|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."|
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.
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.