For 45 years we thought we understood how stars like our Sun rotate. A Japanese supercomputer has just cast doubt on it
Understanding how stars rotate may seem like a technical detail, but it is actually a central piece to understanding their evolution. For 45 years, theoretical models held that Sun-like stars would eventually change the way they rotate as they aged. The idea was that, as it lost speed over billions of years, the spin pattern would reverse and the poles would rotate faster than the equator. Now, new research from Nagoya University suggests that that prediction might not come true. The findings. The work, published in Nature Astronomysuggests that solar-type stars could maintain the same rotation pattern that we observe in the current Sun throughout their lives. That is, the equator would continue to rotate faster than the polar regions even as the star slows down with age. The simulations carried out by the team indicate that magnetic fields play a decisive role and could prevent this regime change that was taken for granted in theoretical models for decades. How a star like the Sun actually rotates. Unlike the Earth, which rotates as a solid body, the Sun is made of extremely hot plasma. That causes different regions to spin at different speeds. In the case of the Sun, the equator completes one revolution approximately every 25 days, while the regions near the poles take about 35 days. This phenomenon is known as solar-type differential rotation. For decades, theoretical simulations predicted that this pattern would not be permanent. As stars age and their global rotation slows over billions of years, the plasma flows within them should reorganize. Predictions indicate that there would come a time when the behavior would be reversed: the equator would rotate more slowly and the poles would rotate faster, a regime that the researchers called differential anti-solar rotation. The unexpected role of magnetism. The new simulations suggest that the scenario predicted by theoretical models for decades may not come to pass. According to the results of the study, stars similar to the Sun would maintain the same type of differential rotation throughout their lives. Even if the star slows down with age, the equator would continue to rotate faster than the poles, rather than reversing the pattern as proposed in previous simulations. A supercomputer on stage. To reach that conclusion, the team turned to FugakuJapan’s most powerful supercomputer, installed at the RIKEN research center in Kobe and operational for shared use since March 2021. With its help, researchers carried out an extremely detailed simulation of the interior of solar-type stars. Each simulated star was divided into about 5.4 billion calculation points, a much higher resolution than that used in previous work. This level of detail is important because previous simulations worked at much lower resolutions. Under these conditions, the magnetic fields tended to disappear artificially within the model, which led to underestimating their influence on the internal dynamics of the star. In the new simulation, however, the magnetic fields remained stable and showed a clear effect: they help prevent the reversal of the rotation pattern. The implications. Understanding more precisely how Sun-like stars rotate is key to interpreting their magnetic activity over time. This aspect is related to well-known phenomena on our own star, such as the approximately 11-year solar cycle that regulates the appearance of sunspots and episodes of magnetic activity. A better understanding of these processes could also help improve stellar evolution models used by astronomers to study distant stars. Images | POT In Xataka | PLD Space has raised 180 million euros with Mitsubishi at the helm: the Spanish space startup grows with Japanese money
