Sunspot problems in older stars

Sunspots caused by magnetic fields have plagued our Sun for ages. Their frequency changes approximately every eleven years, but even in the worst case they never cover more than 0.4 percent of the Sun’s surface. However, the Sun is pretty big, which you can appreciate from the fact that a sunspot can be about as large as a whole cross section of the Earth.

On a cosmic scale, however, our Sun is only a small light and, just like there are people with more or less freckles, there are also stars that have large numbers of spots. Under certain circumstances, our Sun might even become such a spot-studded star in a good five billion years; on average, that happens to about one out of every ten stars with the Sun’s approximate mass.

What happens then? In its inevitable stage as a red giant, in which the Sun will also swallow up the Earth, its core will be made of helium. It will be too cold in there to turn the helium into carbon. Only the hydrogen shell will still generate energy. That’s why a red giant is relatively cool. But at some point, enough helium will collect in the center so that the pressure and temperature will increase enough to also generate carbon. There will be a sudden helium flash, and, voila, fresh energy! The old star blooms again, hotter than before, even if a bit smaller. The star has landed on the so-called horizontal branch, which refers to the Hertzsprung-Russell diagram. The horizontal branch lies approximately in the middle and runs, as you might’ve guessed, horizontally.

A star that has come this far usually lands farther to the left and thus burns hotter the lower its metallicity is, that is, the fewer atoms heavier than helium it contains. Extreme cases of the horizontal branch are often found in globular clusters. Astronomers of the European Southern Observatory have made a few interesting observations on these clusters and their findings have now been published in Nature Astronomy. In studying three different globular clusters, the authors determined that many of the extreme horizontal branch stars in these clusters had regular changes in brightness at intervals from only a few days up to several weeks.

“After eliminating all other scenarios, there was only one remaining possibility to explain the observed brightness fluctuations,” said Simone Zaggia, a co-author of the study from the INAF Astronomical Observatory in Padua, Italy and former ESO scholar. “These stars must be covered by spots!”

Spots on extreme horizontal branch stars appear to be much different than the dark sunspots on our Sun. Both, however, are caused by magnetic fields. The spots on these hot, extreme stars are brighter and hotter than the surrounding surface of the star, in contrast to the Sun, where we see its spots, which are cooler than their surroundings, as dark discolorations on the Sun’s surface. The spots on extreme horizontal branch stars are also significantly larger than sunspots and these affected zones cover up to a quarter of the star’s surface.

The spots are incredibly durable and can last for decades, while sunspots only survive for a few days to months. As the hot star rotates, the spots on the surface appear and disappear, causing the visible changes in brightness. Finally, the team also discovered a few extreme horizontal branch stars that exhibited superflares – sudden outbursts of energy and another sign for the existence of a magnetic field. “They are similar to the flares we see on our own Sun, but are ten million times more energetic,” said co-author Henri Boffin, an astronomer at ESO’s headquarters in Germany.

In five billion years, if humankind has migrated to Titan or Triton, it would be good for them if the Sun has not become one of these extreme cases.

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. The spots on these hot, extreme stars are brighter and hotter than the surrounding stellar surface, 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 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 extreme horizontal branch star.

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