Between 4 and 9 solar masses of the progenitor of the Crab nebula supernova are missing, unaccounted for in the filaments and pulsar. Deep spectroscopy and a reanalysis of a direct Schmidt telescope image show a hydrogen-emitting halo around the Crab nebula which can be satisfactorily interpreted as the low-velocity stellar wind of the progenitor. The halo's mass is about 4 solar masses; the progenitor of SN 1054 was thus about 8 solar masses.
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Paul Murdin (1994) studied this question.