I’ve gotten several questions about sunspots and the Sun lately, so I’m going to start off with a post about the basics of sunspots. Enjoy, but remember, NEVER LOOK DIRECTLY AT THE SUN without proper eye-protection.
What is a sunspot?
Let’s use George Fisher’s words: “A sunspot is a dark part of the sun’s surface that is cooler than the surrounding area. It turns out it is cooler because of a strong magnetic field…”
Why is it darker?
Because the sunspot is cooler than the surrounding plasma of the Sun. It is actually still very hot: ‘round about 4000 Kelvin (that’s like 6750 degrees Fahrenheit).
Why is it cooler?
There is a strong loop of a magnetic field going into each pair of sunspots. This makes it harder for the plasma to flow in that area as it does elsewhere on the Sun, so the area cools down. (There’s less convection bringing hotter plasma from deeper in the Sun to the surface there).
Wait, what magnetic field? What? How is a sunspot formed anyway?
Just like the Earth, the Sun has a magnetic field. Neither the Sun’s nor the Earth’s fields are formed the way a regular magnet’s magnetic field is formed – they’re both formed like the magnetic field of an electromagnet. (There are two ways to create a magnetic field: 1. Have a magnet, or 2. Have moving electrons) In the case of the electromagnet (and the Sun and the Earth) electrons are moving, creating a magnetic field.
The Sun is made up of plasma and is very fluid, so as it spins different parts spin at different rates. (Stir your tea tomorrow morning – the tea at the sides of the cup spins around the edge of the cup a lot faster than the tea in the middle. If you have trouble seeing it add a drop of milk or food coloring as it spins. Now imagine how much more complicated that would get if you had a sphere of tea that was spinning and boiling.) That plasma that spins at different speeds is made of the electrons that are causing the Sun’s magnetic field. There are also many other motions going on within the Sun’s plasma. This makes the Sun’s magnetic field get very tangled and twisted.
Whenever a tangled loop of the Sun’s magnetic field pokes through the Sun’s photosphere we get a pair of sunspots: one at one end of the field, one at the other end of the field. These loops (and the associated spots) usually last a couple of weeks.
Why don’t we get “Earthspots”?
The Earth isn’t made of plasma. This makes our magnetic field less prone to tangling, and if a loop of our magnetic field DID poke through our surface we’re not hot enough or made of the right stuff for it to affect the temperature. (I’m pretty sure – maybe we should check with a solar physicist on this one).
Where’d I Get My Info?
Universe Today 
~ A l i c e !