What is a sunspot?

Sunspots, when viewed by visible light, appear as dark blemishes on the Sun. Most sunspots have two physical components: the umbra, which is a smaller, dark, featureless core, and the penumbra, which is a large, lighter surrounding region. Within the penumbra are delicate-looking filaments that extend outward like spokes on a bicycle wheel. Sunspots vary in size and tend to be clustered in groups; many of them far exceed the size of our planet and could easily swallow Earth whole.

Sunspots are the sites of incredibly powerful, magnetically driven phenomena. Even though they look calm and quiet in visible light, pictures of sunspots taken in ultraviolet light and in X-rays clearly show the tremendous energy they produce and release, as well as the powerful magnetic fields that permeate and surround them.

Who first discovered sunspots?

The earliest recorded observations of sunspots go all the way back to the year 28 b.c.e., when Chinese astronomers made note of dark spots on the Sun. In the era of modern, western civilization, the credit goes to the famous Galileo Galilei (1564-1642), who first recognized sunspot activity through his telescope in around 1611 (sources vary, crediting the discovering anywhere between 1610 and 1613). Records show that others, including Johannes Kepler (1571-1630), had observed sunspots before Galileo, but they failed to recognize them for what they were. Kepler, for instance, mistook the spot he saw, several years before Galileo, as the planet Mercury orbiting in front of the Sun.

How big do sunspots get?

Sunspots can range from the relatively small and, except with a telescope, unobservable, to the staggeringly enormous. The biggest sunspots can be more than 100,000 miles (161,000 kilometers) across. Astronomers measure sunspots in "millionths," with each millionth being one millionth of the surface area of the Sun that is facing the Earth. The Earth, if it were a sunspot on the surface of the Sun, would be equal to 169 millionths. Compare that with the typical sunspot, which ranges from 300 to 500 millionths, and you get an idea of how big they are. One of the largest sunspots ever measured was seen in 2001 and was 2,400 millionths. But that does not account for how far the solar flares emanating from sunspots shoot out into space; solar flares can be as long as 100,000 miles (161 thousand kilometers), and some of the energy they emit can literally stretch to Earth's orbit, 93 million miles (150 million kilometers) away.

Is there a sunspot cycle?

Yes. Sunspot activity goes through a two-cycle pattern of high and low activity: one that lasts about 11 years (more accurately, about 10 years, six months), and also an

The Sun

The Sun's chromosphere is captured in this photo by Hinode's Solar Optical Telescope in 2007.The Hinode mission is funded by the United States, Japan, the United Kingdom, and the European Space Agency. (JAXA/NASA)

88-year cycle of highs and lows; and astronomers speculate an even longer pattern might be possible. The first person to observe sunspot cycles was German astronomer Heinrich Samuel Schwabe (1789-1875). Originally a pharmacist, Schwabe became an amateur and then professional astronomer. Wondering whether there was another planet besides Mercury and Venus close to the Sun, he accidentally discovered sunspots and became completely fascinated by them. From 1825 until near the end of his life, he observed the Sun daily, recording the number of sunspots he observed. From these observations, he noticed that there were periods of greater and lesser activity, which he thought came in cycles of about 10 years, a fairly close approximation, given the quality of telescopes at the time.

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