Solar and stellar flares exhibit distinct behaviors

The Sun is not only the nearest star to Earth, but also the one whose physical behavior is best characterized. Long-term observations have shown that it is not a static object: it undergoes alternating periods of high and low activity, its luminosity has increased over geological timescales, and it occasionally releases energetic events such as solar flares that can affect Earth. Because of these well-documented properties, it has often been assumed that other main-sequence stars behave in a broadly similar manner. However, recent findings indicate that this assumption does not always hold when it comes to stellar flaring activity.

Comparing the Sun with other stars

There are clear parallels between the Sun and other stars. Just as the Sun exhibits sunspots, other stars show starspots—regions of intense magnetic activity on their surfaces. In the solar case, the number of sunspots varies over an approximately 11-year cycle, closely linked to changes in magnetic activity and the frequency of solar flares. Although direct detection of starspots is challenging, observations have successfully identified them on roughly 400 stars to date. These measurements reveal that many stars also experience activity cycles, with periods ranging from about 3 to 20 years, depending on stellar properties. Spectroscopic analyses further confirm that magnetic activity in these stars generally follows the same cyclic behavior.

Given that solar flares are powered by magnetic fields, it would be reasonable to expect stellar flares to follow a similar pattern—occurring more frequently in regions with strong magnetic activity and abundant starspots. This expectation is fully consistent with solar observations: sunspots and solar flares tend to appear in the same regions and during the same phases of the solar cycle. However, a recent study published on the arXiv preprint server shows that this relationship does not generally apply to other stars.

How stellar flares were investigated

Because direct imaging of starspots is not feasible for large stellar samples, the researchers employed an indirect observational method. Using photometric data from the Transiting Exoplanet Survey Satellite (TESS), they analyzed variations in stellar brightness over time. Starspots cause a star’s brightness to decrease slightly when they rotate into view and increase again when they rotate out of sight, producing a periodic modulation tied to stellar rotation. In addition, the researchers identified brief, sharp increases in brightness that signal stellar flares. Since flares are only detectable on the hemisphere facing the observer, this approach allows a comparison between flare occurrence and spot-related brightness variations.

The study analyzed data from more than 14,000 stars, identifying over 200,000 stellar flares. The authors then examined whether flares were correlated with the presence of starspots. In the case of the Sun, such a correlation is strong: observing a solar flare almost always implies that sunspots are present on the visible solar disk. For other stars, however, the results were markedly different. The analysis showed that this correlation exists only about half of the time, meaning that the likelihood of starspots being present during a stellar flare is essentially no better than random.

Why the Sun appears different

These findings suggest that the Sun is not a typical representative of stellar behavior in this respect. For most stars, the physical processes responsible for starspots and flares appear to be at least partially decoupled. The reason why sunspots and solar flares are so tightly linked on the Sun remains an open question, highlighting gaps in our current understanding of stellar magnetic activity.

More info: arXiv