Milky Way Grew From the Inside Out

This image shows the latest results as colored dots superimposed on an artist’s conception of the Milky Way. Red dots show stars that formed when the Milky Way was young and small, while blue shows stars that formed more recently, when the Milky Way was big and mature. The color scale shows how many billion years have passed since those stars formed.
G. Stinson (MPIA)

by Irene Klotz 

Scientists have made a cosmic growth chart of the Milky Way galaxy, an innovative blending of data collected by the ongoing Sloan Digital Sky Survey and a new technique to determine the ages of stars.

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As expected, the analysis shows the galaxy’s central disk formed from the inside out, with red giant stars as old as about 13 billion years clustered toward the center and younger stars about 1 billion years old closer to the disk’s edge, astronomer Melissa Ness, with the Max Planck Institute for Astronomy in Heidelberg, Germany, told reporters at the American Astronomical Society meeting in Kissimmee, Florida.

“What we’re able to do … is understand how our galaxy has formed in detail, looking at the dispersion of ages, the gradient of the ages, how the ages change as a function of both the height from the (disk’s) plane and the radius," Ness said. "It’s understanding the details of this inside-out formation that is now possible."

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Unique to the survey is its age-dating technique, which is based on a star’s size. Ness and colleagues used high-quality Sloan survey spectra, which reveals a star’s chemistry, with optical data collected by NASA’s Kepler space telescope to develop a model that can be used to pinpoint a star’s age.
“This is somewhat revolutionary because ages have previously been considered very hard to get, particularly from stellar spectra. They’re important, but they’re difficult,” Ness said.

The key was a newly discovered relationship between a star’s age and its ratio of carbon-to-nitrogen, concentrations of which can be ferreted out by analyzing a star’s spectra.

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Older red giant stars have the highest carbon-to-nitrogen ratios and younger stars the lowest, Ness said.

Working with a sample of 2,000 stars, scientists studied how the ratios change based on a star’s mass. They then incorporated the results into a computer model and used it to calculate masses and ages of all 70,000 red giant stars observed in Sloan’s APOGEE (Apache Point Observatory Galaxy Evolution Experiment) survey.

“We hope there will be many follow-up studies,” Ness told Discovery News. “We’ve only just determined the age catalog ourselves.”