Astronomers have uncovered a surprising twist in the Milky Way’s story.
By determining the ages of more than 100,000 giant stars, an international team of researchers has identified the edge of our galaxy’s star-forming disc for the first time, revealing that the most recent star formation is closer to the center than we expected.
Galaxy models predict new stars are generated progressively out from the core, meaning the ages of stars should decrease as you move towards the edge.
Yet the team observed two distinct trends in their data. One involved the inner disc, where younger stars appear further out. Then, around 40,000 light-years from the middle of the Milky Way, the trend reverses, with older stars appearing further out. Together, these gradients form a U-shaped curve, with the youngest stars concentrated at a specific radius.
Lead author and University of Insubria astrophysicist Karl Fiteni explains, “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.”
Two large stellar surveys, LAMOST-DR3 and APOGEE-DR17, together with AstroNN (a neural network distance estimator) and Gaia astrometric data, were used in the study, which was confined to stars near the galaxy’s midplane with highly circular orbits to isolate the intrinsic properties of the disc.
The authors write, “We chose these samples for the reliability of the age estimations and for good coverage of the outer disc.”
The team combined the ages of giant stars with advanced computer simulations to map how stellar ages change with distance. The result is a cosmic fingerprint of birth, movement, and decline that reveals a remarkable boundary structure in the Milky Way disc between about 35,000 and 40,000 light-years from the galactic center.
University of Insubria
This characteristic is a stable feature in surveys, independent of which survey was used. More generally, the radius corresponds to the transition region where the star-forming activity in the galaxy decreases and where the outer disc becomes increasingly less massive.
Co-author Joseph Caruana, an astrophysicist from the University of Malta, notes, “The data now available allow increasingly precise stellar ages to serve as powerful tools for decoding the story of the Milky Way, ushering in a new era of discovery about our home galaxy.”
But if stellar formation stops at this boundary, why do stars exist outside of it at all? The answer is radial migration.
Stars slowly drift outward as they “surf” on spiral waves sweeping through the galaxy. Just as surfers ride ocean waves toward shore, stars grab onto these spiral arms and are carried away from where they were born.
Because migration is gradual and random, it takes longer for stars to reach farther distances. That’s why the stars found farthest out, beyond the age minimum, are the oldest.
This radius aligns with a sharp change in the galaxy’s stellar density profile known as the break radius, the edge of the star-forming disc. Beyond this point, star formation appears to drop sharply.
Simulations show that this break is a real physical limit, not a statistical illusion arising from differences in assumed solar positions or the small number of sources cataloged for other surveys.
The results validate the belief that the Milky Way is a Type II (down-bending) disc galaxy, where stars are more abundant than what you would expect beyond the break radius. This structural property is produced from the competing effects of a star-formation cut-off and radial migration, resulting in a U-shaped age profile, a fossil record of the merger history.
This not only refines our understanding of how the Milky Way formed but also provides a rule for interpreting disc galaxies in general.
The once-thought-quiescent outer disc is shown to be a dynamic region shaped by gravitational interactions among outwardly migrating stars, orbital resonances, and declining star formation.
This research is published in Astronomy & Astrophysics
Source: Università degli Studi dell’Insubria
Fact-checked by Mike McRae
