Did the Milky Way form inside-out?
New research from the Gaia-ESO project has backed up a past predictions on the "divisions in the chemical composition of the stars that make up the Milky Way's disc," a University of Cambridge news release reported.
The disc is a group of stars and gas clouds that give the Milky Way its "flying saucer" shape.
The research team made their findings by tracking "fast-produced" elements such as magnesium; this allowed them to determine how quickly various parts of the Milky Way evolved.
The researchers concluded the inner regions of the galaxy formed much more quickly than the outer realm; meaning it most likely formed inside-out.
The team used the 8-m VLT to determine the "metallicity" (the amount of elements present other than hydrogen and helium) of stars that varied in age.
In the early days of the universe everything was made up of hydrogen and helium while other "contaminant metals" developed over time. This means older stars would contain less elements and have a lower metallicity.
"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," Professor Gerry Gilmore, lead investigator on the Gaia-ESO Project, said.
Massive stars die young as "core-collapse supernovae" produce magnesium when they expire. This event can cause a neutron star or a black hole to form.
The team suggested that "metal-poor" stars within the Solar Circle ("the orbit of our Sun around the [center] of the Milky Way, which takes roughly 250 million years to complete") have more magnesium than other star populations, suggesting this area had more early deaths.
"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," Maria Bergemann from Cambridge's Institute of Astronomy, who led the study, said in the news release. "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."
The research also helped scientists gain insight into a supposed "double structure" in the Milky Way's disk.
"The thin disc hosts spiral arms, young stars, giant molecular clouds - all objects which are young, at least in the context of the Galaxy," Aldo Serenelli from the Institute of Space Sciences (Barcelona), a co-author of the study, said. "But astronomers have long suspected there is another disc, which is thicker, shorter and older. This thick disc hosts many old stars that have low metallicity." -
"From what we now know, the Galaxy is not an 'either-or' system. You can find stars of different ages and metal content everywhere!" Bergemann said. "There is no clear separation between the thin and thick disc. The proportion of stars with different properties is not the same in both discs - that's how we know these two discs probably exist - but they could have very different origins."
