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Dr. Bozidar Mitrovic: Let me show you a photograph of a sunspot. This is part of the
visible surface of the Sun. It turns out -- as we'll find out --
that the surface temperature of the Sun is about 6,000 degrees. The
sunspots are just parts of the Sun that are colder than the
surrounding area for reasons that we will discuss later on.
Again, there is a similar terminology used here. These things have
nothing to do with the shadow but nevertheless, the terminology is
that the central darker part of the sunspot is called "umbra" and
the outer edge, which is not as dark, is called "penumbra." But
that has nothing to do with shadows cast by an object. Basically
what it is, is that the brightness is proportional to the
temperature to the fourth power. The sunspots are just areas on the surface
of the Sun where the temperature is lower than the surrounding,
and because the brightness scales with the fourth power of
the temperature, they appear to be less bright and we perceive them
as dark spots.
It's not the hole in the Sun or anything like that. It's just the
same surface but because it turns out that the magnetic field of
the Sun is streaming out of these regions, the temperature is
reduced and we perceive them as less bright.
Their number, when the Sun is very active like it is right now, you
probably read in the papers or heard on the news or read on the
web, there are about tremendous solar flares that are happening
right now. When the Sun is very active, when it has these huge
flares, it's also the time when the number of sunspots is near to
maximum.