Cooler-than-average region of gas on the Sun 's surface associated with strong local magnetic activity.
Sunspots appear as dark spots, but only in contrast with the surrounding photosphere , which is several thousand degrees hotter. Spots several times larger than Earth are visible to the unaided eye (viewed through a filter); very small ones are hard to see even with a telescope. They come and go as part of the solar cycle , usually in pairs or groups, and may last for months; their cause appears to be related to the magnetic field reversals that occur every 11 years. The reality of these apparent flaws in the Sun was generally accepted only с 1611. Periods of high sunspot activity are associated on Earth with brighter aurora s and interference with radio signals.