The oldest known stars of Population II have been shining for about 13 billion years, so by some measures the Milky Way isn’t much younger than the universe itself. However, the oldest disc stars are much younger – around 9 billion years old. Based on the process of galaxy formation, we can see in the distant early universe that it’s likely the Milky Way began life in the merger of several smaller irregular galaxies. Rippling shock waves compressed their gas to trigger enormous waves of star formation and create supersized star clusters, which eventually became today’s globular clusters. But the shock waves also heated the gas clouds until they were too hot and fast-moving to coalesce under gravity, expanding instead to form a hot halo around the chaotic central ball of stars. As shorter lived stars with higher masses aged and died off, only the lower mass Population II stars were left.
From about 10 billion years ago, the cooling halo gases were able to coalesce once more around the core of the galaxy. Collisions herded them into a flattened disc, and the process of star formation re-ignited to create Population II stars. Each successive generation of these stars processes more of the galaxy’s lightweight raw materials to form heavier elements, and their deaths – particularly the supernova explosions that mark the end of the most massive stars – enrich the interstellar medium with these elements, which are then incorporated into the next wave of stars. Our own Sun formed around 4.6 billion years ago.