Sunspots are cooler areas of the sun's Photosphere. They also have very strong magnetic
fields, up to 10,000 times that of Earth's, and up to 3000 times the rest of the sun.
Sunspots usually occur in pairs, and can be as large as the planet Earth. They are about
5,000 K (8,500 °F), which is 6,400 K (11,500 °F) cooler than the rest of the Photosphere.
They have been known since ancient times. They were not found to be caused by strange
magnetic fields until 1908, by George Ellery Hale.
Sunspots have an eleven-year activity cycle. They peaked last in 2001, and had their last
big dip in 1995. If you own a telescope and a solar filter, you should see a decreasing
amount of sunspots right now, with the next peak in 2012. No one yet knows why the sun
has this "internal clock."
When sunspots are at their peak, the sun actually becomes brighter. This is because
magnetically brighter areas surround each sunspot, more than making up for the dimmer
Along with the eleven-year sunspot cycle, there appear to be huge lows where sunspot
activity is extremely minimal, lasting about a century every 200-300 years. The last one
was between 1640 and 1715. There are highs, too, with the last one occurring in the
One way that astronomers know this is from past observation. Also, astronomers can tell
how much the magnetism dropped or rose by studying the radioactive carbon-14
preserved in the 8,000-year-old trees.