Milky Way Grew From Inside-Out

By tracking fast-produced elements, and specifically magnesium, astronomers have uncovered evidence that stars in the inner regions of the Milky Way's disc formed first.

The discovery, published online and submitted to the journal Astronomy and Astrophysics, supports the idea that the galaxy grew from the inside-out.

The international team of researchers looked at a wide range of stars of different age and location within the galactic disc to determine their metallicity, or the proportion of hydrogen and helium to other chemical elements. Because the universe consisted almost entirely of hydrogen and helium shortly after the Big Bang, older stars have lower metallicity, or fewer elements besides the two.

"The different chemical elements of which stars - and we - are made are created at different rates - some in massive stars which live fast and die young, and others in sun-like stars with more sedate multi-billion-year lifetimes," said Gerry Gilmore, lead investigator on the European Southern Observatory's Gaia-ESO Project, which supplied the data for the new study.

Massive stars live short lives, producing large amounts of magnesium when they die. In the new study, the researchers show that metal-poor stars inside the Solar Circle, or the Sun's orbit around the Milky Way's center, are much more likely to possess high levels of magnesium. This suggests that this region was once home to a plethora of stars that lived intense, short lives.

Those stars in the outer regions of the Milky Way, and outside the Solar Circle, are largely younger and both metal-rich and metal-poor. All of them, however, have low magnesium levels when compared to the metallicity, suggesting star formation took longer in the outer regions outside the Sun's orbit.

"We have been able to shed new light on the timescale of chemical enrichment across the Milky Way disc, showing that outer regions of the disc take a much longer time to form," said Maria Bergemann from Cambridge's Institute of Astronomy, who led the study. "This supports theoretical models for the formation of disc galaxies in the context of Cold Dark Matter cosmology, which predict that galaxy discs grow inside-out."