An international team of astronomers has determined that the Milky Way's star-forming disk ends around 35,000 to 40,000 light-years from the galactic center. Using stellar age mapping, they found a U-shaped pattern where star formation drops sharply beyond this boundary. Stars farther out are mostly older migrants drifting from inner regions.
Researchers analyzed ages of more than 100,000 giant stars using data from the LAMOST and APOGEE surveys, combined with measurements from the Gaia satellite. This revealed that stellar ages decrease with distance from the center up to about 35,000 to 40,000 light-years, then increase again, forming a U-shaped profile. The pattern indicates inside-out growth of the galaxy, with star formation ceasing abruptly at the edge, as confirmed by galaxy evolution simulations. Dr. Karl Fiteni, lead author now at the University of Insubria, stated, 'The extent of the Milky Way's star-forming disc has long been an open question in Galactic archaeology; by mapping how stellar ages change across the disc, we now have a clear, quantitative answer.' Beyond this boundary, stars exist due to radial migration, where they drift outward on nearly circular orbits influenced by spiral arms. Prof. Victor P. Debattista of the University of Lancashire noted, 'A key point about the stars in the outer disc is that they are on close to circular orbits, meaning that they had to have formed in the disc.' These are not stars scattered by external collisions. Prof. Joseph Caruana of the University of Malta added, 'The data now available allow increasingly precise stellar ages to serve as powerful tools for decoding the story of the Milky Way.' The findings, detailed in a study published in Astronomy, highlight Gaia's role in mapping the galaxy's history, with Prof. Laurent Eyer of the University of Geneva saying, 'Gaia is delivering on its promise.' Future surveys like 4MOST and WEAVE will refine these insights.