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As Dearborn told us, "The energy that is produced in our sun is produced right down in its heart, and that energy has to be transported out, otherwise it doesn't heat the surface. Most of the way out, that energy is transported by diffusion of radiation, as one molecule radiates energy to the next, which absorbs it and radiates it on to the next. But at a certain point...in the outer 20% or so of the radius of the sun, the energy starts to be transported by convective motion, this buoyancy of bubbles of gas that carry excess energy up to the surface, and then cooler portions of gas, which have come to the surface and radiated, drop back down. So there you have mechanical motion, and that mechanical motion is involved in generating the magnetic fields that cause sunspots."

Relatively Cool
As well as being a darker area on the sun, a sunspot is an area that temporarily has a concentrated magnetic field. This magnetic force inhibits the convective motion, which ordinarily brings hot matter up from the interior of the sun, so the area of the sunspot is cooler than the surrounding plasma and gas. But as Fisher points out, sunspots are actually quite hot. "Instead of being about 5700 degrees kelvin like the rest of the photosphere, the temperature of a sunspot is more like 4000 degrees kelvin. But that is still very hot, compared to anything here on the earth."

So while sunspots are about one-third cooler than the surrounding materials, they are much darker. Dearborn explains, "If you have a piece of gas or iron and you heat it up and ask how much light it emits, you can measure it. If you then double the temperature, the amount of light that's emitted...is almost eight times as much. The amount of light that's emitted is a very steep function of the temperature." So while sunspots are moderately cooler they are considerably darker. Still, if you could cut out a moderately-sized sunspot and place it in the night sky, it would be about as bright as a full moon. the umbra and penumbra
 

Sunspots are made up of two parts: a dark, roughly circular central disk called the umbra, and a lighter outer area called the penumbra. The term "umbra" means "shade" in Latin, "penumbra" means "almost shade." The granular appearance of the area outside of the sunspot is the result of convection. The center of each granule is hot material rising from the sun's interior, while the edges are cooler material sinking downward.

 


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Observatory  1998 The Exploratorium.