For many years, astronomers have wondered where the Milky Way truly “ends.” Unlike a solid object, our galaxy does not have a clear boundary.
Its outer regions slowly fade into space, making it difficult to say exactly where its main structure stops.
Now, a new study has provided the clearest answer yet by identifying the outer limit of the Milky Way’s star-forming disc.
The research, published in Astronomy & Astrophysics, shows that most stars in our galaxy form within about 40,000 light-years from the center.
Beyond this distance, star formation drops sharply. This discovery helps scientists better understand how our galaxy grew and evolved over billions of years.
To reach this conclusion, an international team of astronomers studied the ages of more than 100,000 giant stars.
They used data from large sky surveys like Gaia, along with ground-based projects such as LAMOST and APOGEE. By combining these observations with advanced computer simulations, the team was able to map how star ages change across the galaxy.
Galaxies like the Milky Way grow in a process called “inside-out” formation. This means star formation begins in the dense central regions and gradually spreads outward.
As a result, stars closer to the center are usually older, while stars farther out tend to be younger.
The researchers confirmed this general pattern, but they also found something unexpected. At a distance of about 35,000 to 40,000 light-years from the center, the trend reverses. Instead of getting younger, stars begin to get older again. This creates a “U-shaped” pattern in star ages across the galaxy.
This turning point marks the edge of the Milky Way’s active star-forming disc. Beyond this boundary, new stars rarely form. Instead, the stars found there are mostly older ones that have slowly drifted outward over time.
This movement happens through a process known as “radial migration.” Stars can gradually shift their positions by interacting with the galaxy’s spiral arms, similar to how surfers ride waves in the ocean. Over billions of years, some stars travel far from where they were originally born.
Importantly, the study found that these outer stars move in nearly circular paths. This shows they were not thrown outward by violent events like galaxy collisions, but instead moved gradually through natural processes within the galaxy.
Scientists still do not fully understand why star formation drops off at this specific distance. Possible explanations include the influence of the galaxy’s central bar structure or a warp in the outer disc that disrupts the gas needed to form new stars.
Future projects, such as 4MOST and WEAVE, are expected to provide even more detailed data. These efforts will help researchers uncover the exact reasons behind this boundary.
This study highlights how measuring the ages of stars can reveal the hidden history of our galaxy. Step by step, scientists are piecing together the story of how the Milky Way came to be.
