Credit: Halo stars: ESA/Gaia/DPAC, T Donlon et al. 2024; Background Milky Way and Magellanic Clouds: Stefan Payne-Wardenaar;; LICENCE: CC BY-SA 3.0 IGO or ESA Standard License
On the left the halo appears messy and ‘wrinkly’, a sign that a merger has occurred relatively recently. On the right it appears smooth and uniform, a sign that a merger has instead occurred in the ancient past.