The Sun's Largest Known Sunspot Sets a Ceiling on Its Flares

Researchers have put a ceiling on the most energetic flare the Sun could produce, and it reaches into the range of the biggest outbursts seen on other stars like it. Natalie Krivova and Emre Isik of the Max Planck Institute for Solar System Research and colleagues report that an exceptionally large and complex group of sunspots could, in principle, release a few times 10^34 erg in a single flare.

The question is about the Sun as a star. Other Sun-like stars are seen to produce superflares, single flares releasing more than 10^34 erg, about once a century, and whether the Sun itself can reach that scale has remained unsettled.

The team used modern observations to measure how the energy a flare releases scales with the area of its ribbons, the bright strips that light up on the solar surface while a flare is under way. Taking the upper edge of that relation, they extended it to the largest sunspot group recorded since 1859, the Great Sunspot of April 8, 1947. Their paper appeared in Philosophical Transactions of the Royal Society A on Sept. 10, 2026.

The number is a limit on what is possible, not a measurement of a flare that happened. It comes from stretching a relation measured on present-day flares out to a sunspot group bigger than any that modern flare observations cover.

Extreme solar particle events, brief surges of high-energy particles from the Sun, are why the question is open. Those events show the Sun has occasionally produced eruptions considerably more energetic than any observed in the modern era. These extreme bursts originate from powerful coronal mass ejections (vast clouds of plasma thrown off the Sun) that usually accompany large flares.

The paper is part of a Royal Society meeting issue on radiocarbon and cosmic radiation events, and is open access.