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Sunspots are planet-sized concentrations of magnetic flux on the surface of the sun; they are sources of solar flares, coronal mass ejections and intense UV radiation.

They are cooler (and thus darker) regions on the sun where the magnetic field loops up out of the solar surface. Sunspots generally occur in pairs of opposite polarity.

Essentially, a magnetic storm on the the Sun's surface which appears as a dark area. A sunspot is approximately 1,500 to 2,500 degrees Kelvin cooler than the surrounding photosphere. The number of sunspots we see on the Sun at any given time appears to cycle every 11 years.

(Sources: [1] [2] [3] [4].)

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