A cooler darker spot appearing periodically on the surface of the sun; associated with a strong magnetic field; SYN. macula.
Dark patch on the surface of the Sun, actually an area of cooler gas, thought to be caused by strong magnetic fields that block the outward flow of heat to the Sun’s surface. Sunspots consist of a dark central umbra, about 4,000K (3,700şC/6,700şF), and a lighter surrounding penumbra, about 5,500K (5,200şC/9,400şF). They last from several days to over a month, ranging in size from 2,000 km/1,250 mi to groups stretching for over 100,000 km/62,000 mi.
Sunspots are more common during active periods in the Sun's magnetic cycle, when they are sometimes accompanied by nearby flares. The number of sunspots visible at a given time varies from none to over 100 in a cycle averaging 11 years.