Interference filter images and long-slit spectroscopy are presented which show that much of the brighter filamentary gas in the Crab Nebula is involved in large structures consisting of a nearly pure helium band or torus across the neutron star and bipolar, helium-rich lobes. It is inferred that the SN precursor may have initially had a mass of 20-30 solar masses, and that the strongest forbidden Ni II emission coincides with regions of relatively low helium abundance in the north-east part of the nebula. The results support a scenario in which interaction and mixing occurs between an ambient interstellar cloud and material produced in the SN.
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MacAlpine et al. (1989) studied this question.