Sunspots
Sunspots are regions on the solar surface that appear dark because they
are cooler than the surrounding photosphere, typically by about 1500 K (thus,
they are still at a temperature of about 4500 K, but this is cool compared to the
rest of the photosphere). They are only dark in a relative sense; a sunspot
removed from the bright background of the Sun would glow quite brightly.
Closeup of a Sunspot
The adjacent image shows a closeup of a sunspot.
One can also see clearly in this image the granulation of the solar surface.
A typical
sunspot consists of a dark central region called the umbra and somewhat
lighter surrounding region called the penumbra. These can be seen clearly in the image
(click on "Show Labels" to label the image).
Groups of Sunspots
The largest sunspots observed have had diameters of about 50,000 km, which
makes them large enough to be seen with the naked eye. Sunspots often come in
groups with as many as a hundred in a group, though sunspot groups with more than
about ten are relatively rare. A group of sunspots is shown in the adjacent right image.
Sunspot Numbers
Sunspots develop and persist for periods ranging from hours to months, and are
carried around the surface of the Sun by its rotation (a fact known to Galileo; see the right frame).
There are well-established methods for
measuring the
number of sunspots.
Here, for example, is a
table
giving the daily count of observed sunspots in
the year 1996.