they are cooler than the surr- ounding solar surface (a sunspot is about 3,800 to 4,100 degrees Kelvin in its center, compared with the 5,700 degrees Kelvin of nearby material). What is not clear is why they exist at all; why are some areas of the Sun’s surface cooler than others? Some astronomers think sunspots occur when magnetic fields interfere with the flow of energy from the Sun’s depths, although the process is not fully understood. 
    As is apparent from this image, the term “spot” does not fully reflect the complex structure of sunspots. The dark central region, or umbra, is the coolest and has the strongest magnetic fields. Umbrae range in size from 10,000 to 20,000 kilometers —or one to two times