<P> The pulsar's extreme energy output creates an unusually dynamic region at the centre of the Crab Nebula . While most astronomical objects evolve so slowly that changes are visible only over timescales of many years, the inner parts of the Crab Nebula show changes over timescales of only a few days . The most dynamic feature in the inner part of the nebula is the point where the pulsar's equatorial wind slams into the bulk of the nebula, forming a shock front . The shape and position of this feature shifts rapidly, with the equatorial wind appearing as a series of wisp - like features that steepen, brighten, then fade as they move away from the pulsar to well out into the main body of the nebula . </P> <P> The star that exploded as a supernova is referred to as the supernova's progenitor star . Two types of stars explode as supernovae: white dwarfs and massive stars . In the so - called Type Ia supernovae, gases falling onto a' dead' white dwarf raise its mass until it nears a critical level, the Chandrasekhar limit, resulting in a runaway nuclear fusion explosion that obliterates the star; in Type Ib / c and Type II supernovae, the progenitor star is a massive star whose core runs out of fuel to power its nuclear fusion reactions and collapses in on itself, releasing gravitational potential energy in a form that blows away the star's outer layers . The presence of a pulsar in the Crab Nebula means that it must have formed in a core - collapse supernova; Type Ia supernovae do not produce pulsars . </P> <P> Theoretical models of supernova explosions suggest that the star that exploded to produce the Crab Nebula must have had a mass of between 9 and 11 M. Stars with masses lower than 8 M are thought to be too small to produce supernova explosions, and end their lives by producing a planetary nebula instead, while a star heavier than 12 M would have produced a nebula with a different chemical composition from that observed in the Crab Nebula . Recent studies, however, suggest the progenitor could have been a super-asymptotic giant branch star in the 8 to 10 M range that would have exploded in an electron - capture supernova . </P> <P> A significant problem in studies of the Crab Nebula is that the combined mass of the nebula and the pulsar add up to considerably less than the predicted mass of the progenitor star, and the question of where the' missing mass' is, remains unresolved . Estimates of the mass of the nebula are made by measuring the total amount of light emitted, and calculating the mass required, given the measured temperature and density of the nebula . Estimates range from about 1--5 M, with 2--3 M being the generally accepted value . The neutron star mass is estimated to be between 1.4 and 2 M . </P>

The crab nebula is a cold dust and gas region of the oort cloud