The Photosphere
Virtually all of the light we get from the Sun comes from the photosphere, hence the name. The photosphere is only a few hundred kilometers thick. In comparison, that is three times thinner than the skin of an apple. Think about that! All of the sunlight you see comes from that thin skin.
The photosphere has a temperature of just below 6,000 Kelvin. The photosphere displays granulation which is evidence of convection currents below the surface. Here is a nice time lapse movie of solar granules. Granules are generally about the size of the state of Texas. They are not unlike the altocumulus clouds that form in our atmosphere; much larger, of course. Granules in the photosphere occasionally grow up to become sunspots.
Sunspots are somewhat cooler than their surroundings, that's why they appear dark. Just how do sunspots work? Here are some recent ideas.
Every eleven years under the influence of the Sun's magnetic field the number of sunspots increases and decreases. The arrangement of these spots forms an interesting butterfly pattern. We are coming up on the next sunspot maximum. During sunspot maximum, there are always concerns about the charged particles which the Sun will be throwing our way.
We are now well past a sunspot minimum. In fact, the number of sunspots has been unusually low for the last year or so. When will the next maximum be, and how big will it be? Here is a predicition. During times of sunspot maximum, violent flares can erupt on the surface of the Sun. These flares last 15 or 20 minutes or so, and are tremendous releases of magnetic energy on the Sun. Every sunspot cycle sees the magnetic polarity of the spots reverse.
Here is a great image of a sunspot group showing a lot of the detail of umbra and penumbra.
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Updated February 23, 2011