Scientists Map the Milky Way's Hidden Star-Making Boundary at 40,000 Light-Years

Somewhere in the vast turning wheel of our galaxy, at a distance of 40,000 light-years from its center, the Milky Way quietly stops making new stars — a boundary astronomers have now traced for the first time through the ages of more than 100,000 giant stars. By reading the subtle U-shaped curve written into stellar ages, an international team has revealed that our galaxy grows from the inside out, and that the ancient stars haunting its outer reaches are not natives but wanderers, gently shepherded outward by gravity over billions of years. This discovery does not merely mark a line on a map