The photosphere is the bright "surface" of the Sun.  Its temperature is over 10,000° Fahrenheit.  Note the photosphere's granular appearance.  The granules are rising currents of hot gas. They can be up to about 600 miles across and last about 5 minutes before being replaced by another rising current of hot gas. 

    The penumbra is darker than the photosphere because its temperature is several hundred degrees cooler than the photosphere. 

    The umbra appears dark because its temperature is still lower, about 7600°.  Even though the umbra looks quite dark, in reality it would appear dazzlingly bright if you could see it by itself.  It looks dark only because the photosphere is so much brighter! 

    The number of sunspots on the Sun at any one time is quite variable. Sometimes there are none at all.  At other times there may be several hundred. The average number varies over time with a period of approximate 11 years between maximums. 

     The lifetime of a spot can be anywhere from a few hours for a small one to several months for a very large spot. 

       Click button to see NOAO's view of how the number of sunspots have varied over the last 50 years.