Study shows that Galaxy gets its distinctive shape due to outer reaches forming slower than inner areas
New research from the University of Cambridge has determined that our Milky Way Galaxy formed “inside out,” beginning at the Galaxy’s center and growing outwards. The new data come from the Gaia-ESO project using the. 8-m VLT in Chile, one of the world’s largest telescopes. The observations back up theoretically-predicted divisions in the chemical composition of the stars that make up the Milky Way’s disc – the vast collection of giant gas clouds and billions of stars that give our Galaxy its “fried egg” or “UFO” shape.
By tracking elements that are produced quickly (magnesium, in this case), scientists observed that the inner parts of the Galaxy formed more quickly than the outer portions, indicating that most of the Galaxy’s expansion came from the center. Specifically, scientists examined stars to determine their “metallicity,” or the degree to which they contain elements that aren’t hydrogen and helium. Older stars show lower levels of 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,” said Professor Gerry Gilmore, lead investigator on the Gaia-ESO Project.
As massive stars collapse at the end of their relatively short lives, they produce huge quantities of magnesium as part of the catastrophic process. The older stars within the Solar Circle (the path traveled by the Sun around the center of the Galaxy) contain higher levels of magnesium than the younger stars on the outskirts of the Galactic Disk. This would suggest that the center contained more turnover activity from stars that “lived fast and died young” in the past.
“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.
Scientists are quick to point out that there are no clear distinctions between the thicker, faster-forming “inner disc” and the thinner, slower-forming “outer disc:” Stars or all ages and all levels of metallicity can be found throughout the Galaxy; they just appear in higher concentrations in their respective discs.
The astronomers theorize that the older, thicker interior disc could have formed a number of ways, including by cannibalizing other galaxies. “The Milky Way has cannibalised many small galaxies during its formation. Now, with the Gaia-ESO Survey, we can study the detailed tracers of these events, essentially dissecting the belly of the beast,” said Greg Ruchti, a researcher at Lund Observatory in Sweden, who co-leads the project.