The CRAB NEBULA is almost certainly the result of a supernova explosion which was observed by Chinese astronomers on 4 July 1054. This supernova was so bright that it was risible in full daylight for three weeks. Strangely, however, there is no record of its having been seen in any of the European or Arabian observatories known to have been in existence at that time. Possibly the apparition was regarded as too ominous to contemplate, or perhaps Europe had an abnormally cloudy summer that year!

The Crab Nebula is about 2000 pc away and is now about 4pc in diameter. Its mass is about 1M0. Synchrotron radiation is emitted at all wavelengths from radio to X- and possibly gamma rays; the whitish diffuse glow seen in is from this process, and shows a high degree of polarization. The pink-coloured filaments that give the nebula its characteristic crab-like shape and comprise most of its mass, are ionized gas. The pink colour is due to the dominance of the Ha line in the emission-line spectrum of these particular regions. The ionization in the filaments is maintained not by a star, but by the ultraviolet continuum of synchrotron
radiation.

In the ease of the Crab Nebula, we can actually identify the star which exploded to form the supernova, It is arrowed in and was originally identified because ripples of synchrotron light appear to move outwards from it about every three months, presumably carrying new relativistic electrons into the nebula. The extraordinary nature of this object was not fully appreciated until 1007 whoa it was found to be flashing off and on 30 times a second. This star was the first, and remains the only, optical pulsar known, but its pulsations can also be seen at radio, infrared, X-ray and gamma-ray wavelengths. The pulsar is crucial to the nebula since it is the reservoir for the power the nebula radiates into space. A neutron star spinning 30 times a second contains a vast reserve of kinetic, energy. From the rate at which the pulsar is slowing down we can calculate the rate at which the energy is being lost by the pulsar. We find that this power is equal to the total power radiated by the nebula (mainly at X-ray wavelengths). By some process which we do not understand the rotating neutron star is able to accelerate streams of electrons to almost the speed of light. These electrons then carry energy away from the pulsar and radiate it as synchrotron radiation in the outer regions of the nebula. The presence of this internal energy source, in the form of a pulsar, is probably the main reason that the Crab Nebula appears so different from the shell-shaped remnants such as Cassiopeia A.

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