Recent advances in supercomputing have allowed scientists to tackle a long-standing question in astronomy. Researchers have been trying to understand why the chemical makeup at the surface of red giant stars changes as these stars evolve.
For many years, scientists struggled to connect what happens deep inside a red giant to what is observed at its surface. Nuclear reactions in the core alter the star’s internal composition, but a stable layer separates this region from the outer convective envelope. How material manages to cross this barrier and reach the surface remained unclear.
In a new study published in Nature Astronomy, researchers from the University of Victoria’s (UVic) Astronomy Research Centre (ARC) and the University of Minnesota have now found the answer.
Stellar Rotation Drives Element Mixing
The key factor is stellar rotation.
“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,” says Simon Blouin, lead researcher and postdoctoral fellow at UVic. “Stellar rotation is crucial and provides a natural explanation for the observed chemical signatures in typical red giants. This discovery is another step forward in understanding how stars evolve.”
Scientists have long known that stars like our Sun expand dramatically once they run out of hydrogen in their cores, becoming red giants that can grow up to 100 times their original size. Since the 1970s, astronomers have detected changes in their surface chemistry during this phase, including shifts in carbon-12 to carbon-13 ratios. These changes suggest that material from deep inside the star must be transported outward, but the exact mechanism had not been confirmed.
“We knew that internal waves, generated by churning motions in the convective envelope, were able to pass through this barrier layer, but previous simulations found that these waves transported very little material. We were able to show that the rotation of the star dramatically amplifies how effectively these waves can mix material across the barrier, to an extent that matches the observed changes in surface composition,” explained Blouin.
Blouin and his colleagues found that rotation can boost mixing rates by more than 100 times compared to stars that are not rotating. Faster rotation leads to even stronger mixing. Because our Sun will eventually become a red giant, these findings also provide insight into its future evolution.
Advanced Simulations Reveal Hidden Processes
To uncover this process, the team relied on hydrodynamical simulations, which model how material flows inside stars in three dimensions. These simulations are extremely complex and require powerful computing systems, making the discovery possible only with recent advances in supercomputing.
“Until recently, while stellar rotation was thought to be part of solving this conundrum, limited computing abilities prevented us from quantitatively testing the hypothesis,” says Falk Herwig, principal investigator and director of ARC. “These simulations allow us to tease out small effects to determine what actually happens, helping us to understand our observations.”
The researchers used computing resources from the Texas Advanced Computing Centre at the University of Texas at Austin and the Trillium supercomputing cluster at SciNet at the University of Toronto. Trillium, launched in August 2025, is among the most powerful systems available in Canada for large-scale academic simulations and is part of the Digital Research Alliance of Canada. Its enhanced processing capabilities played a crucial role in enabling this work.
“We were able to discover a new stellar mixing process only because of the immense computing power of the new Trillium machine. These are the computationally most intensive stellar convection and internal gravity wave simulations performed to date, ” said Herwig.
Broader Impact and Future Research
The methods used in this study extend beyond astrophysics. The same computational approaches can help scientists better understand fluid motion in many systems, including ocean currents, atmospheric patterns, and blood flow. Herwig is collaborating with researchers in these areas to build shared tools and infrastructure for large-scale simulations.
Blouin plans to continue exploring how stellar rotation affects different types of stars. Future work will examine how varying rotation patterns influence mixing efficiency and whether similar processes occur in other stages of stellar evolution.
This research was supported by the Natural Sciences and Engineering Research Council (NSERC), the National Science Foundation (NSF) and the US Department of Energy.