Solar flares and stellar flares behave very differently across the galaxy

The Sun is the most closely studied star in existence, and its behavior has shaped much of what astronomers expect from other stars. Centuries of observation show that solar activity follows cycles of calm and intensity, marked by changes in temperature, magnetic behavior, and the appearance of sunspots. When solar flares erupt, they almost always occur near those sunspots.

This tight connection has long been treated as a basic rule of stellar physics. However, a new large-scale study shows that this rule does not apply to most stars.

Using data from the Transiting Exoplanet Survey Satellite operated by the National Aeronautics and Space Administration, researchers analyzed more than 218 000 stellar flares from 14 163 stars. Their conclusion is direct and statistically robust. Stellar flares occur with no preference for whether starspots are visible or not. In numerical terms, flares happened when stars were brighter than average 49.97% of the time, effectively random.

This finding overturns a core assumption built from solar experience. While solar flares and sunspots are strongly linked, stellar flares across the galaxy are generally not.

On the Sun, sunspots are cooler, darker regions formed where strong magnetic fields suppress convective heat flow. Solar flares arise when magnetic field lines above these regions reconnect violently, releasing vast amounts of energy. Because of this, flares tend to cluster in active regions rich in sunspots. The correlation is so reliable that sunspot counts have been used for decades as a predictor of solar flare activity.

It seemed reasonable to assume that the same physics governs other main-sequence stars. Other stars have starspots, which show magnetic cycles lasting from about 3–20 years, and their magnetic fields are generated by stellar dynamos similar in principle to the Sun’s. If magnetic fields drive flares, then starspots should signal where flares are most likely to occur.

The new study demonstrates that this assumption does not hold when tested across a large and diverse stellar population.

Because astronomers cannot directly resolve starspots on distant stars, the researchers relied on indirect but powerful measurements. As a spotted star rotates, its brightness changes slightly. When starspots face the observer, the star appears dimmer. When those spots rotate out of view, the star brightens. This rotational brightness modulation serves as a proxy for spot coverage.

At the same time, stellar flares appear as sudden spikes of brightness lasting minutes to hours. Since flares can only be detected on the side of the star facing the observer, their timing can be compared directly to the star’s brightness at that moment. If flares prefer spotted regions, they should cluster during dimmer phases of the rotation. They do not.

Across three large stellar catalogs and after extensive filtering to remove noise, binaries, and poorly modeled systems, flares were distributed evenly between brighter and dimmer phases. Individual samples showed slight deviations in opposite directions, but when combined, they canceled out. The final result remained consistent with pure chance.

This outcome held even after rigorous bias testing. The researchers injected artificial flares into real light curves and attempted to recover them using the same detection pipeline. These tests confirmed that the analysis method itself did not favor bright or dim phases. Any small residual biases were measured and accounted for, leaving the central result unchanged.

One potential complication is the role of faculae, which are bright magnetic regions that often accompany spots. On the Sun, faculae cover much more surface area than sunspots and can partially offset their dimming effect. Over the full 11-year solar cycle, this causes the Sun to become slightly brighter at solar maximum by about 0.1%.

If faculae dominate brightness variations on other stars, then rotational light curves might not map cleanly onto spot coverage. In such a case, flares correlated with spots could still appear random when viewed only through brightness changes.

The study directly tested this possibility. If some stars were spot-dominated and others faculae-dominated, then flare behavior should split into two groups. One would show flares mostly during dim phases, the other during bright phases. Instead, most stars showed flare rates close to 50%, indicating no hidden bimodal behavior.

This strengthens the conclusion that the lack of correlation is real, not an artifact of competing surface features.

The stellar population studied is dominated by M dwarfs, small and cool stars that are prolific flare producers. Some M dwarf flares exceed the energy of the Carrington Event of 1859, the strongest solar storm ever recorded, yet they occur far more frequently. In extreme cases, a single flare can temporarily double the star’s visible brightness.

On the Sun, the most energetic flares are the ones most strongly tied to sunspots. For M dwarfs, this connection appears to break down. The magnetic environments of these stars may store and release energy in ways that do not require large, long-lived spot groups.

This suggests that stellar flares are not simply scaled-up or scaled-down versions of solar flares. Instead, different stellar masses, rotation rates, and internal structures may produce fundamentally different magnetic architectures.

The study also acknowledges remaining uncertainties. Some stars classified as spotted singles may be close binaries whose brightness variations arise from tidal distortion rather than magnetic spots. Pulsating stars and contamination from nearby sources within the TESS pixel scale of about 21 arcseconds can also introduce confusion. None of these effects, however, appears large enough to erase a strong underlying correlation if one existed.

The broader implication is striking. The Sun, long treated as the template for stellar behavior, may be atypical when it comes to how flares relate to surface magnetism. Understanding why the Sun shows such a tight spot flare connection while most stars do not could reveal new details about stellar dynamos and magnetic reconnection.

This also affects how astronomers assess flare hazards for exoplanets. Spot coverage alone may not be a reliable indicator of flare risk around other stars, particularly around active M dwarfs that host many known exoplanets.

In the end, the result is clear. Across more than 14 000 stars and over 200 000 flares, stellar flares show no meaningful relationship with starspots. What appears to be a defining feature of solar activity is not a universal stellar rule.

References:

¹ Starspots and Flares are Generally Not Correlated – Andy Zhang et al. – Arxiv – December 2, 2025 – https://arxiv.org/pdf/2512.01051 – OPEN ACCESS

Reet Kaur

I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.

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