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Using the Stefan-Boltzmann Law we can explain why sunspots appear dark even though they are exceedingly hot.
Sunspots have temperatures of around 4,500 K. A comparison of the power radiated per unit area of a sunspot and the surrounding surface of the sun can be made using the Stefan-Boltzmann Law.
We have (brightness of sun)/(brightness of sunspot) = (5,800 K/4,500 K)4 = 2.8 -- that is, the sun is roughly 2.8 times as bright as a sunspot.
February 28, 2013 4:17 PM PST
Photo by: NASA/SDO
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