Red giants map how the Milky Way grew

KISSIMMEE, Fla. — Our galaxy was built from the inside out. That’s the clear conclusion from an unprecedented survey of the ages of tens of thousands of the galaxy’s stars, reported January 8 at a meeting of the American Astronomical Society. “The Milky Way grew up by growing out,” Melissa Ness, an astronomer at the Max Planck Institute for Astronomy in Heidelberg, Germany, said at a news conference.

Ness and her colleagues developed a computer program that analyzed the light emitted by red giants —bright stars that started out like the sun but exhausted their hydrogen fuel — to determine the stars’ masses and ages. Although scientists were pretty sure that galaxies grow outward, this new census of the galactic interior to the outskirts will help researchers chart that development in impressive detail. “It’s a galactic archaeology project,” says Mario Pasquato, an astrophysicist at Yonsei University in Seoul, South Korea, who was not involved in the research.

Most stars don’t easily divulge their ages. Red giants are slightly more helpful because their age depends on their mass — but determining mass isn’t so easy either. Ness and her colleagues hit on a clever trick to figure out masses and ages by encompassing data from two telescopes, one in space and one on the ground. NASA’s Kepler space telescope, best known for spotting distant planets, had previously delivered accurate mass readings for about 2,000 red giants. Meanwhile, using a small telescope in New Mexico, the Sloan Digital Sky Survey precisely measured the light from those Kepler stars plus that from about 150,000 others.

The researchers trained a computer program to learn how the amount of light emitted at different wavelengths by the Kepler stars varied depending on the stars’ mass. Once the algorithm had determined that relationship, the researchers simply plugged in Sloan light measurements to determine the masses, and thus the ages, of about 70,000 galactic red giants. The ages are accurate to within about 40 percent, which Pasquato says is admirable because of the difficulties in aging stars. As expected, the Milky Way’s oldest stars reside in the center of the galaxy, while the youngest generation lives in the distant suburbs.

This year, a new Sloan telescope in Chile will begin scanning the Southern Hemisphere skies, potentially adding many more red giants to the galactic age catalog.