Sunspots, such as this one, appear dark:
Why?
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$\begingroup$ Considering how bright the sun is, the sunspots could appear relatively dark and still be quite bright. $\endgroup$– trevorKirkbyCommented Dec 8, 2013 at 6:07
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$\begingroup$ Plotting them in dark, is just a representation, not of visible light, but of relative temperature (infra-red light). $\endgroup$– harogastonCommented Jul 5, 2014 at 5:48
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7$\begingroup$ What prior research was done here? It is easy to answer based on wikipedia and other readily available resources that are just a simple google search away. Indeed you appear to have answered your own question using just such resources. It is baffling to me that such a simple question (and your answer) have received so many upvotes. $\endgroup$– ProfRobCommented Mar 4, 2016 at 14:50
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1$\begingroup$ @RobJeffries It might have received so many upvotes because it appears on the first page of Google for "sunspots dark". It's probably helping people, which isn't a bad thing. Also, this is a self-answered question, a feature of the system which users are encouraged to utilize. If you have more information, please feel free to write your own answer. $\endgroup$– user19Commented Mar 4, 2016 at 15:08
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$\begingroup$ I see. However, the question could be answered by minimal research effort. Indeed the second block quotation that you use is actually from the wikipedia page for sunspots, not from the NASA resource you cite. $\endgroup$– ProfRobCommented Mar 4, 2016 at 15:59
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2 Answers
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Typical sunspots have a dark region (umbra) surrounded by a lighter region, the penumbra. While sunspots have a temperature of about 6300 °F (3482.2 °C), the surface of the sun which surrounds it has a temperature of 10,000 °F (5537.8 °C).
From this NASA resource:
Sunspots are actually regions of the solar surface where the magnetic field of the Sun becomes concentrated over 1000-fold. Scientists do not yet know how this happens. Magnetic fields produce pressure, and this pressure can cause gas inside the sunspot to be in balance with the gas outside the sunspot...but at a lower temperature. Sunspots are actually several thousand degrees cooler than the 5,770 K (5496.8 °C) surface of the Sun, and contain gases at temperature of 3000 to 4000 K (2726.9 - 3726.8 °C). They are dark only by contrast with the much hotter solar surface. If you were to put a sunspot in the night sky, it would glow brighter than the Full Moon with a crimson-orange color!
Sunspots are areas of intense magnetic activity, as is apparent in this image:
You can see the material kind of getting stretched into strands.
As for the reason it is cooler than the rest of the surface:
Although the details of sunspot generation are still a matter of research, it appears that sunspots are the visible counterparts of magnetic flux tubes in the Sun's convective zone that get "wound up" by differential rotation. If the stress on the tubes reaches a certain limit, they curl up like a rubber band and puncture the Sun's surface. Convection is inhibited at the puncture points; the energy flux from the Sun's interior decreases; and with it surface temperature.
All in all, the sunspots appear dark because the are darker than the surrounding surface. They're darker because they are cooler, and they're cooler because of the intense magnetic fields in them.
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1$\begingroup$ Is there any image of the sunspot alone, so that we can see that it's actually pretty bright? $\endgroup$ Commented Mar 1, 2014 at 14:43
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$\begingroup$ That would be a neat picture. I'll look for some, although I somewhat doubt I'll find any. $\endgroup$– user19Commented Mar 1, 2014 at 14:44
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$\begingroup$ I asked because my searches failed. It would be interesting to see the structure of the area which is black on most pictures. $\endgroup$ Commented Mar 1, 2014 at 14:49
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Sunspots are cooler because their magnetic fields inhibit the replenishing of heat from convective flows (due to the interaction between plasma and magnetic fields). This allows them to cool radiatively. The rest of the solar surface is constantly being replenished by convective cells that reheat it.
Solar plasma at the photosphere radiates roughly as a black-body, meaning that the energy (and wavelength) spectrum of radiation follows the Planck function. The Planck function scales up and down with temperature at all energies (and wavelengths). The brightness of a blackbody at a given energy (or wavelength) is determined by the value of the Planck function at that energy.
The image you showed is taken in the "photospheric continuum", which is a black-body dominated part of the radiation spectrum. So, because sunspots are cool (compared to their surroundings), this means that their Planck function is lower than their surroundings, and hence their brightness is lower, causing them to appear dark in the image.
