The Crab Nebula (Messier 1, NGC 1952) is the object which started Charles Messier logging non-cometary objects on his Messier Catalog. It is the expanding cloud of gas thrown off in the explosion associated with the July 4, 1054 supernova recorded by Chinese astronomers, now more than 6 light years across (the nebula is currently expanding at 1000 km/sec and the total mass of ejected material is about 0.1 solar masses). The supernova which produced it was bright enough to be visible to the naked eye during the daytime, rivalling the planet Venus. The Crab Nebula is located approximately 6,500 light years away, in the constellation Taurus. Thus the cosmic event itself happened 6,500 years earlier, ca. 5400 BC.
The Crab Nebula. In the nebula's very center lies a pulsar: a neutron star rotating 30 times a second.
The Crab pulsar. The image combines optical data from Hubble (in red) and X-ray images from Chandra (in blue)
At the centre of the nebula is the Crab Pulsar, a neutron star remnant of the supernova which is roughly 10 kilometers in diameter. It was discovered in 1969. The Crab Pulsar rotates once every 33 milliseconds, or 30 times each second, and the beams of radiation it emits interact with the nebular gasses to produce complex patterns of wind and fluorescence. The most dynamic feature in the inner part of the Crab is the point where one of the pulsar's polar jets runs into the surrounding material 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.
The Crab nebula is often used as a calibration source in X-ray astronomy. It is very bright in X-rays and the flux density and spectrum are known to be constant, with the exception of the pulsar. The pulsar provides a strong periodic signal that is used to check the timing of the X-ray detectors. In X-ray astronomy, 'Crab' and 'milliCrab' are sometimes used as units of flux density. Very few X-ray sources ever exceed one Crab in brightness.