At the heart of Omega Centauri, the most massive globular cluster in our Galaxy, thousands of stars are packed so tightly that their light merges into a single glowing blob when seen from Earth. Separating this swarm into individual stars and determining the composition of each is a task akin to sorting through an overfilled anthill. Astronomers armed themselves with the MUSE spectrograph, which captures thousands of spectra at once, and the neural network DD-Payne, trained to recognize the lines of different elements in them.

In the end, they measured the abundances of carbon, nitrogen, oxygen, magnesium, sodium, calcium, and barium in 7302 red giants within half a light radius of the cluster's center. That's like a chemist taking samples from every resident of a large city and sorting the results by the age of the neighborhoods. It turned out that the stars in Omega Centauri are not all the same: some were born earlier and are poorer in heavy elements, while others formed later and are richer. The difference is especially striking for nitrogen: younger stars have more of it, while for calcium and barium the link with age is weaker.

Such a detailed chemical map allows us to test scenarios for the cluster's birth. For example, if Omega Centauri really was the core of a dwarf galaxy, then stars of different generations should have formed from gas that was gradually enriched in heavy elements by supernova explosions. The observed composition–age trends are consistent with this picture: the younger the stars, the more barium and calcium they contain — elements that are forged in the interiors of old stars and ejected into space when they die.

The work shows that even in the crowded stellar core, the history of each generation can be discerned. The swarm ceases to be a featureless blob and becomes a chronicle, where each star is a line about the gas from which it was born.