkles that still ripple through different families of stars, affecting how they move and behave in space. ESA’s Gaia space telescope aims to unravel the history of our galaxy by studying these wrinkles – and new research from the mission now reveals that the most recent of these galactic collisions took place billions of years later than we thought.
This slider visualises the Milky Way and its surrounding ‘halo’ of stars. Most stars in the Milky Way lie in the disc (like the Sun, for example), but stars from past collisions end up in the halo, a large ‘cloud’ of stars that extends outwards in all directions. These halo stars have been enhanced in this image, but in reality would be very dim compared to the disc.
On the left the halo appears messy and ‘wrinkly’, a sign that a merger has occurred relatively recently (as found by Gaia). On the right it appears smooth and uniform, if our galaxy looks like this it would indicate that a merger has instead occurred in the ancient past.