Our galaxy grew upside down: young stars settled into ever-thinner layers

The Milky Way's disk did not form the way you might picture — orderly and flat from the start. Using precise stellar ages from the Gaia space telescope, David Mirabal of the Instituto de Astrofísica de Canarias found a sharp turn about 10.8 billion years ago: the layer in which new stars were being born suddenly thinned, from more than 4,100 light-years deep to about 1,600, and kept thinning over time to roughly 400 light-years today. In other words, the galaxy settled as it aged — each generation of stars born into a calmer, flatter home than the one before. The reconstruction, in The Astrophysical Journal Letters, gives theorists a hard timeline to match.