The nebular spectra of supernovae differ from those of better-known emission nebulae in that many of the emission lines are optically thick. Here we sketch the theory for interpreting such spectra, and show how it can be used to interpret prominent emission line systems in the spectrum of SN 1987A. As examples, we describe: (1) a simple method to infer the density of O I from observations of the evolution of the doublet ratio in [OI]λλ6300; (2) new kind of hydrogen recombination line spectrum; (3) an analysis showing that the Ca II infrared emission lines must come from primordial, not newly-synthesized, calcium; (4) a theory for the Fe/Co/Ni emission lines that shows that the inner envelope of SN 1987A must have a foamy texture, in which low density radioactive bubbles of Fe/Co/Ni reside in a massive substrate of hydrogen, helium, and other elements.

Introduction

Conventional wisdom holds that supernova explosions produce most of the heavy elements in the universe, and a major goal of astronomy is to test this hypothesis through observations of supernova spectra. For this purpose, SN 1987A should be a Rosetta Stone. We have observed its spectrum in far greater detail than that of any other supernova: at wavelength bands, such as gamma rays and far infrared, where no other supernova has been observed; with almost daily (nightly!) observations continuing for more than seven years after outburst; and with unprecedented spectral resolution (McCray 1993).