A sunspot is caused by strong magnetic activity on the surface of the sun. It appears dark because it's about 1,500 degrees Kelvin colder than the area around it, but it's still so hot that if you were looking at it anywhere else but on the sun, it would be brighter than the Moon. Sunspots like this one can be used to predict space weather, letting astronauts know when to grab their radiation-proof space umbrellas. This image was taken by the new solar telescope at Big Bear Solar Observatory in California, and you can find a full resolution image at the link below.
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