Superflares 10m times more powerful than the sun’s spotted on strange stars

2 Jun 2020

An artist’s impression of what one of these stars, and its giant white-ish spot, might look like. Image: ESO/L Calçada, INAF-Padua/S Zaggia

Astronomers looking at extreme horizontal branch stars have detected giant magnetic spots and powerful superflares.

A discovery found in a particular type of star could help solve a number of elusive mysteries in stellar astronomy. These stars, referred to as extreme horizontal branch stars, have about half the mass of our own sun but are up to five times hotter.

Now, astronomers using European Southern Observatory (ESO) telescopes have noticed odd behaviour among these stars that suggests they are plagued by enormous magnetic spots and superflares millions of times more powerful than any seen on the sun.

One of the astronomers, Henri Boffin, said: “They are similar to the flares we see on our own sun, but 10m times more energetic.

“Such behaviour was certainly not expected and highlights the importance of magnetic fields in explaining the properties of these stars.”

Extreme horizontal branch stars are known to bypass the final stage of a star’s life, dying prematurely, and are generally associated with having a companion star. However, the vast majority of stars seen in a recent survey, when observed in tightly packed stellar groups called globular clusters, do not appear to have companions.

Significantly different to the sun

Writing in Nature Astronomy, the astronomers led by Yazan Momany from the INAF Astronomical Observatory in Italy described how many of the extreme horizontal branch stars within these clusters showed regular changes in their brightness over the course of just a few days to several weeks.

Simone Zaggia, co-author of the study, said: “After eliminating all other scenarios, there was only one remaining possibility to explain their observed brightness variations; these stars must be plagued by spots!”

These magnetic spots appear to be quite different to the ones seen on the sun, being both hotter and brighter than the surrounding stellar surface. On the sun, spots are seen as dark stains on the solar surface as they are cooler than the surrounding matter.

The spots on the extreme horizontal branch star are also much larger than sunspots, covering up to a quarter of the star’s surface and lasting for decades. The researchers were also surprised to discover some of this star-type showed superflares.

Artist's illustration of the sun being compared with horizontal branch stars.

Spots on extreme horizontal branch stars (right) appear to be quite different from the dark sunspots on our own sun (left), but both are caused by magnetic fields. Image: ESO/L Calçada, INAF-Padua/S Zaggia

These latest findings could help explain the origin of strong magnetic fields in many white dwarfs, objects that represent the final stage in the life of sun-like stars and show similarities to extreme horizontal branch stars.

According to astronomer David Jones, it could help put together a “bigger picture” of stellar science as a whole.

“Changes in brightness of all hot stars – from young sun-like stars to old extreme horizontal branch stars and long-dead white dwarfs – could all be connected,” he said.

“These objects can thus be understood as collectively suffering from magnetic spots on their surfaces.”

Colm Gorey was a senior journalist with Silicon Republic