How an ancient collision with another galaxy transformed the Milky Way

Astrophysicists have found evidence that the Milky Way collided and merged with a dwarf galaxy around 12 billion years ago. Using the Hubble Space Telescope, the team found that the makeup of some globular clusters — groups of stars held tightly together by gravity — doesn’t match that of those that are thought to be native to the Milky Way or that arrived during a later merger with another dwarf galaxy known as the Gaia–Sausage–Enceladus. “We do not know how our house was built,” says astrophysicist and study co-author Davide Massari. But these findings point to “where the first significant batch of bricks came from”.