Sunspots are comparatively dark, irregularly shaped areas that appear on the visible surface of the sun, called its photosphere, for varied amounts of time. They can form and remain for just a few hours or for anywhere up to a few months. Frequently they form in pairs or groups and larger ones can be visible to the human eye when wearing appropriate sun-filters, while the smallest may be barely perceivable by telescope. Anywhere other than the sun’s photosphere they would actually appear bright, but in comparison to the rest of the surface they seem dark.
The comparative darkness is due to the sunspots having a lower temperature. The incandescent gases at the visible surface of the sun are 5800 degrees Kelvin on average. (The Kelvin temperature scale places the lowest theoretical temperature for a thermodynamic system at zero, generally referred to as “absolute zero”, which is minus 273.15 degrees Celsius or minus 459.67 degrees Fahrenheit.) The average temperature at the center of a sunspot is 4300 K, regardless of its size, so that is 1500 degrees cooler in Kelvin, Celsius or Centigrade or around 2520 degrees cooler in Fahrenheit. Whichever way you measure it, it’s a lot cooler than the rest of the surface.
Why it is so much cooler is related to the sunspot’s other distinguishing feature, very strong magnetic field strength and fluctuating activity. Magnetic field strengths 6000 times greater than that measured at the Earth’s poles have been measured centered on sunspots. The temperature at the bottom of the sun’s photosphere is approximately 11000 K, so this results in heat rising towards the surface in plumes of slowly cooling gas. When a rising plume reaches the magnetic field it is blocked and flows sideways. This results in the central region of the sunspot, called the umbra, being coolest and therefore darkest, while the outer rim of the sunspot, the penumbra, appears as brighter, thread-like filaments with dark centers. Making the sunspot’s physical appearance something like a roughly rounded pillow with a fringe.
There are two main hypotheses on why the umbra is cooler. The first proposes that the magnetic field blocks some heat from radiating through it. While this would account for a cooler center, it implies that the heat should flow out beyond the field so that the photosphere surrounding the sunspot should be noticeably hotter. Except for the filaments in the penumbra, this doesn’t seem to occur. But the sheer size, sunspots are in the order of tens of thousands of kilometers across, and interaction with cooled gaseous volumes that are now sinking may distort or hide this.
The second hypothesis suggests some heat energy is used to power the magnetic field itself. That a rising plume encounters a slightly stronger than normal magnetic field and feeds it, increasing its strength and pushing it closer to the surface where it becomes a sunspot. This may fit better, accounting for the lower temperature and the fairly consistent temperature at the center of sunspots of any size.
The actual reason may prove to be a combination of the two or something yet to be discovered. Research on sunspots continues, as well as on the sunspot cycles that occur approximately every 11 years and how the cycles in the northern and southern hemispheres get a little out of time with each other and then resynchronize. When sunspot formation is particularly strong it can generate solar flares potentially dangerous to man-made satellites and ongoing space missions, as well as interfering with electromagnetic signaling such as radio transmissions, so it is in our interests to learn as much as we can about these solar phenomena.