Milky Way's star-forming disk ends 40,000 light-years from the center, baffling astronomers

Astronomers mapped ages for about 100,000 bright stars across the Milky Way’s disk and found that active star formation effectively ends at roughly 40,000 light-years from the Galactic center, creating a U-shaped age profile where younger stars lie inward and older ones outward. The outer stars likely reach those distances via radial migration along spiral waves rather than in situ formation. The boundary’s origin could relate to the Milky Way’s bar or disk warp, with Gaia data combined with ground-based spectroscopy (LAMOST and APOGEE) and simulations helping explain the phenomenon, though the exact mechanism remains under study.