Why sunspots appear as dark areas

Sunspots are cooler because their magnetic fields inhibit the replenishing of heat from convective flows due to the interaction between plasma and magnetic fields.

This allows them to cool radiatively. The rest of the solar surface is constantly being replenished by convective cells that reheat it. Solar plasma at the photosphere radiates roughly as a black-body, meaning that the energy and wavelength spectrum of radiation follows the Planck function.

The Planck function scales up and down with temperature at all energies and wavelengths. The brightness of a blackbody at a given energy or wavelength is determined by the value of the Planck function at that energy. The image you showed is taken in the "photospheric continuum", which is a black-body dominated part of the radiation spectrum. So, because sunspots are cool compared to their surroundings , this means that their Planck function is lower than their surroundings, and hence their brightness is lower, causing them to appear dark in the image.

Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Why do sunspots appear dark? Ask Question. Asked 8 years, 1 month ago.

Active 5 months ago. Viewed 23k times. Improve this question. Undo Undo 4, 4 4 gold badges 33 33 silver badges 58 58 bronze badges. Scientists measure them by comparing them to the sun's visible area that is, the half of the sun that we can see from Earth. The biggest recorded sunspot in the last century occurred in It covered 6, millionths of the sun's visible area -- roughly 18 times the Earth's surface area [source: European Space Agency ]. Sign up for our Newsletter! Mobile Newsletter banner close.

Mobile Newsletter chat close. Mobile Newsletter chat dots. Mobile Newsletter chat avatar. Mobile Newsletter chat subscribe. Prev NEXT. The Solar System. Sunspots have a lighter outer section called the penumbra, and a darker central region named the umbra.

Sunspots are caused by disturbances in the Sun's magnetic field welling up to the photosphere, the Sun's visible "surface". The powerful magnetic fields in the vicinity of sunspots produce active regions on the Sun, which in turn frequently spawn disturbances such as solar flares and coronal mass ejections CMEs.

Because sunspots are associated with solar activity, space weather forecasters track these features in order to help predict outbursts of "solar storms". Sunspots form over periods lasting from days to weeks, and can persist for weeks or even months before dissipating.