sunspots

Regions on the solar surface that appear dark because they are cooler than the surrounding photosphere, typically by about 1500 K (thus, they are still at a temperature of about 4500 K, but this is cool compared to the rest of the photosphere). Dark spots, some as large as 50,000 miles in diameter, move across the surface of the sun, contracting and expanding as they go.

Citation: 
http://www.exploratorium.edu/sunspots/index.html and http://csep10.phys.utk.edu/astr162/lect/sun/sunspots.html
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