Sunspots are dark areas on the surface of the Sun, formed where the magnetic field is particularly strong, blocking heat from rising to the surface. While the temperature of the Sun may be 6,000° Celsius (about 11,000° F), sunspots are about 1,500° Celsius cooler. Sunspot activity may appear in 11-year cycles, and in some cases may be visible without the aid of telescopes.
Pictured: Images of a sunspot group captured by the Mt. Wilson Observatory on August 3, 5, 7 and 9, 1915.