sentences:  Estimates of the total mass of the nebula are important for estimating the mass of the supernova's progenitor star.The amount of matter contained in the Crab Nebula's filaments (ejecta mass of ionized and neutral gas; mostly helium) is estimated to be .One of the many nebular components (or anomalies) of the Crab Nebula is a helium-rich torus which is visible as an east-west band crossing the pulsar region.