Milky Way's true edge defined: star formation stops about 40,000 light-years out

Astronomers mapped the Milky Way’s star-forming disk by analyzing stellar ages with Gaia data and ground-based surveys (LAMOST, APOGEE) and simulations. They found the outer edge of active star formation at about 35,000–40,000 light-years from the Galactic Center, marked by a U-shaped age pattern where ages dip closest to the center and rise beyond the edge. Beyond this boundary, most stars migrated outward rather than formed in situ, explaining why older stars populate the far outer disk. This defines the true edge of the Milky Way’s star-forming region and highlights inside-out growth and radial migration as key Galactic processes.