The researchers tracked late-in-life stars known as red giants, spending years gathering spectral information spanning 50,000 light years — half the size of the Milky Way. The age of red giants can be reverse engineered based on their mass (which they calculated by combining data from the Apache Point Observatory Galaxy Evolution Experiment and images from the Kepler space telescope), so the team was able to use these old-timers to determine how long ago different regions of the galaxy were churning out stars.
“Close to the center of our Galaxy, we see old stars that were formed when it was young and small. Farther out, we see young stars. We conclude that our Galaxy grew up by growing out,” Ness said in a statement. “To see this, we needed an age map spanning large distances, and that’s what this new discovery gives us.”
The findings confirm what scientists already suspected, with the oldest stars clustered in the center of the galaxy and the edges bustling with young stellar activity. What started as a small disk over 13 billion years ago grew outward, accumulating gas and dust to form the arms of stars that would one day make a galactic spiral full of countless alien worlds.