Stellar structure theory predicts the radius of a star from its mass, composition, and age. For low-mass stars — red dwarfs and the smallest M dwarfs — the prediction is precise: they're fully convective, their interiors are well-mixed, and the equations of stellar structure admit clean solutions. But eclipsing binary observations systematically show radii 5–15% larger than predicted. This radius inflation problem has persisted for decades, with proposed explanations including spots, tidal heating, and magnetic activity. The corrections are modest: 10% here, 15% there.
Mullan and MacDonald (arXiv 2602.23147, February 2026) find that some stars may be inflated far beyond this. Using magneto-convective models applied to 88 stars in 44 Kepler eclipsing binaries, they show that internal magnetic fields can support stars against gravitational collapse — magnetic pressure supplements thermal pressure, allowing the star to expand significantly beyond its non-magnetic equilibrium.
For stars above 0.6 solar masses, the observed inflation is consistent with internal magnetic fields of about 10,000 gauss — strong but not extraordinary for an active star. The magnetic pressure contributes modestly to the hydrostatic balance, producing the familiar 10–15% radius excess.
For stars below 0.4 solar masses, the required magnetic fields jump to 100,000–300,000 gauss. At these field strengths, the magnetic pressure is no longer a perturbation — it becomes a dominant structural element. The star inflates dramatically, potentially reaching twice the radius predicted by non-magnetic models. This is hyper-inflation: the star's structure is fundamentally altered by its magnetic field.
The mechanism is straightforward. In a non-magnetic star, gravitational compression is balanced by thermal pressure from nuclear burning. Add a strong magnetic field, and the Lorentz force provides additional outward support. The star expands until the three forces — gravity, thermal pressure, and magnetic pressure — reach a new equilibrium at a larger radius. The stronger the field, the larger the star.
The effect intensifies toward lower masses because convective dynamo efficiency increases relative to gravitational binding energy. Smaller stars are more magnetizable per unit of gravitational confinement. The field doesn't just puff them up — it transforms their structure from thermally supported to magnetically supported objects.
Some of these stars may not be what they appear. A hyper-inflated 0.3-solar-mass star at twice its expected radius mimics a larger, more massive star in photometric surveys. The magnetic field hides the true mass behind a bloated surface.