First age map of red giants in Milky Way shows galaxy grew from inside out

January 9, 2016

A new study shows that the Milky Way galaxy grew from the inside out. By measuring the composition and masses of red giants, astronomers determined their ages and found that older stars saturate the center of the galaxy, whereas younger ones lie near the edges.

Age map of the Milky Way shows location and age of red giants. Red dots are old stars at the center, blue dots are younger stars toward the edge. Credit: G. Stinson (MPIA)

A team of scientists led by Melissa Ness, of the Max Planck Institute for Astronomy, Heidelberg, Germany, observed 70,000 red giant stars in our galaxy in an effort to determine their ages. They measured carbon and nitrogen abundance in stars. By the process of nuclear fusion, stars create heavy elements from hydrogen and helium. The older the star, the more time it had to produce those elements like carbon and nitrogen.

To obtain this chemical information about observed stars, team used Sloan Digital Sky Survey observatory (SDSS) and its APOGEE spectrograph. 

However, other piece of the puzzle came with courtesy of Kepler space telescope. For red giants, the relationship between age and mass is known, but measuring one or the other is not so easy. 

The team was supported by Marie Martig of the Swinburne University of Technology in Melbourne, Australia. She used Kepler data that contains exact masses and ages of 2000 stars. Then, she combined these values with above mentioned chemical information from Ness and her team.

Martig was able to define how mass, age and abundance of carbon and nitrogen of a red giant correlate. Martig and Ness’ team then calculated mass and age of the 70,000 stars and created the age map pictured above.

"This is somewhat revolutionary, because ages have previously been considered very hard to get," Ness said. "Measuring the individual ages of stars from their spectra and combining them with chemical information offers the most powerful constraints in the galaxy. This is key to understanding galaxy formation." 

The age map shows that our galaxy started as a group of stars that are now 13 billion years old. With time, Milky Way grew by collecting more gas and dust. This new inflow of gas and dust initiated the birth of the next generation of stars near galaxy’s edges, whereas old stars now lie near the center.

"Our galaxy grew, and it grew up by growing out," Ness added.

The result was made public at a press conference on January 8, 2016, at the 227th Meeting of the American Astronomical Society in Kissimmee, Florida, USA.

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