Astronomy Jargon 101: Sunspots

In this series we are exploring the weird and wonderful world of astronomy jargon! You’ll feel a little cooler after reading about today’s topic: sunspots!

Sunspots are regions on the surface of the Sun that appear darker than the surrounding area. They are caused by the Sun’s massive magnetic field bundling up and punching through the surface. sunspots tend to come and go following an 11-year cycle that tracks the Sun’s magnetic activity.

Astronomers since ancient times have known about sunspots, though they were difficult to observe before the invention of the telescope. Galileo Galilei made many observations of them, and used their motion to prove that the Sun was rotating.

A single sunspot lasts anywhere from a few days to a handful of months. But they rarely come alone, almost always appearing in groups. A group on one half (either north or south of the equator) of the Sun will usually find a matching group on the opposite side. These groups will last up to a few months. After a few years of intense sunspot activity, they will start to dwindle. Sometimes the Sun remains free of blemishes for years before they appear again.

For centuries astronomers have recorded a regular 11-year pattern to the disappearance and resurgence of sunspots. Through careful observation, measurement, and laboratory experiments, astrophysicists have determined that the activity of these spots is connected to the Sun’s magnetic field. When the magnetic field is weak and untangled, few spots appear. Over time, the Sun’s rotation tangles up the magnetic field lines, causing them to burst out of the surface, like worms popping out of a rotten apple. The spots appear where the field lines exist and reenter the surface.

Sunspots appear darker because they are cooler than the surrounding surface, typically by a few thousand Kelvin. The strong magnetic fields at the location the spots prevent the normal cycles of heat convection near the surface, causing the cooling effect.

Other stars also host spots of their own, in which case astronomers call them starspots.