Why are sunspots black?

A high-resolution GREGOR image of a sunspot, a cool, dark magnetic storm on the sun. (Image credit: KIS)

The sun is a great ball of fiery, electrically charged gas. As the sun advances through its regular 11-year solar cycle, electromagnetic activity on the star's surface gets more and more chaotic. This turbulence inevitably leads to the appearance of sunspots — dark, planet-size regions that form in the sun's lower atmosphere as a result of intense magnetic disturbances.

To most visible-light telescopes, sunspots appear black. But why do they look this way, and are they really black?

It turns out that sunspots aren't actually black. Rather, the darkness is just an optical illusion created by the contrasting heat of sunspots and their surroundings.

"Sunspots are only dark in contrast to the bright face of the sun," according to the University Corporation for Atmospheric Research (UCAR). "If you could cut an average sunspot out of the Sun and place it elsewhere in the night sky, it would be about as bright as a full moon."

The two massive sunspot groups, known as AR 2993 and AR 2994, became visible a few days ago at the northeast limb of the sun after becoming active while still hidden by the sun's disk. — The two massive sunspot groups open on the northeast limb of the sun. (Image credit: Langkawi National Observatory, MYSA/MOSTI)

The reason sunspots appear so much darker than the rest of the sun's visible surface, or photosphere, is because they are much cooler, and the gas underneath a sunspot emits about 25% as much light as the rest of the sun, according to NASA.

Sunspots are still scorching hot — astronomers estimate that the temperature of a typical one is about 6,300 degrees Fahrenheit (3,500 degrees Celsius) — but the surrounding photosphere blazes at roughly 10,000 F (5,500 C), according to the National Weather Service (NWS).

Sunspots are cool because they form in regions where magnetic fields are especially strong — roughly 2,500 times stronger than Earth's, and far stronger than anywhere else on the sun, according to the NWS. This increases the magnetic pressure exerted on sunspots, thereby inhibiting the flow of heat from the sun's interior to the surface and leaving the region cooler than its surroundings.

The pent-up magnetic energy of sunspots can have some spectacular — and dangerous — side effects. When the magnetic-field lines around sunspots become too tangled, they can snap into new configurations, releasing sudden bursts of magnetic energy. This energy can interact with the surrounding plasma — hot, electrically charged gas that makes up much of the sun — and create an explosion of energy known as a solar flare.

The largest sunspot group in 24 years appeared on the sun in October 2014. The sunspots sit below the bright active region in the middle of the sun here. — Sunspots crackle with solar flares. (Image credit: NASA/SDO/LMSAL)

Solar flares always occur in active regions near sunspots, which means the more sunspots there are on the sun at a given time, the more likely a flare is to erupt. Sunspots are more likely to occur near the climax of the sun's 11-year activity cycle, also known as the "solar maximum," when magnetic activity peaks.