Credit: NASA, ESA, STScI, and C. Johnson

ur Milky Way galaxy is shaped like two fried eggs glued back-to-back. A central bulge of stars sits in the middle of a sprawling disk of stars. Though this is a common feature among myriad spiral galaxies, astronomers have spent decades puzzling out how and when the Milky Way’s central bulge might have formed. Were the stars within the bulge born early in our galaxy’s history, 10 to 12 billion years ago? Or did the bulge build up over time through multiple episodes of star formation?

Some studies have found evidence for at least two star-forming bursts, leading to stellar populations as old as 10 billion years or as young as 3 billion. Now, a comprehensive new survey of millions of stars instead finds that most stars in the central 1,000 light-years of the Milky Way’s hub formed when it was engorged with infalling gas more than 10 billion years ago. This process might have been triggered by simple accretion of primordial material, or something more dramatic like merging with another young galaxy.

“Many other spiral galaxies look like the Milky Way and have similar bulges, so if we can understand how the Milky Way formed its bulge then we’ll have a good idea for how the other galaxies did too,” said co-principle investigator Christian Johnson of the Space Telescope Science Institute in Baltimore, Maryland.

“This survey gives us a big picture view of the bulge in a way that many previous surveys have not been able to do,” added co-author Caty Pilachowski of Indiana University in Bloomington, Indiana.

Looking Younger than their Age

To reach their conclusion, the team studied the stars’ chemical compositions. Like many Hollywood stars, stars in the galactic bulge look like they’ve undergone a cosmic Botox treatment – they appear younger than they are. That’s because they contain about the same amount of heavy elements (heavier than hydrogen and helium) as the Sun – what astronomers call metals. That’s surprising because metals take time

Provider: Space Telescope Science Institute

Image Source: https://webbtelescope.org/contents/news-releases/2020/news-2020-56

Curator: STScI, Baltimore, MD, USA

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