4 July 2014

Sunspots are dark regions that appear on the surface of the Sun. Despite their appearance, sunspots aren’t actually dark. They are cooler than the surrounding surface, which means they are less bright. When an image of the Sun is made dim enough to view directly, either by viewing the Sun through a filter or by projecting the Sun’s image onto a surface, the cooler, dimmer sunspots appear dark.

Sunspots are actually depressions in the Sun’s surface. They are caused by magnetic fields, which interact with the ionized plasma of the Sun. Because the Sun rotates differentially (that is, its polar regions rotate more slowly than its equatorial region), the magnetic field of the Sun winds around the star until it snaps back into alignment, which gives rise to sunspots. We can actually measure the strength of magnetic fields near sunspots due to the Zeeman effect.

Sunspots also appear and disappear over time. Their numbers rise and fall with a cycle of about 11 years, and was the first evidence of varying solar activity known as the solar cycle. We’ve been tracking the sunspot cycles since the 1600s. Of course now we can observe their formation in real time, as seen in the video. Each frame of this video is about a minute apart, and it tracks the formation of sunspots over about two weeks.