What is this image?
Credit: NOAO/AURA/NSF (http://www.noao.edu/kpno/40th/mcpierce.html)
This is an image of the optical spectrum of the Sun. What Jean likes about it is that it's a great simple example of a spectrum. It's a beautiful spectrum that contains an enormous amount of information.
The Sun generates a fairly smooth continuum, or rainbow, of light. But as the light from the Sun's surface interacts with the atoms above the Sun's atmosphere, specific wavelengths are absorbed. This produces the dark lines in the image.
Each atom absorbs particular, characteristic wavelengths that depend on the physical conditions in the atmosphere. By studying the absorption lines you can learn which and how much or each of the elements are in the Sun and the temperature and density structure of the solar atmosphere.
Read more about this image on Astronomy Picture of the Day (http://apod.gsfc.nasa.gov/apod/ap030629.html).