New research from the Gaia-ESO project has revealed that the Milky Way likely formed in the middle of the galaxy first, with the outer regions forming more slowly. This groundbreaking study was led by Maria Bergemann of the University of Cambridge and has been submitted to the journal Astronomy and Astrophysics.
The Gaia-ESO survey seeks to better understand how the Milky Way galaxy formed an estimated 13.6 billion years ago. As some of the earliest globular clusters began to merge, the blossoming galaxy began to take on a spherical shape. This is known as the stellar halo. As it continued to gain mass, the halo began to spin and the angular momentum caused a disk to form around it. Our sun and many other younger stars were formed in this disk. Eventually, the galaxy flattened out into the spiral shape we know today.
When the first stars began to appear in the Universe, they were composed almost solely of hydrogen and helium, as nothing else was readily available. “Metallicity” is the term used to describe the amount of elements found in a star that aren’t hydrogen or helium. Other elements, which the team refers to as “contaminant metals” would be created later as stars forge heavier elements in their core and then spew them out as they collapse into supernovae. Core-collapse supernovae, which come from massive, short-lived stars, produce a great deal of magnesium that is projected outward and could go on to form new stars.
During its survey, the Gaia-ESO team found that the stars with the largest concentrations of magnesium are actually found toward the center of the galaxy, suggesting that massive stars were forming and dying early in the galaxy’s history. Even younger stars with high metallicity that were formed out in the disk don’t have much magnesium, relatively speaking. This indicated to the researchers that stars that came from the disk took longer to form than the stars in the center. Thus, the Milky Way formed from the inside out.
The study also brings new insight to the debate that Milky Way’s disk has a dual structure: a thin disk composed of the youngest elements of the galaxy, and a thick disk composed of older, low metallicity stars. While the team did find that newer stars indeed had much higher metallicity overall than older stars, they did not find any evidence that the stars were segregated into thin or thick regions by age or metal content.