How far the Milky Way's disc extends has long been difficult to define — it doesn't end sharply, but fades away gradually at its outer edges. Now, for the first time, an international team of astronomers has identified the edge of the Milky Way's star-forming disc by studying the ages of stars, revealing that the bulk of our Galaxy's star formation occurs within 40,000 light-years of the Galactic Centre.
he researchers employed a new approach combining analysis of the ages of bright, giant stars with advanced computer simulations of galaxy evolution. This method unveiled a "U-shaped" pattern in the distribution of the ages of stars, which pinpoints the edge of the Milky Way's star forming regions.
“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”, remarked the paper’s lead author, Dr Karl Fiteni, now based at the University of Insubria.
Building From the Inside Out
Galaxies do not form stars uniformly across their discs. Instead, they build themselves from the centre outward. Star formation began in their dense central regions and gradually extended outward over billions of years, a process astronomers call "inside-out" growth. This means that, in general, on average, stars are younger the further they are from the centre, since the outer disc is where star formation has only recently reached.
The research revealed that in the Milky Way, the average age of stars does indeed decrease away from the centre, which is exactly what is expected from inside-out growth. However, at around 35,000 to 40,000 light-years from the Galactic Centre, the trend reverses; stars start getting older again with increasing distance. This reversal creates a characteristic “U-shaped” age profile. By comparing this signature with state-of-the-art galaxy simulations, the team showed that the age minimum marks a sharp drop in star formation efficiency, confirming it as the true boundary of the Milky Way's star-forming disc. “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", commented Prof. Joseph Caruana, co-author and supervisor of the project based at the University of Malta.
Why are there stars beyond the edge?
If star formation drops off sharply at this boundary, why are there any stars beyond it at all? The answer lies in a process called "radial migration" — stars gradually drifting outward from where they were born by surfing on spiral waves that sweep through the Galaxy. Just as ocean surfers catch waves that carry them toward shore, stars can catch rides on spiral arms, gradually moving outward from their birthplaces.
Beyond this edge, stars get there primarily by migrating outward from where they formed. Since migration is a slow, random process, with stars catching different spiral waves at different times, it takes progressively longer for stars to reach ever larger distances beyond the radius where star formation ends. This creates the observed pattern where the very furthest stars, beyond the age minimum, are the oldest.
Crucially, these stars move on nearly circular orbits, ruling out the possibility that they were flung to large radii by a collision with another galaxy. Their presence in the outer disc is the quiet, cumulative result of internal Galactic dynamics alone. Prof. Victor P. Debattista, co-author and co-supervisor of the study at the University of Lancashire, explained: "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 that have been scattered to large radii by an infalling satellite galaxy."
Mapping the boundary
The team analysed over 100,000 giant stars using data from the spectroscopic LAMOST and APOGEE surveys, combined with accurate measurements from the Gaia satellite, an ambitious program to map the stars in the Milky Way. By focusing on stars orbiting in the Galaxy's main disc, they isolated the signal of inside-out growth from other Galactic processes. Prof. Laurent Eyer, a co-author from the University of Geneva, remarked: “Gaia is delivering on its promise: by combining its data with ground-based spectroscopy and galaxy simulations, it allows us to decipher the formation history of our Galaxy.”
To confirm that the U-shaped pattern marks the true limit of efficient star formation, the researchers used state-of-the-art galaxy simulations. These models revealed that the age minimum coincides with a sharp drop in the star formation efficiency, demonstrating that the observed pattern requires the outward migration of older stars.
“In astrophysics, we use simulations run on supercomputers to identify the physical mechanisms responsible for the features we observe in galaxies”, explained co-author Dr. João A. S. Amarante, from Shanghai Jiao Tong University. In this study, he added, “they allowed us to demonstrate how stellar migration shapes the age profile of the disc and to identify where the star-forming region ends.”
Why the majority of star formation drops off at this particular radius from the Galactic Centre remains uncertain. The leading suspects are the Milky Way's central bar, whose gravitational influence may cause gas to pool at a certain radius, or the Galaxy's outer warp, where the disc bends and potentially disrupts star formation. While the exact mechanism remains unknown, the research confirms that the U-shaped age profile is a clear signature of the Milky Way's well-defined star-forming boundary.
Looking Ahead
As next-generation surveys like 4MOST and WEAVE will provide even more detailed data, astronomers will be able to refine these measurements and potentially identify which physical processes are responsible for defining the boundary of our Galaxy's star-forming disc. The research also demonstrates how stellar ages, once difficult to measure precisely, have become a powerful tool for Galactic archaeology, allowing astronomers to trace how the Milky Way assembled and evolved over billions of years.
Quotes:
"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." — Dr. Karl Fiteni, University of Insubria
"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." — Prof. Joseph Caruana, University of Malta
"Gaia is delivering on its promise: by combining its data with ground-based spectroscopy and galaxy simulations, it allows us to decipher the formation history of our Galaxy." — Dr Laurent Eyer, University of Geneva
“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 that have been scattered to large radii by an infalling satellite galaxy.” — Prof. Victor P. Debattista, University of Lancashire
"In astrophysics, we use simulations run on supercomputers as a tool to identify the physical mechanisms responsible for creating the features we observe in galaxies, such as the Milky Way. In our current study, for example, these simulations helped us to demonstrate how stellar migration shapes the stellar age profile of galaxies, allowing us to identify the edge of our Galaxy's star-forming disc." — Dr. João A. S. Amarante, Shanghai Jiao Tong University
Astronomy and Astrophysics
Data/statistical analysis
Not applicable
The edge of the Milky Way’s star-forming disc: Evidence from a ’U-shaped’ stellar age profile
13-Apr-2026