The Groundbreaking Simulation from Nagoya University
Nagoya University's latest research has turned the tables on decades-old astrophysical theory. A team led by Professor Hideyuki Hotta and Researcher Yoshiki Hatta from the Institute for Space-Earth Environmental Research (ISEE) utilized Japan's flagship supercomputer, Fugaku, to model the interiors of solar-type stars with unprecedented detail. Published in Nature Astronomy on February 25, 2026 (DOI: 10.1038/s41550-026-02793-x), their study reveals that these stars likely retain their solar-type differential rotation—faster at the equator and slower at the poles—throughout their entire lifetimes, defying predictions that they would flip to an anti-solar pattern as they age.
This discovery bridges a long-standing gap between theoretical models and astronomical observations. For over 45 years, simulations suggested that as stars slow their overall rotation due to magnetic braking from stellar winds, internal gas flows would reverse the differential rotation pattern. However, telescopes like Kepler and TESS, using asteroseismology to probe stellar interiors via sound waves, have never detected such anti-solar rotation in solar-like stars. Nagoya's high-resolution magnetohydrodynamic (MHD) simulations provide the missing piece, showing strong magnetic fields suppress the turbulent convection needed for the flip.
The implications extend beyond stellar physics, influencing models of magnetic activity, sunspot cycles like our Sun's 11-year dynamo, and even planetary habitability over billions of years. As Japan solidifies its leadership in computational astrophysics, this work highlights ISEE's pivotal role in space-earth environmental studies.
Understanding Differential Rotation in Stars
Solar-type stars, defined as main-sequence G-type stars with masses and temperatures similar to our Sun (approximately 1 solar mass and 5,500–6,000 K surface temperature), are not rigid bodies like planets. Composed of plasma—a hot, ionized gas—they exhibit differential rotation, where different latitudes rotate at varying speeds. In solar-type differential rotation, the equatorial regions complete a full turn in about 25 days, while polar regions take around 35 days. This arises from the interaction of convection—rising hot gas plumes and sinking cooler gas—with the Coriolis force from the star's rotation.
Contrast this with anti-solar rotation, theorized for rapidly rotating stars like young F-type stars or aged, slow rotators. Here, poles would spin faster than the equator. The transition was expected as stars age, slowing from initial rapid spins (periods of days) to mature rates like the Sun's ~25-day equatorial period, due to angular momentum loss via magnetized stellar winds—a process called magnetic braking.
- Solar-type: Equator fast, poles slow—driven by convection anisotropy.
- Anti-solar: Poles fast, equator slow—predicted when convection weakens relative to rotation effects.
These patterns profoundly affect the stellar dynamo, generating magnetic fields that drive activity like flares and spots, observable via spectroscopy and asteroseismology.
The 45-Year Theoretical Puzzle
Since the 1980s, low-resolution simulations predicted a rotation flip in slowly rotating solar-type stars. Early models, limited by computational power, overlooked sustained magnetic fields, assuming turbulence alone dictated flows. As rotation slowed (Rossby number Ro >1, where convection time exceeds rotation period), anti-solar DR was expected, reviving dynamos in old stars.
Yet, asteroseismic data from Kepler (2009–2018) and TESS (2018–present) contradicted this. Measurements of p-modes (acoustic waves) inferred rotation profiles via rotational splittings, consistently showing solar-like DR even in evolved subgiants. No anti-solar signatures emerged, prompting questions about model inaccuracies or observational limits.
"Theoretical studies suggested this DR becomes anti-solar in slowly rotating stars," notes the paper's abstract, highlighting the discrepancy resolved by Nagoya's work.
Nagoya's Fugaku-Powered Breakthrough
ISEE at Nagoya University, dedicated to integrated space-earth research since 2017, spearheaded this via its stellar magnetism group. Prof. Hotta, expert in solar convection simulations, and Hatta, specializing in dynamo theory (also affiliated with NAOJ), leveraged Fugaku—world's fastest supercomputer until 2022, now Japan's top at RIKEN Kobe.
