Other
Planetary Nebulae Observer Home Page
This home page is linked to detailed descriptions of nebulae types, shapes, and structures and their relationship to stellar evolution, a historical perspective on the discovery of nebulae, a gallery of images, and other related resources.
Khan Academy
Khan Academy: Absorption/emission Lines
An article discussing why absorption lines of Fraunhofer lines appear.
Wolfram Research
Wolfram Science World: Gustav Kirchhoff
This site from ScienceWorld provides a short but factual biographical sketch of the German physicist, Gustav Kirchhoff and describes his most notable discoveries and contributions to the realm of science.