Superflares Millions of Times More Energetic Than Similar Explosions From the Sun Detected on Extremely Hot Stars

Researchers have discovered massive magnetic spots on the surface of extremely hot stars, some of which have produced superflare explosions millions of times more energetic than similar eruptions on the sun.

For a study published in the journal Nature Astronomy, an international team of astronomers examined a group of stellar objects called "extreme horizontal branch (EHB) stars" found in spherical collections of hundreds of thousands of stars known as "globular clusters."

EHB stars tend to have around half the mass of our sun, but are four or five times hotter with temperatures ranging between 20,000 and 30,000 Kelvin (around 35,000 and 53,000 Fahrenheit.) For decades, understanding the formation of hot EHB stars has remained one of the biggest mysteries in our knowledge of stellar evolution.

"These hot and small stars are special because we know they will bypass one of the final phases in the life of a typical star and will die prematurely," Yazan Momany, an author of the study from the INAF Astronomical Observatory of Padua in Italy, said in a statement.

"Investigating EHB stars is like having a sneak peek of what a low-mass star like our Sun might become in around five billion years from now—one step short from heading towards the stellar graveyard," Momany told Newsweek.

"Overall, EHB stars represent a true mystery in the stellar evolutionary theory which is summarized as follows: how and why do stars like our Sun manage to lose as much as 30-40 percent of their mass during their final evolutionary stages?"

Conducting observations with several telescopes operated by the European Southern Observatory (ESO,) the astronomers found that many EHB stars in three different globular clusters were exhibiting regular changes in their brightness. These changes in brightness had timescales ranging between a few days to several weeks.

Analyzing these results, the team determined that the most plausible explanation for this phenomenon was that the surface of these EHB stars must be covered in magnetic spots. The changes in brightness observed by the astronomers are the result of the spots appearing and disappearing, according to the study.

"We attribute the first variability phenomenon to the presence of giant superficial spots that come and go as the star rotates and thereby induce the changes in the stellar luminosity," Momany said.

These spots, which are caused by intense magnetic fields, are brighter and hotter than the surrounding areas, the researchers said. They are huge in size, covering up to one quarter of the star's surface.

starspot, star
An artist's illustration of a spot on an EHB star. ESO/L. Calçada, INAF-Padua/S. Zaggia

These spots differ from those found on our sun, which tend to be darker than the surrounding areas and, thus, are cooler. On our star, sunspots are a temporary phenomenon, which increase and decrease in frequency over an average solar cycle of 11 years.

"The spots on EHB stars are up to 3,000 times the size of a typical sunspot, which is close to an Earth-sized planet," Momany said. "They are also long-lived phenomenon, confirmed to be stable up to decades, whereas sunspots come and go on timescales of up to a few months at most."

The team also observed irregular, abrupt jumps in the luminosity of some of the EHB stars taking place on timescales of hundreds of days.

"We attribute the second phenomenon to eruptive manifestations of magnetic fields, similar to the flares occurring on our sun, but far more energetic," Momany said.

Among all the EHB stars they observed, the scientists detected two incredibly powerful "superflare" events—massive stellar explosions that arise from starspot regions.

"They are similar to the flares we see on our own sun, but ten million times more energetic," says study co-author Henri Boffin, another author of the study from ESO's headquarters in Germany said in a statement. "Such behavior was certainly not expected and highlights the importance of magnetic fields in explaining the properties of these stars."

Superflares are many times more energetic than the typical solar flares that our sun produces. Traditionally, it has been assumed that older, less active stars like the sun did not produce these extremely powerful flares. However, some evidence has emerged recently to suggest that they might occur, albeit very rarely.

This article was updated to include additional comments from Yazan Momany.