A burst of star formation that peaked two billion to three billion years ago spangled the Milky Way with a new generation of stars.

To understand how the Galaxy formed and evolved, astronomers need to know the rate at which its stars are born and how that rate has changed over time. But there is no way to measure the age of individual stars directly.

Roger Mor at the University of Barcelona in Spain and his colleagues turned to data from the Gaia satellite, which precisely measures the distance from Earth to millions of stars. These measurements allow researchers to calculate a star’s true brightness and size, which can be fed into models to infer its age.

The team simulated star formation in the Milky Way over time, and found it was in steady decline until roughly five billion years ago, when production suddenly ramped up. The researchers estimate that half the total mass of all the stars ever created in the Milky Way’s thin disk — which contains most of the Galaxy’s stars — was produced during this period.