Stars like the Sun have dark spots on their surfaces. These dark spots are important for astronomers, because they allow us to track how quickly a star is spinning, which is related to the age of a star. Since Earth is close to the Sun, spots on the Sun are easy to see, but for distant stars, our telescopes are not powerful enough to see starspots. That is why astronomers have begun to use planets to study starspots. When a planet eclipses its star, the amount of light that goes missing depends on whether the planet is blocking out a bright part of the star or a dim part of the star, like a starspot. Using this technique, astronomers have determined that HAT-P-11, a star 20% less massive than the Sun, has spots just like the Sun, but its surface is 100 times more spotted than the Sun's.

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Go to source: https://kids.frontiersin.org/article/10.3389/frym.2019.00139