Sunspots are areas on the sun's photosphere, or surface, which appear darker than the rest of the sun. The photosphere of the sun, or any gaseous celestial body, is the layer of gases which makes up the visible surface. On the sun, the photosphere is extremely deep, stretching for hundreds of miles. Beneath the photosphere lies the solar core, the powerhouse of the sun, which produces energy and heat. Primarily this is accomplished by converting hydrogen into helium in a form of nuclear reaction. Sunspots stand out strongly against the bright surface of the sun, and have been observed for thousands of years.
In the 1600s, sunspots were more closely documented and generally accepted. Numerous writings and drawings from previous periods show that observers saw areas of apparent darkness on the surface of the sun. Sunspots appear dark because they are cooler than the rest of the surface of the sun, but they are actually extremely bright. They are irregularly shaped, and tend to appear in clusters or groups, always within five to 35 degrees North and South of the sun's equator.
The cause of sun spots is a magnetic storm. The spots serve as a visual indicator of increased magnetic activity on that area of the sun, and they are commonly accompanied by coronal mass ejections, better known as solar flares. Heavy magnetic activity can impact life on Earth by disrupting communications and the weather, and may have led to climate anomalies in the past.
The sun is not the only celestial body to have characteristic spots marking magnetic activity. On other stars, the dark marks are known as “starspots.” In all cases, they appear to run in cycles. The sun has an 11 year solar cycle with periods of increased activity at the beginning and the end of the cycle. This cycle has been documented and studied since the 1800s, when astronomers began to probe more deeply into sunspots and the solar system in general.
Sometimes, sunspots are so large that they can actually be seen with the naked eye, although this would cause retinal damage. However, using filters and a weak telescope can reveal sunspots during periods of high activity. Astronomy observatories use heavy duty telescopes and roving space craft to aid them in their studies of sun spots. Published images of sun spots can be found, often with special filters applied to reveal solar flares and magnetic hot spots associated with the sunspots.