The Source of the Sun’s Dark Spots

A new study provides insight into how convection creates sunspots

A sunspot is bigger than the Earth, as demonstrated in this colorized image.
A sunspot is bigger than the Earth, as demonstrated in this colorized image. The Royal Swedish Academy of Sciences, V.M.J. Henriques (sunspot). NASA Apollo 17 (Earth)

From here on Earth, a spot on the Sun is barely a speck on that bright yellow ball in the sky. But sunspots are huge, bigger than our planetary home, and they have a complex structure that can’t be seen without some kind of help. Close up, you’ll see that a sunspot has a central dark area—called the umbra, which appears dark because it’s cooler than the rest of the Sun’s surface—with some lighter spots, and that center is surrounded by a lighter filamentary area, called the penumbra, that flows outward.

Scientists used the Swedish 1-m Solar Telescope (which, despite its name, is on the Canary Island of La Palma, off the coast of Africa) to observe a sunspot near the middle of the Sun on May 23, 2010, and look at how gas flowed in and out; they then used those observations to create computer simulations of sunspots. Their results appear this week in Science.

The study supports the idea that sunspots occur as hot gases rise to the surface of the Sun and spread out, cool and then flow back into the star. This convective flow creates the pimple-like pattern of a sunspot, according to the study. And the filaments of the penumbra are actually columns of gases.

We care about what’s happening with the Sun because our nearest star can sometimes turn on us, blasting powerful solar storms toward Earth that “could zap satellites, disable communication networks and GPS systems and fry power grids at a cost of $1 trillion or more,” Rob Irion noted in his April story for Smithsonian, “Something New Under the Sun.” Scientists are using a suite of satellites aimed at the star as well as telescopes here on Earth to learn more about the Sun. And it’s increasingly important work: as solar and space physicist Daniel Baker of the University of Colorado noted in Irion’s story, “The Sun is a highly variable star…. We live in its outer atmosphere, and the cyber-electric cocoon that surrounds Earth is subject to its whims. We’d better come to terms with that.”