GoKawiilAstronomers have located the edge of the Milky Way’s star-forming disk for the first time, showing that star formation is focused within 40,000 light-years of our galactic center.
An artist's illustration of the Milky Way's warped stellar disk, based on Gaia data.
Stefan Payne-Wardenaar; Magellanic Clouds: Robert Gendler / ESO
Disk galaxies like the Milky Way form stars “inside-out” — starting from the center and working outwards through the disk. So, as a general rule, the farther out astronomers look, the younger the stars are.
Now, a team led by Karl Fiteni (then at University of Malta), carried out under the supervision of Joseph Caruana and Victor Debattista, has analyzed more than 100,000 giant stars. By coupling observations with advanced computer simulations, the astronomers show that this inside-out pattern reverses at between 35,000 and 40,000 light-years from the Milky Way’s center. Beyond this distance, the stars are older again.
Inside-out growth and stellar migration in the Milky Way: Inside the star-forming disc (within ~12 kpc), abundant cold gas fuels continuous star formation, producing young stars. Beyond this break radius, star formation drops sharply, and the outer regions are instead dominated by stars that formed in the inner disk and later migrated outward.
Joseph Caruana et al. / Astronomy & Astrophysics 2026
The sudden switch-up creates a U-shaped age profile. The nadir of this curve marks a steep decline in star formation, and that marks the edge of the Milky Way’s star-forming disk. “The extent of the Milky Way’s star-forming disk has long been an open question in galactic archaeology,” Fiteni says. “By mapping how stellar ages change across the disc, we now have a clear, quantitative answer.”
The analysis involves data from the LAMOST and APOGEE spectroscopic surveys, as well as measurements from the European Space Agency’s Gaia satellite. Fiteni’s team focused on red giant branch stars, whose ages can be estimated with relatively high precision. The results are published in Astronomy & Astrophysics.
The stars beyond this boundary probably weren’t formed in situ. But they didn’t come from infalling satellite galaxies either. Instead, they likely migrated outward over time. “A key point about the stars in the outer disk is that they are on close-to-circular orbits, meaning that they had to have formed in the disk,” says team member Victor Debattista (University of Lancashire, UK).
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