| MadSci Network: Astronomy |
Like everything else in the solar system, the Sun rotates. However, unlike the Earth, the Sun is not a solid body, but is composed of (very hot!) gas. So the rotation rate changes depending on where you are on the Sun. At the equator, it rotates about once every 27 days or so. Actually, one way to measure the rotation rate is to watch sunspots and see how long they take to go around, kind of like watching the Earth from space and seeing how long it takes an island grouping to go around once.
The last theory I heard about sunspots is that they are localized strengthenings in the Sun's magnetic field. For some reason, the Sun generates a local magnetic field that is very strong near the surface. Now, near the surface, the main way heat is transported from the Sun's interior is by convection. This is just like the way clouds form on the Earth, or bubbles rise in a pot of boiling water: hot gas rises, and cool gas sinks. Now it turns out that a strong magnetic field suppresses this convection. So on some spot on the surface of the Sun, heat is not being brought up as well from the interior, and that part cools a bit. Being cooler, it does not give off as much heat as the surface surrounding it, and it looks black.
This also explains why sunspots always appear in groups of two; one for each magnetic pole. They tend to be "stuck" to one point on the surface of the Sun; I have never heard of them wandering. They do change shape as the gas they are in moves, and the magnetic fields shift.
Sunspots are linked to solar activity and aurorae on the Earth too. You can get lots more information at the following web sites: