New simulations challenge decades- old theory of stellar rotation

For decades, astronomers have expected the Sun to one day turn its rotation pattern inside out. New supercomputer simulations now suggest that moment will never come. Astronomers have long known that the Sun does not rotate as a solid body: its equator completes a rotation in roughly 25 days, while its poles lag behind at around 35 days. Unlike solid planets, stars are vast spheres of turbulent plasma and do not rotate uniformly. This solar- type differential rotation is a hallmark of our home star. However, for 45 years, theoretical models suggested that this pattern was only temporary. As stars like the Sun age and spin down, they were expected to undergo a ‘rotation flip’, in which the poles begin to rotate faster than the equator — a state known as anti-solar differential rotation.

New research from Nagoya University, published in Nature Astronomy, now challenges this long-standing prediction. Using the Fugaku supercomputer, Japan’s most powerful system, Hideyuki Hotta and Yoshiki Hatta carried out some of the highest-resolution simulations of stellar interiors yet attempted. Their models divided a simulated star into 5.4 billion grid points, allowing the complex interplay of gas flows and energy transport to be followed in unprecedented detail. The results point to a very different picture of how stars evolve.

The simulations identify a crucial ingredient missing from earlier work: magnetic fields. In previous, lower-resolution models, magnetic fields were too weak to be properly resolved and often dissipated artificially. The new study shows that magnetism plays a central role in regulating stellar rotation. Magnetic fields and turbulent flows act together to maintain faster rotation at the equator than at the poles throughout a star’s lifetime, preventing the predicted rotation flip even as the star gradually slows with age.

The findings help resolve a long-standing mismatch between theory and observation. Despite decades of searching, astronomers have not convincingly identified stars beyond the Sun that exhibit anti-solar differential rotation. By accounting for magnetic effects, the new simulations bring theory into closer agreement with what is seen.

Beyond addressing a decades-old problem, the results have wider implications for stellar evolution and the environments of exoplanets. Improved models of stellar interiors allow astronomers to better predict long-term magnetic activity and starspot cycles – key factors in shaping the radiation environment around a star. These, in turn, influence whether orbiting planets can remain hospitable to life over billions of years. For now, it appears that the Sun, and stars like it, are likely to retain their familiar rotational pattern throughout their lives.