Most of us have marveled at the striking images of planetary nebulae taken with the Hubble Space Telescope, and wondered how evolved red giant stars manage to throw off their outer layers into such intricate forms and symmetries. What is not widely appreciated is that this glowing, ionized gas represents only a small part of a much more massive, extended structure. The ionized gas (seen in the optical emission lines) of most planetary nebulae is surrounded by the partly atomic, sometimes partly molecular, fossil shell of a mass-loss phase that lasted for thousands of years. This material is virtually invisible in the optical spectral region (though faint giant halos are seen in some deep optical pictures). In this talk, I will introduce you to a variety of spectroscopic methods for probing and studying these neutral layers, which range from optical and UV absorption-line spectroscopy to mid and far-infrared emission line observations, and summarize what these diverse methods have told us about the true nature of planetary nebulae.
Reference: "A Survey of Planetary Nebulae at Na D: Evidence for Neutral Envelopes," Dinerstein, Sneden, and Uglum 1995, ApJ, 447, 262