These are two pictures of different sunspots. The picture on the left shows the whole Sun with some large sunspot groups on it. The sunspot groups in that picture are as big as the giant planet Jupiter. The picture on the right is a closeup of some other sunspots. The large sunspot near the middle of the righthand picture is bigger than Earth!
Click on image for full size
Images courtesy SOHO (NASA & ESA) and the Royal Swedish Academy of Sciences.
Sunspots are dark, planet-sized regions that appear on the "surface" of the Sun. Sunspots are "dark" because they are colder than the areas around them. A large sunspot might have a temperature of about 4,000 K (about 3,700° C or 6,700° F). This is much lower than the 5,800 K (about 5,500° C or 10,000° F) temperature of the bright photosphere that surrounds the sunspots.
Sunspots are only dark in contrast to the bright face of the Sun. If you could cut an average sunspot out of the Sun and place it in the night sky, it would be about as bright as a full moon. Sunspots have a lighter outer section called the penumbra, and a darker middle region named the umbra.
Sunspots are caused by the Sun's magnetic field welling up to the photosphere, the Sun's visible "surface". The powerful magnetic fields around sunspots produce active regions on the Sun, which often lead to solar flares and Coronal Mass Ejections (CMEs). The solar activity of flares and CMEs are called "solar storms".
Sunspots form over periods lasting from days to weeks, and can last for weeks or even months. The average number of spots that can be seen on the face of the Sun is not always the same, but goes up and down in a cycle. Historical records of sunspot counts show that this sunspot cycle has an average period of about eleven years.
Our Sun isn't the only star with spots. Just recently, astronomers have been able to detect "starspots" - "sunspots" on other stars.
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