Astronomers have found what they describe as “a plague of magnetic spots” on the surface of extremely hot stars hidden in stellar clusters.
A team led by Yazan Momany from the INAF Astronomical Observatory of Padua, Italy, used the European Southern Observatory’s telescopes to take a closer look at extreme horizontal branch stars – objects with about half the mass of the Sun but four to five times hotter.
“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,” Momany says.
When looking at three different globular clusters, he and colleagues found that many of the extreme horizontal branch stars within them showed regular changes in their brightness over the course of just a few days to several weeks.
After eliminating other scenarios, there was only one possible explanation for these observed brightness variations, says co-author Simone Zaggia from INAF. “These stars must be plagued by spots.”
The spots are brighter and hotter than the surrounding stellar surface, the researchers say, unlike on the Sun, where we see spots as dark stains that are cooler than their surroundings.
They are also significantly larger than sunspots, covering up to a quarter of the star’s surface, and are very persistent, lasting for decades, while individual sunspots last for a few days to months.
As the hot stars rotate, the spots on the surface come and go, causing the visible changes in brightness.
Some of these stars also experience superflare events – explosions of energy several million times more energetic than similar eruptions on the Sun.
“Such behaviour was certainly not expected and highlights the importance of magnetic fields in explaining the properties of these stars,” says the ESO’s Henri Boffin.
This finding 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, the researchers suggest.
“The bigger picture… is that 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,” says David Jones from Spains’s Instituto de Astrofísica de Canarias.
“These objects can thus be understood as collectively suffering from magnetic spots on their surfaces.”
The findings are published in the journal Nature Astronomy.