Galactic break: In our Milky Way, new stars do not form everywhere – there is an outer limit of the star-forming zone. Astronomers have now determined exactly where this is. Their analyzes show that the age distribution of stars forms a U-shaped curve – first the age decreases towards the outside, then increases again. The kink in this curve is around 36,000 to 39,000 light-years from the galactic center, the team determined. This marks the long, indeterminate outer edge of the star-forming zone.
Our sun and most of the other stars in the Milky Way lie in the “thin disk” – the main plane of our galaxy. The spiral arms and the bar are located in this disk of stars, which is around 1,000 to 1,300 light-years thick and is also where star formation takes place. As with most galaxies, it has evolved from the inside out: most of the star formation initially took place in the dense central region of the Milky Way and has since migrated further outwards.
Does the Milky Way also have a “break”?
This “inside-out” growth of galaxies is reflected in the age distribution of their stars: As you move outwards from the center, the stars get younger and younger on average – but only up to a certain point. Beyond this limit of active star formation, the age distribution is reversed again: the stars get older the further out they are. Although they originated internally, they migrated externally over the course of billions of years. Overall, this creates a U-shaped age distribution of the stars.
The big question, however: Does our Milky Way also have such an outer limit of active star formation and a U-shaped curve? And where is it? “While such breaks are common in disk galaxies, it was still unclear whether the Milky Way also has such a profile,” explain Karl Fiteni from the University of Malta and his colleagues. “Our position in the galactic disk and the obscuring dust make it difficult for us to determine this precisely.”
Kink in the curve
In order to provide greater clarity, Fiteni and his team have now evaluated data from the two large spectroscopic star mapping systems LAMOST and APOGEE and supplemented these with data from the Gaia satellite. “These independent data sets use different methods to determine the age of stars,” explain the astronomers. For their analysis, they evaluated the age, position and movement of around 100,000 stars at a distance of around 22,000 to 65,000 light-years from the center of the Milky Way.
The result: “Both catalogs independently show a negative age gradient in the inner region of the star disk up to around 32,000 light-years from the center,” report the astronomers. “This is consistent with the inside-out growth scenario.” As expected, the stars become increasingly younger as the distance from the center of the Milky Way increases. But beyond a distance of around 39,000 light-years this is reversed.
Milky Way is a Type II galaxy
This results in the following: The outer edge of the star-forming disk of our Milky Way lies between 36,700 and 39,600 light-years from the galactic center. This zone marks the region in which star formation drops to a minimum and from which the age distribution of the stars reverses. “The presence of such a positive age gradient in the outer region of the stellar disk is of great importance for the large-scale structure of the Milky Way disk,” explain the astronomers.
The new data confirms that our Milky Way also has a U-shaped age distribution of its stars. This also makes it one of the so-called Type II galaxies, which show a clear break in star formation in its disk. According to studies, around 50 to 60 percent of all spiral galaxies in the local universe correspond to this type, as Fitani and his colleagues explain.
What causes the termination?
“A key question in this context, however, is which factors determine where the break radius lies,” write Fiteni and his team. This has not yet been clearly clarified. “One possibility would be that the location of this break-off zone is linked to the galactic bar,” say the astronomers. The bar forms an important transport route for gases and therefore has a strong influence on star formation and the structure of a galaxy.
Alternatively, transitions in the temperature and density of the galactic gases or “bumps” in the star disk could also influence this break in star formation. “In the Milky Way, the reason for the drop in star formation beyond the break radius remains unclear,” explain the astronomers. They hope that future Milky Way mapping, but also data from the Gaia satellite’s fourth star catalog, can provide more clarity here.
Source: Karl Fiteni (University of Malta) et al, Astronomy and Astrophysics, 2026; doi: 10.1051/0004-6361/202558144