It has always been difficult to define where the Milky Way ends, because the Milky Way disk does not suddenly stop, but gradually disappears into space. Now, for the first time, an international team of astronomers has determined the boundaries of a galaxy’s star-forming disk by examining the ages of stars. Their findings show that most star formation in the Milky Way takes place within about 40,000 light-years from the galactic center.
To reach this conclusion, the researchers combined measurements of the ages of bright giant stars with sophisticated simulations of galaxy evolution. This approach reveals a distinct “U-shaped” pattern in how stellar ages are distributed, marking the outer limits of active star formation in the Milky Way.
“The extent of the Milky Way’s star-forming disk has long been an unanswered question in galactic archeology. By mapping how the ages of stars vary across the disk, we now have a clear, quantitative answer,” said lead author Dr Carl Fiteni, currently based at the University of Insubria.
Inside-out growth shapes the galaxy
Galaxies do not form stars evenly across their disks. Instead, they grow outward from the center. Star formation begins in a dense central region and slowly spreads outward over billions of years. This process is known as “inside-out” growth. As a result, stars generally become younger the farther they are from the center, since stars began to form more recently in the outer regions.
The Milky Way follows this pattern to some extent. The study shows that, as expected, the age of stars decreases as they move away from the center. However, approximately 35,000 to 40,000 light-years away from the galactic center, this trend reverses. Beyond this region, the stars age again with increasing distance, forming a characteristic U-shaped age profile.
By comparing this pattern with detailed galaxy simulations, the researchers found that the point at which stars are youngest corresponds to a sharp drop in star formation efficiency. This confirms that it is the true boundary of the Milky Way’s star-forming disk. “With the data now available, increasingly accurate stellar ages serve as a powerful tool for deciphering the story of the Milky Way, ushering in an era of new discoveries about our home galaxy,” commented Professor Joseph Caruana, co-author and supervisor of the project, based at the University of Malta.
Why do stars exist beyond the star formation edge?
Such a sharp decline in star formation at this boundary raises obvious questions. Why are there still stars beyond?
The answer lies in a process called “radial migration.” Stars gradually move outward from their birthplace by interacting with spiral waves within the galaxy. Like a surfer riding an ocean wave, stars gain momentum from their spiral arms and can drift farther over time.
Beyond the edge, few stars formed locally. Instead, they slowly moved outward. This process is gradual and random, so it takes longer for the star to reach further away. This explains why the stars furthest across the boundary tend to be the oldest.
Importantly, these stars follow nearly circular orbits. This rules out the idea that they were thrown outward by collisions with other galaxies. Their presence in the outer disk reflects the stabilizing influence of internal galactic dynamics. Professor Victor P. DeBattista, co-author and co-supervisor of the study from the University of Lancashire, explained: “The important thing about stars in the outer disk is that they are in near-circular orbits, which means they must have formed inside the disk. These stars are not stars that have been scattered over a large radius by falling satellite galaxies.”
Mapping the Milky Way using stellar data
To uncover this boundary, the research team analyzed more than 100,000 giant stars. They used spectroscopic data from the LAMOST and APOGEE surveys, as well as precise measurements from the Gaia satellite, which is mapping stars across the Milky Way galaxy in unprecedented detail.
By focusing specifically on stars orbiting within the galaxy’s main disk, the researchers were able to identify signatures of inside-out growth. This allowed them to isolate it from other processes that could affect the motion and distribution of stars. “Gaia is delivering on its promise. By combining its data with ground-based spectroscopy and galaxy simulations, we will be able to decipher the formation history of galaxies,” said co-author Professor Laurent Eyer of the University of Geneva.
The team then used advanced simulations to confirm that interpretation. These models showed that a U-shaped age pattern naturally arises when star formation declines rapidly and older stars move outward, strengthening the idea that this marks the true edge of the star-forming disk.
“In astrophysics, we use simulations run on supercomputers to identify the physical mechanisms responsible for the features observed in galaxies,” explains co-author Dr. Joanne AS Amarante from Shanghai Jiao Tong University. With this study, he added, “we were able to demonstrate how stellar migration shapes the disk’s age profile and identify where star-forming regions end.”
What controls the star formation boundaries of galaxies?
Although the location of the boundary is now clear, it is still unclear why star formation decreases at this distance. One possibility is the Milky Way’s central bar, whose gravitational pull could cause gas to accumulate in a certain radius. Another is distortions in the outer parts of galaxies, which could bend the disk and disrupt the conditions necessary for star formation.
Although the exact cause is still being investigated, this study confirms that the U-shaped age pattern is a reliable indicator of the Milky Way’s star formation limit.
Looking to future discoveries
Future surveys such as 4MOST and WEAVE will provide even more detailed observations, helping astronomers refine these measurements and better understand what shapes the structure of galaxies.
The study also highlights that measuring the ages of stars, once a major challenge, has become a powerful tool for exploring the history of galaxies. Scientists are developing a clearer picture of how the Milky Way formed by tracing how stars formed and moved over billions of years.
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“The extent of the Milky Way’s star-forming disk has long been an unanswered question in galactic archaeology. Mapping how stellar ages vary across the disk now provides a clear, quantitative answer.” — Dr. Karl Fiteni, University of Insubria
“With the data now available, increasingly accurate stellar ages serve as a powerful tool for deciphering the story of the Milky Way, ushering in an era of new discoveries about our home galaxy.” — Professor Joseph Caruana, University of Malta
“Gaia is delivering on its promise. By combining its data with ground-based spectroscopy and galaxy simulations, we will be able to decipher the formation history of galaxies.” — Dr. Laurent Ayer, University of Geneva
“The important thing about stars in the outer disk is that they are in nearly circular orbits, which means that they had to form inside the disk. These stars are not stars scattered over a large radius by falling satellite galaxies.” — Professor Victor P. Debattista, University of Lancashire
“Astrophysics uses simulations run on supercomputers as a tool to identify the physical mechanisms that produce the features observed in galaxies such as the Milky Way. For example, in this study, these simulations helped demonstrate how stellar migration shapes the stellar age profile of a galaxy, allowing us to identify the edge of a galaxy’s star-forming disk.” — Dr. Joao AS Amarante, Shanghai Jiao Tong University
