The Milky Way is a very chilled galaxy. It meekly produces stars at a rate of one solar mass per year, and its supermassive black hole is not having any major outbursts – but it’s not always been like this. During its formative years, from 13 to eight billion years ago, our galaxy was a chaotic and exciting place.

Maosheng Xiang and Hans-Walter Rix from the Max Planck Institute for Astronomy detailed the teenage years of the Milky Way, its collision with other galaxies, its production of stars, and the creation of heavy elements that will over time make planets and eventually us.

The team used the data from the European Space Agency’s Gaia telescope that worked out the position and motion of 1.5 billion stars, plus the Large Sky Area Multi-Object Fibre Spectroscopic Telescope in China that has measured the chemical composition of 9 million stars.

Together, the team extracted the location and motion of 250,000 sub-giant stars, whose age can be determined with high accuracy. A fantastic index of what happened, when, and where, in the history of the Milky Way.

The oldest stars found formed about 800 million years after the Big Bang. The Milky Way started from the collision of small, gas-rich protogalaxies merging to form a flattened shape, 100,000 light-years across and 6,000 light-years thick. This is called the thick disk. The Sun is located in the thin disk, which is within the thick disk and evolved much later.

Based on the observations from this study, published in Nature, the thick disk formed from the get-go of the Milky Way 13 billion years ago. That's two billion years earlier than previously thought. And it also had a very specific peak in star formation roughly 11 billion years ago. This event seems to be connected to the merger between the Milky Way and the Gaia Enceladus Sausage, a satellite galaxy of the Milky Way.

The gravitational disruption caused by the high-speed collision pushed gas clouds together, making it easier to create new stars. Even after the merger phase passed, the thick disk continues to be gas-rich and form new stars.

Increased star formation led to the creation of heavier elements, and the subsequent spread as stars die. These tend to “pollute” newer stars and accumulate in the central regions of the galaxy. The observations show instead that the stars in the thick disk have roughly the same composition, suggesting that the gas was well mixed.

Around 8 billion years ago, the gas contracted to what we call today the thin disk. There’s still star formation happening there, but it’s more moderate, the sign that the galaxy entered a calm and more mature era of development.