Astronomers using advanced supercomputer simulations have discovered that stellar rotation enables material from the deep interiors of red giant stars to reach their surfaces. Researchers from the University of Victoria and the University of Minnesota identified this mechanism, resolving a puzzle that has puzzled scientists since the 1970s. The findings, published in Nature Astronomy, explain observed changes in surface chemistry.
Red giant stars, which expand dramatically after exhausting hydrogen in their cores, exhibit puzzling changes in surface composition as they evolve. Since the 1970s, astronomers have noted shifts such as alterations in carbon-12 to carbon-13 ratios, indicating that material altered by nuclear reactions in the core must somehow cross a stable barrier layer to the outer convective envelope. A new study has pinpointed stellar rotation as the key driver of this mixing process. Simon Blouin, lead researcher and postdoctoral fellow at the University of Victoria's Astronomy Research Centre, explained: > Using high-resolution 3D simulations, we were able to identify the impact that the rotation of these stars was having on the ability for elements to cross the barrier. Stellar rotation is crucial and provides a natural explanation for the observed chemical signatures in typical red giants. The team found that rotation amplifies the mixing effect of internal waves by more than 100 times compared to non-rotating stars, with faster rotation enhancing it further. Previous models showed waves transporting minimal material, but rotation changes that dramatically. Falk Herwig, principal investigator and ARC director, noted the role of recent supercomputing advances: > Until recently, while stellar rotation was thought to be part of solving this conundrum, limited computing abilities prevented us from quantitatively testing the hypothesis. Simulations ran on the Texas Advanced Computing Centre at the University of Texas at Austin and Canada's Trillium supercluster at SciNet, University of Toronto, launched in August 2025. Herwig highlighted Trillium's power: > We were able to discover a new stellar mixing process only because of the immense computing power of the new Trillium machine. The research, supported by NSERC, NSF, and the US Department of Energy, offers insights into the Sun's future as a red giant and has applications to fluid dynamics in oceans and atmospheres.