Two nearly identical simulations of a galaxy. The orange and red dot represent the same star in two simulations that differ only minimally from each other. That small difference grows over time into a clearly divergent position. Credit: UL/Portegies Zwart/Asano. Astronomers who simulate galaxies do not always get the same result, even when they start from identical conditions. New research from Leiden University shows that this is not a flaw, but a consequence of how galaxies behave—and how they are modeled. The findings offer, for the first time, a way to address a long-standing question: how chaotic is a galaxy like the Milky Way really? The computer simulations by Tetsuro Asano and Simon Portegies Zwart (Leiden Observatory) will soon be published in Astronomy & Astrophysics and are available now on the arXiv preprint server. The researchers created hundreds of models of Milky Way-like galaxies: flat disks of stars, embedded in a large, invisible cloud of dark matter that holds the system together.…