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Analysis and Interpretation of Astronomical Spectra 107
Planetary nebulae usually pass through all 12 excitation classes, following the evolution of
the central star.
In this regard the SNR are also a highly complex special case. By very young SNR, eg the
Crab Nebula (M1), dominate higher excitation classes whose levels are not homogeneously
distributed within the nebula, according to the complex filament structure [231]. The diag-
nostic line He II at λ 4686 is therefore a striking feature in some spectra of M1, see [33],
Table 85.
