There is a class of stars that have perplexed scientists for six decades, known as extreme horizontal branch (EHB) stars. They are half as massive as the Sun but four to five times hotter. And based on our theories, they are too hot for what they should be.
Scientists suspected that something else was going on but they weren’t quite sure what. New observations, published in Nature Astronomy, have unveiled some vital clues. The team looked at EHB stars in globular clusters, tight collections of stars that orbit around the Milky Way. They highlighted variability in the stars’ brightness and discovered another peculiar thing: they were alone.
“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,” lead author Dr Yazan Momany, from the INAF Astronomical Observatory of Padua, said in a statement. “In our galaxy, these peculiar hot objects are generally associated with the presence of a close companion star.”
The lack of a companion is very important. Scientists have considered that their extreme properties could be explained as a product of the interactions with a partner star. But without one, the cause must be firmly an internal process. According to the team, the period and intensity of the variation can be explained by strong magnetic processes.
These give rise to gigantic starspots, covering up to a quarter of the star. Quite the upgrade compared to the more minute Sunspots. They also last for longer. While sunspots might disappear in a few weeks, these persist for decades.
“After eliminating all other scenarios, there was only one remaining possibility to explain their observed brightness variations,” explained co-author Dr Simone Zaggia, also from the INAF Astronomical Observatory of Padua. “These stars must be plagued by spots!”
In two observed stars, the team reports the presence of superflares. A sudden release of energy like the solar flares released by our Sun but 10 million times more energetic. Flares are also linked with magnetic field variations, another implication that whatever caused the emergence of these hot small star is to be found in the intensity of the magnetic field.
“The bigger picture though,” co-author Dr David Jones from the Instituto de Astrofísica de Canarias said, “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. These objects can thus be understood as collectively suffering from magnetic spots on their surfaces.”
The observations were conducted with several of the telescopes that belong to the European Southern Observatory consortium, including the Very Large Telescope and the New Technology Telescope.
Spots on the Sun vs spots on extreme 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. ESO/L. Calçada, INAF-Padua/S. Zaggia