There is no sharp line where the Milky Way simply stops. But astronomers say they have now pinned down the edge of the galaxy’s star-forming disc, and it sits about 35,000 to 40,000 light-years from the Galactic Center.
The finding comes from an international team that mapped the ages of stars across the Milky Way’s disc and identified a distinct U-shaped pattern. The study found stellar ages get younger with distance from the centre, up to a point. Then, at roughly 35,000 to 40,000 light-years out, the pattern flips and stars get older again.
Researchers said that turning point marks a sharp drop in star formation efficiency and identifies the true boundary of the Milky Way’s star-forming region.
“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,” lead author Dr Karl Fiteni of the University of Insubria said.
The team analysed more than 100,000 giant stars using spectroscopic data from the LAMOST and APOGEE surveys, along with measurements from the Gaia satellite. They then compared the observations with simulations of galaxy evolution.
The researchers said the Milky Way follows an inside-out growth pattern, with star formation beginning in dense central regions and spreading outward over billions of years. That means stars are generally younger farther from the centre, until the outer limit of active star formation is reached.
Beyond that edge, stars are still present, but the study says most of them did not form there. Instead, they gradually migrated outward from their birthplaces through interactions with spiral waves in the galaxy.
Because that process is slow and random, the most distant stars beyond the boundary tend to be the oldest, according to the study.
“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,” co-author Prof Victor P. Debattista of the University of Lancashire said.
The team said those near-circular orbits rule out the idea that the stars were flung outward by collisions with other galaxies.
Prof Joseph Caruana of the University of Malta said improved stellar age data is changing what astronomers can do with the Milky Way.
“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,” he said.
Co-author Dr Laurent Eyer of the University of Geneva said Gaia had been central to the work.
“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,” he said.
The simulations also helped the team test what they were seeing in the data.
“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 of Shanghai Jiao Tong University said.
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