Most of us are familiar with sunspots – dark, cool blemishes on the surface of the Sun that appear and disappear. Now, astronomers using the European Southern Observatory telescopes announced that they have found stellar spots that are hotter and brighter than the surface of their stars. The stars, in this case, are “extreme horizontal branch stars”. They are half the mass of our Sun but four to five times hotter. 

Usually, we observe these types of stars in our own galaxy with a companion star. They can also be found in globular clusters, and generally don’t seem to have companions. They do, however, change in brightness, and that is what attracted the attention of astronomers, who conducted long-term observations of these stars. The changes in brightness lasted days to weeks, and scientists like Simon Zaggia of the INAF Astronomical Observatory of Padua concluded “these stars must be plagued by spots!”

The spots on these extreme horizontal branch stars are much bigger than sunspots. They can take up a quarter of the surface of the star, which is why the star’s brightness changes as it rotates. Additionally, these stars showed signs of “superflares” – stellar flares similar to those we observe on the Sun but with ten million times the energy. Flares such as these are compelling evidence for magnetic fields in these smaller, hotter stars.

The big picture here is that, as former ESO fellow David Jones said, “changes in brightness of all hot stars … could be connected” and may help explain the existence of magnetic fields in other stars like white dwarfs.

This is the start of understanding one of the most complicated topics in astrophysics – magnetic fields. We’re glad someone other than us is making sense of these twisting fields.

More Information

European Southern Observatory press release 

“A Plague of Magnetic Spots Among the Hot Stars of Globular Clusters,” Y. Momany et al., 2020 June 1, Nature Astronomy (Preprint from ESO)