The Milky Way has been around for at least 13.7 billion years, but it has its younger and older areas. Now, researchers from the Max Planck Institute for Astronomy in Heidelberg, Germany have created a map that shows how our galaxy grew in time, confirming the belief that it grew from the inside out.

Age distribution of red giant stars from the galactic center to the Milky Way’s outskirts. Blue are the youngest stars, while towards the middle, the red stars are the oldest. Image Credit: M. Ness & G. Stinson / MPIA

The gist of it is very simple: there are older star at the middle of the galaxy and younger ones towards the extremities, to it seems that our galaxy aged by growing up. The chart considered over 70,000 stars extending half-way through the Milky Way.

“Close to the centre of our galaxy, we see old stars that were formed when it was young and small. Farther out, we see young stars. We conclude that our galaxy grew up by growing out,” said Melissa Ness from the Max Planck Institute for Astronomy in Heidelberg in Germany, who led the study. “To see this, we needed an age map spanning large distances, and that is what this new discovery gives us,” she added.

In order to determine these ages, they studied red giants. Red giants are giant stars of low or intermediate mass (roughly 0.3–8 solar masses ) in a late phase of stellar evolution – they’re old. They’re also easy to observe, which makes them an ideal candidate for this type of study.

“If we know the mass of a red giant star, we know its age by using the fusion clock inside every star. Finding masses of red giant stars has historically been very difficult, but surveys of the galaxy have made new, revolutionary techniques possible,” said Marie Martig, co-author of the study.

The map, which was unveiled at the 227th annual meeting of the American Astronomical Society used data from the APOGEE survey and the Kepler Space Telescope.

“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,” said Steve Majewski, principal investigator of the APOGEE survey. “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,” he added.

Not only was APOGEE ideal, but it was actually vital for this type of study. Even five years ago, when it wasn’t available,  such a map wouldn’t have been possible.

“The Kepler dataset gives you the mass of stars,” Ness said. “From mass, you can deduce age using stellar models.” That’s exactly what Ness and her colleagues did, for approximately 30,000 red giant stars in the Kepler dataset.

In the future, astronomers want to refine the results even more and create a more detailed map of how the galaxy evolved.

“APOGEE has nineteen elements for each star that it observes, and each of those is telling you about the conditions that the star formed from,” she said. “When the APOGEE 2 data is available, we want to analyze these elemental abundances along with ages and couple the information.”