Jens Asbjørn Bøgen
New research sheds light on how galaxies like the Milky Way grow and why their stars display unexpected chemical signatures, offering fresh insight into the forces that shaped our cosmic neighbourhood.
Astronomers have long noticed that stars around the Sun fall into two well-defined chemical groups, divided by their levels of magnesium and iron.
The separation is striking enough to appear as two clear tracks on chemical-abundance diagrams, yet the reason behind this “bimodality” has remained unanswered.
Galaxy chemistry puzzle
A study published in Monthly Notices of the Royal Astronomical Society analyses this mystery using high-resolution Auriga simulations, digital models that mimic the birth and evolution of galaxies similar to the Milky Way.
Researchers from the Institute of Cosmos Sciences at the University of Barcelona (ICCUB) and France’s Centre national de la recherche scientifique (CNRS) examined 30 such simulated systems to trace how their chemical fingerprints emerged.
The team emphasises that decoding the Milky Way’s chemical record is essential for understanding not only our own galaxy’s past but also the formation of others, such as Andromeda, where a comparable split in stellar chemistry has not been observed.
“This study shows that the Milky Way’s chemical structure is not a universal blueprint,” said lead author Matthew Orkney of ICCUB and the Institut d’Estudis Espacials de Catalunya (IEEC) in an article from the Royal Astronomical Society.
He added: “Galaxies can follow different paths to reach similar outcomes, and that diversity is key to understanding galaxy evolution.”
Paths to bimodality
According to the researchers, the simulations reveal that two separate chemical sequences can arise through a variety of evolutionary routes.
Some galaxies undergo intense bursts of star birth that later stall, while others show a gradual shift driven by changes in the supply of gas flowing in from their surroundings.
The findings also challenge earlier assumptions that a dramatic collision — such as the Milky Way’s past encounter with the dwarf system known as Gaia-Sausage-Enceladus — was required to generate the chemical divide.
Instead, the authors point to infalling metal-poor gas from the circumgalactic medium as a decisive factor in forming a second branch of stars.
Looking ahead
The shapes of these chemical tracks, the study notes, align closely with each galaxy’s star-formation timeline, reinforcing the idea that chemical patterns provide a window into long-term galactic behaviour.
“This study predicts that other galaxies should exhibit a diversity of chemical sequences. This will soon be probed in the era of 30m telescopes where such studies in external galaxies will become routine,” said Dr Chervin Laporte of ICCUB-IEEC, CNRS-Observatoire de Paris and Kavli IPMU.
He added: “Ultimately, these will also help us further refine the physical evolutionary path of our own Milky Way.”
Future observations from telescopes such as JWST, along with missions including PLATO and Chronos, are expected to test these predictions with unprecedented precision.
Sources: ICCUB, CNRS, Monthly Notices of the Royal Astronomical Society
