The Sun
The internal structure is not relevant for the purposes of this site, so I will not be explaining it here.
Moving out from there, the photosphere is where the light photons we see with our eyes are originally emitted. The temperature decreases as the radius increases, changing from about 9400 K to 4400 K. Next is the chromosphere. The intensity of this layer is one thousand times less than the intensity of the photosphere. Unlike the photosphere, the temperature increases as radius increases within this layer, maximizing around 10,000 K. The density also decreases with an increase in radius. Not pictured here is the transition region. A small change in radius of 100 km results in a substantial increase in temperature to about 100,000 K. Then it increases more gradually to about 1,000,000 K. The outermost layer of the sun is the corona. The corona is one million times less intense than the photosphere. As a result, the corona's emission is only visible during a total solar eclipse when the moon completely blocks the light from the photosphere. Unlike the others, this layer has no clear outer boundary. |
Sunspots
Sunspots are the dark spots on the photosphere of the sun that look like blemishes. These spots are only dark because the gas in the area is cooler and less bright relative to the surrounding gas, so it looks dark by comparison.
Sunspots are a result of the odd magnetic field of the sun. Its magnetic field would look a lot like the earth's, going north and south, except that its rotation drastically affects the field. Differential rotation "wraps" the magnetic field around the sun, making the field lines more parallel to the equator rather than perpendicular. Convection pulls the field lines toward the photosphere surface, sometimes causing the lines to come out from under the surface. These occurrences produce sunspot pairs. |
As a result, the magnetic field is typically 1000 times greater in sunspots than in the surrounding surface area. Scientists believe this is the cause of the lower temperatures; the strong magnetic field likely redirects the flow of hot gas. At these locations the field lines are roughly perpendicular to the surface and either have an outward directed field (like the south pole of a magnet) or an inward directed field (like the north pole of a magnet). As the image below suggests, the sunspot pairs in the northern hemisphere are all directed one way, while those in the southern hemisphere are directed the opposite way. The direction switches every cycle, and the location of these sunspots also changes throughout the cycle (see "where do they occur?" here for more about sunspots and solar flares).