The Milky Way grew by devouring galaxies, leaving a trail of ghost stars

There have always been stories about them. Australian Aboriginal tales tell of an old couple called the Jukara, while for Chile’s Mapuche people they are Rvganko or water ponds. To modern astronomers, they are the Small and Large Magellanic clouds, two of the closest galaxies to us that are in the process of being swallowed by the Milky Way.

They are evidence of a kind of snowball effect that characterises the formation and evolution of galaxies, which scientists believe are the result of a series of violent collisions. As galaxies fall into one another, the smaller ones are ripped apart by tidal forces of the larger, leaving behind streams and clusters of stars that don’t move in lockstep with the rest of the galaxy. They also stand out for their chemical compositions, which allow astronomers to group stars that were born together and piece back their rough ages.

And together, age, chemistry, and motion tell astronomers different things about stars. Galactic archaeologists use precise measures and larger sample sizes afforded by new missions to tease apart minute differences between groups of stars, revealing the history of the Milky Way going back almost 12 billion years.