Recent advances in supercomputing have enabled scientists to tackle long-standing questions in astronomy. Researchers have been trying to understand why the chemical composition of red giant stars’ surfaces changes as they evolve.
Scientists have struggled for years to connect what’s happening deep inside red giant stars with what they see on their surfaces. Nuclear reactions within the core change the star’s internal composition, but this region is separated from the outer convective envelope by a stable layer. How material crosses this barrier to reach the surface remained unclear.
In a new study published in natural astronomyresearchers at the Astronomical Research Center (ARC) at the University of Victoria (UVic) and the University of Minnesota have now found the answer.
Rotation of stars promotes mixing of elements
An important factor is the rotation of the stars.
“Using high-resolution 3D simulations, we were able to determine how the rotation of these stars affects the ability of elements to pass through the barrier,” said UVic principal investigator and postdoctoral researcher Simon Blouin. “Stellar rotation is extremely important and naturally explains the chemical signatures observed in typical red giant stars. This discovery is another step in our understanding of how stars evolve.”
Scientists have long known that stars like our Sun expand dramatically when 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 surface chemistry during this stage, such as changes in the ratio of carbon-12 to carbon-13. These changes suggest that material deep within the star must be transported outward, but the exact mechanism was not confirmed.
“We knew that internal waves generated by stirring motion within the convective envelope could pass through this barrier layer, but previous simulations showed that these waves transported only a small amount of material. We were able to use the rotation of the star to dramatically amplify how efficiently these waves can mix material across the barrier, to an extent that is consistent with the observed changes in surface composition,” Blouin explained.
Blouin and colleagues found that rotating stars increase mixing rates by more than 100 times compared to non-rotating stars. The faster the rotation, the more intense the mixing. These discoveries also provide insight into its future evolution, as our Sun will eventually become a red giant star.
Uncover hidden processes with advanced simulations
To uncover this process, the team relied on hydrodynamic simulations to model how material flows through the star’s interior in three dimensions. These simulations are extremely complex and require powerful computing systems, making their discoveries possible only with recent advances in supercomputing.
“Until recently, stellar rotation was thought to be part of the solution to this challenge, but limited computing power prevented us from testing the hypothesis quantitatively,” said Falk Herwig, principal investigator and director of ARC. “These simulations allow us to uncover small effects and determine what is really happening, which helps us understand our observations.”
The researchers used computing resources from the Texas Advanced Computing Center at the University of Texas at Austin and the Trillium Supercomputing Cluster at the University of Toronto’s SciNet. Launched in August 2025, Trillium is one of the most powerful systems available for large-scale academic simulation in Canada and is part of the Canadian Digital Research Alliance. Its enhanced processing power played a key role in making this work possible.
“We were able to discover new stellar mixing processes thanks to the enormous computational power of the new Trillium machine. These are the most computationally intensive simulations of stellar convection and internal gravitational waves ever performed,” said Herwig.
Wider implications and future research
The techniques used in this study go beyond astrophysics. The same computational approach can help scientists better understand the movement of fluids in many systems, such as ocean currents, atmospheric patterns, and blood flow. Herwig is collaborating with researchers in these fields to build shared tools and infrastructure for large-scale simulation.
Blouin plans to continue investigating how stellar rotation affects different types of stars. Future studies will examine how changes in rotational patterns affect mixing efficiency and whether similar processes occur at 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 U.S. Department of Energy.
