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.
To reach their conclusion, the team studied the stars’ chemical compositions. Stars in the galactic bulge 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. That’s surprising because metals take time to accumulate. They must be created by earlier generations of stars, ejected through stellar winds or supernovas, and then incorporated into later generations.
Our Sun, at 4.5 billion years old, is a relative newcomer, so it makes sense that it would be replete in metals. In contrast, most old stars within our galaxy are lacking in heavy elements. And yet bulge stars are metal-enriched despite their advanced age.
The team used the measured brightness of stars at different wavelengths of light, particularly in the ultraviolet, to determine their metal content. Stars forming at different times would be expected to have different metallicities on average. Instead, they found that stars within 1,000 light-years of the galactic center showed a distribution of metals clustered around a single average. This suggests that those stars formed in a brief firestorm of star birth.