Funded by JSPS KAKENHI and MEXT's Fugaku program (hp220173 etc.), simulations modeled solar-type stars at rotation rates 1/10th to 1/100th the Sun's, dividing interiors into 540 million+ grid points (5.4 billion in some reports)—orders higher than prior ~10^6 grids.
These MHD sims solved Navier-Stokes equations coupled with induction equations, capturing turbulence, Lorentz forces from B-fields, and buoyancy.
Simulation Methods: Step-by-Step Insight
The process unfolded as follows:
- Model Setup: Spherical domain mimicking solar-type star (radius R=1, density profile from standard solar model). Initial conditions: weak seed magnetic field, polytropic equation of state for stratification.
- Rotation Variation: Parameterized by Rossby number, simulating young (fast) to old (slow) rotators.
- High Resolution: 540M grids ensure resolving smallest eddies (Taylor microscale), preventing artificial B-field quenching.
- Equilibration: Ran thousands of dynamical times (~10^5 rotation periods) until statistically steady state.
- Analysis: Computed angular velocity Ω(θ, r), magnetic energy, turbulent anisotropy via Legendre polynomials (a3 coefficient matching Kepler data).
"The simulation can reproduce the Sun’s observed rotation pattern almost perfectly," Hatta stated.
Photo by Markus Winkler on Unsplash
Key Findings: No Flip, Persistent Solar-Type DR
Results unequivocally showed solar-like DR persists:
- Equatorial acceleration maintained by equatorial convection cells, suppressed at poles by magnetism.
- Magnetic fields (10^4 G near-surface) stronger than prior models, monotonically declining without revival.
- No anti-solar DR even at ultra-slow rates (P_eq >250 days).
- Turbulence anisotropy favors solar-like regardless of rotation; B-fields enforce it.
Figure 2 illustrates Ω profiles: cylindrical isorotation bands shear equatorward, akin to Sun's tachocline.
Implications for Stellar and Solar Physics
This paradigm shift refines gyrochronology (age-rotation relations), dynamo models, and exoplanet habitability assessments. Persistent DR implies steady magnetic braking decline, matching gyrochrones from clusters like Praesepe. For the Sun, explains why no cycle changes despite 4.6 Gyr age.
Broader: Enhances forecasts of stellar winds impacting exoplanet atmospheres. Read the full Nature Astronomy paper.
Nagoya's feat underscores Japan's computational edge; Fugaku enabled resolutions infeasible elsewhere.
Reconciling Simulations with Observations
Asteroseismology via rotational splittings (Kepler legacy, TESS) reports solar-like DR in thousands of solar analogs. Nagoya sims' a3 coefficients align precisely (Fig. 4), validating against real data. No need for elusive anti-solar detections— they don't exist.
Nagoya University's Astrophysics Excellence
ISEE integrates solar-terrestrial research, with stellar magnetism as core. Hotta's group pioneers MHD sims; past Fugaku works include solar convection. Collaborations with RIKEN, NAOJ bolster Japan's astro leadership. Explore research positions in Japan's top astrophysics hubs.
Japan's Supercomputing Prowess in Astronomy
Fugaku (442 petaFLOPS) powers diverse astro sims, from galaxy formation to stellar interiors. Nagoya- RIKEN ties exemplify national strategy. Future: Post-Fugaku era with Frontier systems.
Check Nagoya's press release for visuals.
Photo by Artyom Korshunov on Unsplash
Future Directions and Open Questions
Next: Multi-D progenitors, exoplanet host stars, 3D dynamo inversions. Implications for habitable zones: stable magnetism shields atmospheres longer.
"This highlights magnetism as key in stellar evolution," per abstract.
Japan's youth in astrophysics: Pursue PhDs at ISEE; postdoc opportunities abound. Career advice for stellar researchers. University jobs in Japan.
