Astronomers have identified the Milky Way’s first two building blocks

When galaxies collide and merge, several processes happen in parallel: each one carries with it its own reservoir of hydrogen gas; when they merge, these clouds destabilise and many new stars form within them.
“Of course, interacting galaxies already have their own stars, and when they merge, they blend together. In the long run, such ‘families’ will form part of the stellar population of the newly formed galaxy. Once the merger is complete, it may seem hopeless to determine which predecessor galaxy the particular star streams belonged to, but there are actually some ways to trace their lineage,” the study authors explained.