BOTW Directory - Science > Astronomy and Space > Interstellar Medium

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National Optical Astronomy Observatory - Article on the history of planetarty nebula. Comes with images and links to related pages.
Penn State University - Variability of Planetary Nebula Central Stars - Features article from the institution's Department of Astronomy and Astrophysics on the variability in the central stars of Galactic planetary nebulae.
Planetary Nebulae Gallery - Provides photos and information on the various types of planetary nebulae.