According to George Fischer, a solar astronomer at the University of California, "A sunspot is a dark part of the sun's surface that is cooler than the surrounding area. It turns out it is cooler because of a strong magnetic field there that inhibits the transport of heat via convective motion in the sun. The magnetic field is formed below the sun's surface, and extends out into the sun's corona."

Four Facts about Solar Activity

1. The number of sunspots varies with an 11 year period called the solar cycle. At the cycle's peak -- last in mid-2000 -- solar activity is very high.
2. Sunspots have strong magnetic fields, thousands of times stronger than Earth's. Magnetic fields keep sunspots cool and also serve as a reservoir of energy for explosive events like solar flares and coronal mass ejections.
3. A geomagnetic storm is a disturbance in Earth's magnetic field triggered by solar activity. Aurorae, power outages, and satellite failures can all be caused by such storms.
4. Coronal holes are magnetic gaps in the Sun's outer atmosphere. Without a strong magnetic field to contain the corona, solar wind particles flow out of coronal holes and can strike our planet.

On Sept. 14 1999, the space-based SOHO observatory photographed a huge eruptive prominence escaping the Sun, seen in the upper right corner of the image. Prominences are loops of magnetic fields with hot gas trapped inside. Sometimes, as the fields become unstable, the prominence will erupt as this one did and become part of a coronal mass ejection. Beautiful prominences like these become more common as we approach solar maximum.