A new scientific study suggests that a 45-year-old theory about how stars like the Sun rotate over time may need to be reconsidered. Researchers from Nagoya University used one of the world’s most powerful supercomputers to simulate the internal behaviour of sun-like stars and discovered that their rotation pattern may remain stable throughout their lifetime.
For decades, astronomers believed that aging stars eventually change the way they rotate. The widely accepted theory suggested that as stars slow down over time, their rotation pattern flips. In such a scenario, the poles would spin faster than the equator, a phenomenon known as anti-solar differential rotation.
However, the new study indicates that this reversal may not occur.
“The simulation can reproduce the sun’s observed rotation pattern almost perfectly. When we apply it to slower-rotating stars, it also matches astronomical observations and shows no anti-solar rotation,” Yoshiki Hatta, study co-author and a professor at NU, said.
Instead of switching to anti-solar rotation, the simulations show that the equator continues to rotate faster than the poles even when the star becomes very slow. The results suggest that magnetic fields inside stars play a much more important role than previously believed.
Why scientists expected stars to change their rotation
Unlike Earth, stars are made of extremely hot gas. This allows different regions within a star to rotate at different speeds, a phenomenon known as Differential rotation.
In the Sun, for instance, the equator completes one rotation in about 25 days, while the polar regions take about 35 days. Scientists long assumed that this pattern would eventually change as stars age and gradually lose rotational speed over billions of years.
Earlier theoretical models suggested that slower rotation would reorganise gas flows inside stars. These internal movements were expected to make the poles rotate faster than the equator.
However, astronomers have never clearly observed such stars in reality. The predicted rotation pattern appeared in computer models, but observational evidence remained limited.
High-resolution simulations reveal the role of magnetism
To investigate the discrepancy, researchers used powerful numerical simulations based on magnetohydrodynamic calculations, which analyse both plasma motion and magnetic field behaviour.
The calculations were performed on Fugaku supercomputer. Each simulated star was divided into about 5.4 billion grid points, allowing scientists to capture extremely small turbulent motions and magnetic structures inside the stellar interior.
Earlier models used far fewer grid points, which caused magnetic fields to weaken artificially during simulations. As a result, previous studies underestimated the influence of magnetism.
With higher resolution simulations, magnetic fields remained strong and stable. The findings showed that magnetic forces together with turbulent gas flows keep the equator rotating faster than the poles, even when stars rotate slowly.
“We found that these two processes, turbulence and magnetism, keep the equator spinning faster than the poles throughout the star’s life, not just when the star is young. So even though stars do slow down, the switch doesn’t happen because magnetic fields, which previous simulations missed, prevent it,” Hideyuki Hotta, one of the lead researchers and a professor at Nagoya, said.
The simulation also reproduced the Sun’s rotation pattern with strong accuracy. When the same model was applied to stars rotating more slowly than the Sun, the rotation pattern still remained solar-like.
The research may explain why astronomers have struggled to find evidence of anti-solar rotation in real stars. The simulations also showed that a star’s magnetic field gradually weakens as it ages.
“Our results show that the magnetic field monotonically decreases over the stellar lifetime,” the study authors note.
If confirmed through future observations, these findings could change current understanding of stellar evolution. Stellar rotation affects magnetic activity and the emission of energetic particles, which in turn influence the environments of planets orbiting those stars.
However, the results are based on simulations rather than direct measurements. Observing internal stellar rotation remains extremely difficult, and future astronomical observations will be needed to test these predictions.
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