Astronomers uncover the engine behind the most powerful stellar flares in the Kepler field

A new study led by researchers at the Konkoly Observatory of the HUN-REN Research Centre for Astronomy and Earth Sciences (HUN-REN CSFK) in Budapest has revealed what makes a distant red giant star the most explosively active in the entire Kepler field—and the answer lies in a surprisingly subtle interplay between a close stellar companion, vast dark starspots and a highly efficient internal magnetic dynamo.

A star that outshines all others in fury

KIC 2852961 is not the largest or the brightest star NASA’s Kepler space telescope ever watched—but it is, by a wide margin, the most violent. This red giant produces superflares: eruptions of energy so enormous they dwarf even the most powerful solar flares our sun can muster. Understanding what fuels them has been an open question since the star first drew attention.

“We wanted to find out what makes this particular star so extreme,” said lead author Zsolt Kővári, of HUN-REN CSFK. “Is it the nearby companion star pulling on it? Or something happening deep inside? The answer turned out to be more nuanced than either option alone.”

The same team first brought KIC 2852961 to wider attention in a 2020 study published in the journal Astronomy & Astrophysics, in which they cataloged 59 (super)flares from the full Kepler data set and showed that flares and superflares across very different types of stars likely share the same underlying physical mechanism.

The new paper, accepted for publication in Astronomy & Astrophysics, builds directly on that foundation—now with radial velocity measurements that clinch the binary nature of the system and allow a more detailed picture of what drives its extreme behavior.

A binary star system shaped by tidal forces

Using ground-based spectroscopic observations combined with Kepler’s space photometry and data from the European Gaia mission, the team confirmed that KIC 2852961 is not alone: It is the primary component of a close binary system, where the secondary star is a faint dwarf. This proximity means the two stars exert powerful tidal forces on each other—similar in principle to how the moon raises ocean tides on Earth, but far more dramatic in scale.

As the primary component entered the red giant phase—a process spanning tens or even hundreds of millions of years—it expanded and its rotation slowed; however, under the influence of tidal forces, the red giant’s rotation became increasingly synchronized with its orbital motion, and the orbit became increasingly circular.

The circularization timescale, t_c, scales steeply with the ratio of the orbital separation a to the stellar radius R, in the form of t_c ~ (a/R)⁸, so the same tidal forces that were negligible on the main sequence (i.e., before entering the red giant state) became overwhelmingly effective once the giant swelled to its current ~12 solar radii. The researchers found that KIC 2852961 is nearly, but not completely, there. And that “almost” turns out to matter enormously.

Differential rotation: the hidden driver

Unlike solid bodies, stars generally do not rotate as a single, rigid unit. Different latitudes spin at different speeds—a phenomenon known as differential rotation. On the sun, the equator rotates faster than the poles. In KIC 2852961, tidal forces have strongly constrained this differential rotation, but have not eliminated it entirely.

That residual differential rotation appears to be the key. It drives a powerful magnetic dynamo inside the star—a self-sustaining process that continuously generates and amplifies magnetic fields. The stronger and more complex those fields become, the more magnetic energy can be stored and ultimately released in explosive flares.

“Differential rotation plays a dual role,” explained Kővári. “On one hand, it helps generate and sustain huge starspots on the surface. On the other, it twists and shears the coronal magnetic field lines above those spots—and when these tangled, highly stressed magnetic field lines reconnect, the result is a superflare.”

Giant starspots as magnetic energy reservoirs

One of the most striking features of KIC 2852961 is how much its brightness fluctuates as it rotates. These large-amplitude light variations—clearly visible in Kepler data—are caused by large dark starspots covering substantial fractions of the stellar surface. These are regions of intense magnetic activity analogous to sunspots, but on a vastly larger scale.

The study shows that these large starspots serve as vast reservoirs of stored magnetic energy. The larger the spot coverage, the more energy is available—and the more powerful the flares that can result. This finding strengthens an emerging picture in stellar physics: The relationship between starspot size and flare energy is not coincidental, but causal.

The companion star: important, but not the trigger

One might expect that a nearby companion star would directly trigger flares through magnetic interactions—for example, when the magnetic field lines of the two stars connect with each other. The team tested this idea carefully, looking for whether the flares tend to occur at specific points in the orbital phase. They did not find such a pattern.

The companion’s role is therefore indirect but foundational: By exerting tidal forces that both spun up and partially synchronized the giant’s rotation, it set up the exact conditions—near-synchronization without full locking—that allow differential rotation to operate so effectively. The companion helped build the engine; however, the red giant holds the ignition key.

Why does this matter?

Red giants are often assumed to be magnetically quiet stars, their activity having wound down with age. However, KIC 2852961 is part of a broader picture: If a red giant—spun up by a nearby companion—remains rapidly rotating, it can become the most magnetically active giant star in the Kepler field.

For the broader field of stellar astrophysics, the results help disentangle a longstanding puzzle: When a star in a binary system flares intensely, is it the binary interaction that drives the activity, or is it the star’s own internal dynamics? For KIC 2852961, the answer is clearly the latter—shaped, but not controlled, by its companion.

The results are also significant for understanding space weather around stars other than our sun. The general level of flare activity in the KIC 2852961 binary system—specifically, the frequency and characteristic energy of the flares—can only be accurately understood if we form a detailed picture of the star’s internal rotation and magnetic state, in addition to knowing the orbital environment and the evolution of the binary system.

Who’s behind this story?

Swati Mestri

Swati Mestri holds a bachelor’s degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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