Sunspots are a vortex of gas on the surface of the Sun associated with strong local magnetic activity. Spots look dark only by contrast with the surrounding photosphere, which is several thousand degrees hotter. The dark centre of a spot is called the umbra; the outer, lighter ring, the penumbra. Spots may be several times larger than the Earth or so small that telescopic observation is difficult. They may last for months. Single spots do appear, but most are in pairs or groups, with the members of a pair (leader and follower in respect to the direction of the Sun's rotation) having opposite magnetic polarity. This polarity reverses from one solar cycle to the next; i.e., if leaders in one cycle are north magnetic poles, leaders in the succeeding cycle will be south poles. Leaders and followers in one hemisphere of the Sun are almost always opposite in polarity from their counterparts across the equator.
Some large spots are visible to the unaided eye when the Sun is seen through clouds or in a camera-obscura image. But general acceptance of the reality of these apparent flaws in the Sun came only about 1611, when systematic study was begun independently by Galileo, Thomas Harriot, Johannes Fabricius, and Christoph Scheiner. S.H. Schwabe in 1843 announced discovery of the solar cycle, in which the number of spots reaches a maximum about every 11 years on the average, as do the effects of solar disturbances (e.g., auroras) on the Earth.
By observing spots, R.C. Carrington found (c. 1860) that the Sun rotates not as a solid body but differentially, fastest at the equator. Sunspots are never seen exactly at the equator or near the poles. Vilhelm Bjerknes theorized in 1926 that spots are the erupting ends of magnetic vortices broken by the Sun's differential rotation. Various elaborations on this idea have been proposed, but the cause of sunspots is still uncertain. George Ellery Hale in 1908 discovered their magnetic fields; John Evershed in 1909 detected the radial motion of gas away from sunspot centres. Annie Russel Maunder in 1922 charted the latitude drift of spots during each solar cycle. Her chart is sometimes called the butterfly diagram because of the winglike shapes assumed by the graph. Each solar cycle begins with small spots appearing in middle latitudes of the Sun. Succeeding spots appear progressively closer to the Sun's equator as the cycle reaches its maximum level of activity and declines.
Excerpt from the Encyclopedia Britannica without permission.