Scientists uncover the physics underlying stars' extraordinary flares

6th December, 2023

This article underwent a thorough review process according to the editorial standards and policies of Science X, with factors such as fact-checking, peer review, reliable sources, and proofreading being particularly emphasised by the editors. The University of Hawaii at Manoa contributed heavily in this regard.

It is a well-known fact that our sun emits solar flares able to significantly impact the Earth, the most powerful of which have the potential to cause blackouts and disrupt communication channels worldwide. However, these solar flares pale in comparison to the 'super flares', thousands of which NASA's Kepler and TESS missions have witnessed, that originate from stars with a brightness level 100–10,000 times that of solar flares.

A sudden release of magnetic energy is believed to be responsible for both the solar flares and the super flares. Even though the super-flaring stars are equipped with more potent magnetic fields, which result in brighter flares, some of these stars display an uncommon behavior—a brief initial brightness enhancement, followed by a longer but less intense flare.

An explanation for this phenomenon was provided by the model developed by a team led by Kai Yang, a Postdoctoral Researcher from the Institute for Astronomy at the University of Hawaiʻi, and Associate Professor Xudong Sun. The Astrophysical Journal published this model today.

'We could identify the physics that drive these flares by applying the knowledge we acquired about the sun to other, cooler stars. Although we could not directly observe them, the changing brightness of these stars over time helped us 'see' these flares that are essentially too small to see,' Yang stated.

The prevailing view is that the visible light from these flares is produced solely by the lower layers of a star's atmosphere, which are heated by particles that rain down from the star's hot corona (outer layer), energized by magnetic reconnection. Researchers recently proposed that the emission from coronal loops on super-flaring stars—hot plasma confined by the star's magnetic field—might also be detectable, but only if the density within these loops is incredibly high. Regrettably, there is currently no method to test this theory since we can only observe these loops on our sun.

Some astronomers found stars that exhibit a strange light curve—a 'peak-bump', or rise in brightness, similar to data from Kepler and TESS telescopes. It was discovered that this light curve closely resembles a certain solar occurrence where a more gradual peak follows an initial burst.

'The light curves resembled a phenomenon we've observed on the sun, referred to as solar late-phase flares,' Sun explained.

The researchers speculated if an identical process, involving energized, larger stellar loops, could create similar late-phase enhancements in visible light. This theory was tested by Yang, who modified fluid simulations commonly used for solar flare loops, adjusting the loop length and magnetic energy. The result revealed that the energy input from large flares propels a significant amount of mass into the loops, causing dense, bright, visible-light emissions—exactly as predicted.

The studies indicated that such 'bump' flare light is only visible when the super-hot gas at the highest part of the loop cools down. This glowing material then descends due to gravity, forming a 'coronal rain' frequently witnessed on the sun. This outcome strengthens the team's confidence in the model's realism.

Additional information related to this research is available in the Astrophysical Journal.

This study was facilitated by the University of Hawaii at Manoa.