When a star like our Sun reaches the end of its main sequence, it enters the red giant branching phase, expanding to several times its original size. During this time, the star undergoes internal chemical changes that change the composition of its surface layer. For decades, researchers have wondered how changes in the internal chemical composition cause changes in the upper layers. At the heart of this problem is the stabilizing layer that connects the core and the outer layer and acts as a barrier between the two.
How elements produced by internal nuclear fusion pass through that layer remains a mystery. Using advanced supercomputers, University of Victoria Astronomy Research Center (UVic-ARC), the University of Minnesota appears to have found the answer. The key, according to their research, which was supported by the Natural Sciences and Engineering Research Council (NSERC), the National Science Foundation (NSF), and the U.S. Department of Energy, is the rotation of the star.
Since the 1970s, scientists studying red giant stars have noticed changes in the surface layers of these stars as they expand. One in particular is that we observed a decrease in the ratio of carbon-12 to carbon-13. The only possible explanation for this behavior is the movement of matter from within, but researchers have so far been unable to show how this happens. A research team led by Simon Blouin, a UVic postdoctoral researcher at ARC, conducted large-scale 3D hydrodynamic simulations to model how material moves inside stars.
A slice of the simulated interior of a red giant star. Credit: Blouin, S. et al. (2025)
To run the simulation, the team Texas Advanced Computing Center (TACC) at the University of Texas at Austin, and the newSciNet’s Trillium Supercomputing Clusterat the University of Toronto. They found that stellar rotation dramatically increases the effectiveness of waves to mix material across this barrier, and that the mixing rate can exceed that for non-rotating stars by a factor of 100 or more, and increases with faster rotation. Bruin said: UVic Newsrelease:
Using high-resolution 3D simulations, they were able to determine how the rotation of these stars affects the ability of elements to pass through the barrier. Stellar rotation is extremely important and provides a natural explanation for the chemical features observed in typical red giant stars. This discovery represents a new step in understanding how stars evolve. 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 is consistent with the observed changes in surface composition.
This confirms what was shown in previous simulations, namely how stirring motion within the convective envelope can pass through the barrier layer. However, these simulations also showed that waves transport little material. This is mainly due to the limited computational resources available to run the simulations. Leveraging recent advances in supercomputing and distributed networks, the team created the first detailed simulation of a star’s internal motion. Falk Herwig, Principal Investigator and Director of ARC, said:
These simulations help us understand observations by revealing small effects and determining what is really happening. We were able to discover a new and exciting mixing process entirely thanks to the enormous computing power of the new trillium machine. These are the most computationally intensive simulations of stellar convection and internal gravitational waves ever performed.
*Graph showing the movement of the solar system’s habitable zone as the sun evolves through the red giant stage. Credit: NASA*
These results provide detailed predictions of what changes the Sun will experience in the future. In about 5 billion years, it will run out of hydrogen fuel and expand outward, potentially consuming Mercury, Venus, and even Earth in the process. At this point, scientists believe that objects that have crossed the frost line (the line at which volatile materials freeze solid) may orbit inside the Earth. The sun’s new habitable zone.
Moreover, the same computational techniques used in this study have numerous applications beyond simulating stellar evolution. It could also be extremely useful for research in areas ranging from ocean currents and atmospheric dynamics to blood flow, with potential benefits for climate science, ocean monitoring, and medicine. Herwig is currently working with researchers in these fields to develop the protocols and infrastructure needed for large-scale simulations.
Blouin plans to continue using this technique to extend this technique to other types of stars and study their rotation in hopes of learning more about how these massive objects evolve.