The Milky Way's ancient stars once belonged to other galaxies instead of being the earliest stars born inside the galaxy when it began to form about 10 billion years ago. Many of the Milky Way's ancient stars are remnants of other smaller galaxies ripped asunder by violent galactic collisions around five billion years ago, according to research that was part of the Aquarius Project, which uses the largest supercomputer simulations to study the formation of galaxies like the Milky Way.