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Question
Why do sunspots appear dark in visible light?
is it because they only appear dark against the brighter surrounding surface of the Sun, but are actually quite bright?
Thanks
Hello.
Sunspots represent cross-sections of large magnetic "tubes" in the Sun, from 5,000 km to 70,000 km diameter. In these tubes, powerful magnetic fields have trapped solar gas, or plasma.
The surface of the Sun proper (photosphere) gets energy from the zone beneath via convection. Within sunspots, this convection of hot gases (acting similarly to when you heat a pan of hot water) is inhibited by the magnetic field action.
This means the trapped gas has to do much more work, say to bring energy to the surface above. With so much energy expended in doing work against the magnetic field, the enclosed plasma must remain at a rather cooler temperature than the surrounding plasma not in magnetic fields.
The difference in temperature between the cooler and hotter gases can range from 1500-1800 F. It is for this reason that sunspots appear dark. Because of the significantly cooler plasma they contain, relative to the surrounding hot gases.
If the whole solar photosphere were at the same temperature as sunspots, it would still be far too bright to look at with the unaided eye.
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Answers by Expert:
Philip Stahl
Expertise
I have forty years of experience in Astronomy, specifically solar and space physics. My specialties include the physics of solar flares, sunspots, including their effects on Earth and statistics as applied to astronomical investigations.
Experience
Astronomy: more than forty years experience starting with construction of my own simple telescopes. Worked at university observatory in college, doing astrographic measurements. M.Phil. degree in Physics/Solar Physics and more than ten years as researcher.
Organizations
American Astronomical Society (Solar Physics and Dynamical Astronomy divisions), American Mathematical Society, American Geophysical Union
Publications
Solar Physics (journal), The Journal of the Royal Astronomical Society of Canada, The Proceedings of the Meudon Solar Flare Workshop (1986), The Proceedings of the Caribbean Physics Conference (1985). Books: 'Selected Analyses in Solar Flare Plasma Dynamics', 'Physics Notes for Advanced Level'.
Education/Credentials
B.A. Astronomy, M. Phil. Physics
Awards and Honors
American Astronomical Society Studentship Award (1984), Barbados Government Award for Solar Research
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