A planetary nebula is a beautiful object created during the final stages of the life of a star whose birth mass was between 1 and 8 solar masses. The wispy, colorful halo of gas making up the nebula and surrounding the dying star is actually material that was originally part of the star itself but has been cast off and is expanding outward into interstellar space. It glows as the result of being heated by the ultraviolet radiation produced by the dying star. The word planetary is really misleading, as these objects have nothing to do with the planets in our solar system. Rather, they acquired the name because when they were first observed in the 19th century their extended appearance (versus the point-like image of a normal star) reminded astronomers of the way planets like Uranus and Neptune appear in a telescope. In a galaxy such as our own Milky Way there are estimated to be several thousand planetary nebulae at any one time. Most of them are concentrated toward the plane of the Milky Way's disk, but a few are also know to exist in the halo and a number have been identified in the bulge of the galaxy as well.
What's so interesting about planetary nebulae? Astronomers are drawn to study these objects because they provide opportunities to analyze material that was once a part of a shining star. For example, by studying the chemical composition of the nebula we can gain an understanding about the material out of which the star originally formed. In addition, the abundances of certain elements such as carbon and nitrogen in the nebula reveal details about the physical processes that occurred within the star during its nuclear fusion lifetime. Studying planetary nebulae helps us to understand how a star changes, or evolves, during its lifetime.
But why and how does a planetary nebula form in the first place? Interestingly enough, it's related to the star's lifelong battle against the relentless force of gravity. In order to keep from collapsing on itself, a star maintains high internal gas pressure by creating its own energy through nuclear fusion. During most of the star's life the principal fuel for fusion is hydrogen, but as the star exhausts it supply of this element, it will rely increasingly on heavier, more complex elements. Ultimately, however, available fuels run out, the star becomes unstable, and it ejects its outer gaseous layers which expand outward and form the nebula. Two primary sources of data are provided here for over 160 planetary nebulae: a spectrum and a digital image. The lines in each spectrum can be analyzed to determine nebular properties such as chemical composition, temperature, and density. The images in turn provide an opportunity to study the morphology of each nebula and, ideally, to be able in the future to correlate it with composition.
We have written three handout exercises making use of this database. The first is a brief introduction to emission lines in planetary nebulae; the exercise uses the maximum ionization levels in three nebulae as a "thermometer" to indicate the relative central star temperatures. The second exercise explains the Balmer decrement and its observed variation caused by interstellar reddening, to allow inferences to be made about the distribution of dust in the Milky Way Galaxy. The third exercise explores the use of the S+ lines to determine nebulae density.