How did the Milky Way galaxy grow into the bright spiral galaxy we live in today? It’s a question a team of astronomers from the Sloan Digital Sky Survey (SDSS) is answering. The American Astronomical Society meeting was held last week. And one of the presentations was an ‘age map’ (or growth chart) of the Milky Way galaxy. You know how parents draw lines on a wall to see how much their kids have grown? Imagine that on a cosmic scale.

Led by Melissa Ness, the astronomers looked at more than 70,000 stars. The mapped stars extend from the heart of the Milky Way to more than 50,000 light years away. Here’s the age map.

Milky Way age map

Each colored dot is a red giant star. The red dots towards the heart of the Milky Way represent stars that formed when the galaxy was much younger. The green and blue dots are stars that formed more recently.

“We conclude that our Galaxy grew up by growing out,” says Ness, lead author of the study. “To see this, we needed an age map spanning large distances, and that’s what this new discovery gives us.”

Ok, but how can astronomers tell the ages when they are looking at more than 70,000 of the same type of star? All astronomers need to know is the mass of the red giant. “If we know the mass of a red giant star, we know its age by using the fusion clock inside every star,” says Marie Martig, co-author of the ‘age map’ study.

Figuring out the mass of red giant stars used to be difficult. The team of astronomers had to combine two techniques to make this ‘age map’ possible. First, they used the Apache Point Observatory Galactic Evolution Experiment (APOGEE) to gather spectra of 70,000 red giant stars. APOGEE was well-suited to the task, and Steve Majewski, principal investigator of the APOGEE survey explains why.

“APOGEE is the ideal survey for this work because it can get high-quality spectra for 300 stars simultaneously over a large area of sky,” says Majewski. “Seeing so many stars at once means getting spectra of 70,000 red giants is actually possible with a single telescope in a few years’ time.”

APOGEE is a fantastic tool, but it’s not enough. The team turned to a telescope that specializes in hunting for exoplanets. The Kepler satellite. Light curves from the Kepler Satellite gave astronomers the last piece of data they needed to determine the age of each red giant star. By seeing how bright a star is, Ness and her team could infer the mass of each star. Piecing data from APOGEE and Kepler together, the team of astronomers created the ‘age map’ seen above.

“In the galaxy we know best – our own – we can clearly read the story of how galaxies form in a Universe with large amounts of cold dark matter,” says Ness. “Because we can see so many individual stars in the Milky Way, we can chart its growth in unprecedented detail. This unprecedented, enormous map really is one for the ages.”

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