Free Astronomy Magazine July-August 2020

JULY-AUGUST 2020 As the hot stars rotate, the spots on the surface come and go, causing the visible changes in brightness. Beyond the variations in brightness due to spots, the team also discov- ered a couple of extreme horizontal branch stars that showed superflares — sudden explosions of energy and another signpost of the presence of a magnetic field. “They are similar to the flares we see on our own Sun, but ten million times more ener- getic,” says study co-author Henri Boffin, an astronomer at ESO’s head- quarters in Germany. “Such behav- iour was certainly not expected and highlights the importance of mag- netic fields in explaining the proper- ties of these stars.” After six decades of trying to under- stand extreme horizontal branch stars, astronomers now have a more complete picture of them. Moreover, this finding could help explain the origin of strong magnetic fields in many white dwarfs, objects that rep- resent the final stage in the life of Sun-like stars and show similarities to extreme horizontal branch stars. “The bigger picture though,” says team member, David Jones, a former ESO Fellow now at the Instituto de Astrofísica de Canarias, Spain, “is that changes in brightness of all hot stars — from young Sun-like stars to old thing more to these mysterious ob- jects. When looking at three differ- ent globular clusters, Momany and his 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 all other scenar- ios, there was only one remaining possibility to explain their observed brightness variations,” concludes Simone Zaggia, a study co-author from the INAF Astronomical Obser- vatory of Padua in Italy and a former ESO Fellow: “these stars must be plagued by spots!” Spots on extreme horizontal branch stars appear to be quite different from the dark sunspots on our own Sun, but both are caused by mag- netic fields. The spots on these hot, extreme stars are brighter and hot- ter than the surrounding stellar sur- face, unlike on the Sun where we see spots as dark stains on the solar surface that are cooler than their surroundings. The spots on extreme horizontal branch stars are also sig- nificantly larger than sunspots, cov- ering up to a quarter of the star’s surface. These spots are incredibly persistent, lasting for decades, while individual sunspots are temporary, lasting only a few days to months. extreme horizontal branch stars and long-dead white dwarfs — could all be connected. These objects can thus be understood as collectively suffer- ing frommagnetic spots on their sur- faces.” To arrive at this result, the astronomers used several instru- ments on ESO’s Very Large Telescope (VLT), including VIMOS, FLAMES and FORS2, as well as OmegaCAM at- tached to the VLT Survey Telescope at Paranal Observatory. They also em- ployed ULTRACAM on the New Tech- nology Telescope at ESO’s La Silla Observatory, also in Chile. The break- through came as the team observed the stars in the near-ultraviolet part of the spectrum, allowing them to re- veal the hotter, extreme stars stand- ing out bright amongst the cooler stars in globular clusters. S pots 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. The spots on these hot, extreme stars are brighter and hotter than the surrounding stellar surface, un- like on the Sun where we see spots as dark stains on the solar surface that are cooler than their surround- ings. The spots on extreme horizon- tal branch stars are also significantly larger than sunspots, covering up to a quarter of the star’s surface. While sunspots vary in size, a typical size is around an Earth-size planet, 3000 smaller than a giant spot on an ex- treme horizontal branch star. [ESO/ L. Calçada, INAF-Padua/S. Zaggia] !