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.

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