How did the Milky Way grow so far beyond where stars form?

The Milky Way's outer disk contains surprisingly young stars at large radii where star formation stopped billions of years ago. By measuring ages and chemical abundances of distant stars, the team shows these stars didn't form in place but migrated outward from the inner galaxy, expanding the disk from 12 kpc to 20 kpc. This radial migration appears to be a common growth mechanism across spiral galaxies.