Astronomical Feature of the Month
Sunspots

Giant dark splotches, some as large as 50,000 miles in diameter, move across the surface of the Sun, swirling around, contracting and expanding as they go. These strange and powerful phenomena are known as sunspots. A sunspot is a dark part of the Sun’s surface that is cooler than the surrounding area. It is cooler because a strong magnetic field in the spot inhibits the transport of heat via convective motion in the sun. The magnetic field forms below the Sun’s surface and extends out into the Sun’s corona. The magnetic field creates something called “magnetic pressure.” The concept of gas pressure is easy to understand (as gas is heated it expands, increasing pressure), magnetic pressure may be a bit more difficult to grasp. Think of it like this, you can feel magnetic pressure when you take two magnets and try to force the ends of the same polarity together. They just won’t go. That’s magnetic pressure. Now, think of a sunspot as a bubble of magnetic pressure surrounded by the gas pressure of the Sun’s photosphere. Outside a sunspot, you have only gas pressure, which is regulated by the temperature. Inside the sunspot you have both gas pressure and magnetic field pressure combined. The pressure inside a sunspot must equal the pressure outside it or the spot would break up or collapse. The magnetic pressure inside the sunspot allows the gas pressure to remain lower than the areas outside of the sunspot. A lower gas pressure means a cooler temperature, which means a darker region. As everyone knows, temperatures and brightness are linked, the hotter something is, the brighter it is, and vice versa, on a geometric scale. So, while sunspots are moderately cooler, they are considerably darker. However, if you could cut out a sunspot and place it in the night sky, it would be about as bright as a full moon. The granular appearance of the area outside of the sunspot is the result of convection. The center of each granule is hot material rising from the Sun’s interior, while the edges are cooler material sinking downward. Scientists have known for over a 150 years that sunspots appear in cycles. The average number of visible sunspots varies over time, increasing and decreasing on a regular cycle of between 9.5 to 11 years, averaging around 10.8 years. The part of the cycle with low sunspot activity is referred to as "solar minimum" while the portion of the cycle with high activity (which we are currently in) is known as "solar maximum.“

Ulysses Studies the Sun
by Lynette Cook, FIAAA