The Sun is a star which is at the center of our solar system. The Earth and other planets, as well as asteroids and other matter, orbit around it because of the pull of its gravity. The Sun is the source of Earth’s energy; this energy is received as insolation from sunlight, which sustains Earth’s climate. Climate models indicate that a relatively small change in the sun’s strength can have a lasting impact on Earth’s temperature, although the resulting change would also be relatively small.
Sunspots, like the one visible on the Sun’s surface in the left image above, are areas of lower temperature on the surface of the sun that appear dark. The large dark sunspot visible in this image is 20 times as large as the Earth! The image on the right shows the Earth and Sun to scale - the Sun is huge! Despite the fact that sunspots are areas of lower temperature on the Sun, they actually increase the amount of solar radiation leaving the Sun. This is because sunspots and solar flares are regulated together by mechanisms within the sun, and when sunspots are very active, more radiation leaves the sun in the form of flares. Sunspots and solar flares follow an 11-year cycle in their activity, known as the solar cycle.
In this graph of the solar cycle since 1975, you can see the 11-year cycle clearly: sunspot and solar flare activity, and irradiance, are at minimum values in 1975, and return to minima in approximately 1986, 1997, and 2008 (estimated). Sunspot activity and solar flares are shown by the blue and green lines, respectively, and irradiance is shown by the red line, which is an annual average, and the yellow lines in the background, which show daily irradiance.