This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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. 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. 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. 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.
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