Secrets of the Stars

As UVA astronomy professor Steven Majewski describes the APOGEE project, his beagle, Max, orbits Majewski’s feet again and again and finally settles in a dog bed beside him.

It’s a comforting, earthly echo of the orbital motions of stars, one of the many things the Apache Point Observatory Galactic Evolution Experiment project has measured in its sweeping exploration of of the Milky Way galaxy. For the past three years Majewski and his APOGEE project team have performed what the professor calls “galactic archaeology.”

The project, which is part of the Sloan Digital Sky Survey, uses a unique spectrograph to spread out starlight by wavelength into its component colors. A UVA team led by astronomy faculty member John Wilson assembled the instrument and delivered it to the Sloan Telescope in New Mexico in 2011. The spectrograph works at infrared wavelengths, allowing for a much clearer view of our dusty galaxy, and uses fiber optics to capture the light from 300 stars at a time.

“Each star has its own chemical fingerprint,” says Majewski, the principal investigator of the project. “The overall goal of APOGEE is to do a systematic census of the chemical compositions of stars, which can tell us how the galaxy evolved,” he says. “The Milky Way and Andromeda are the two largest local galaxies, and both are constantly eating other nearby, smaller galaxies.” Understanding the chemical makeup of each star allows scientists to determine their age and origin and therefore how the galaxy grew.

APOGEE reveals chemical compositions by making detailed infrared rainbows of each star’s light that show telltale patterns of lines corresponding to individual elements like oxygen, carbon and nitrogen. The precise positions of these lines also reveal the Doppler shift of each star, allowing scientists to determine which way the stars are moving in the galaxy—another factor that sheds light on their age and origin.

Majewski and his team are working on a second APOGEE instrument to observe from the du Pont Telescope at Las Campanas Observatory in Chile. The New Mexico telescope can only see one side of the galaxy, Majewski explains; the middle of the galaxy is best seen from the Southern Hemisphere. Chile has the best sites for astronomy in the world; its dry air is good for infrared observations, he says. Most important, the atmosphere there is more stable, which means the light passing through it is less disturbed. “The stars don’t twinkle much in Chile,” Majewski says. “Astronomers like that.”