8 The spectrum of the shell

From the elements we have identified through the emission lines, it is impossible to derive a unique type. Effectively CII, O III, N II and Fe III require a very early type around B0. On the contrary Ca II, Si I, S I correspond to a late A or early F type. As a compromise we may adopt a spectral type of A0 which would correspond to a temperature of 10 000 K. Leibowitz (1977) on theoretical grounds derived a temperature of 8 000 K.