Messier 1, the Crab Nebula, is the most famous and conspicuous supernova remnant in the sky. It is the centuries old wreckage of a stellar explosion first noted by Chinese astronomers in 1054. The nebula was observed later by English astronomer John Bevis in 1731. It was the first astronomical object identified with a historical supernova explosion. The nebula is constantly expanding at a high rate, currently 1500 km/s or 0.5% of the speed of light.

The most unique feature of the nebula is The Crab Pulsar, discovered in 1968, a neutron star rotating about 30 times per second, lying at the centre of the nebula. The mass of the neutron star is almost double that of the Sun, but its diameter is only 30 km, which is half the diameter of the M25. This means that its density is about 300 million million times greater than that of water and very similar to the density of an atomic nucleus. The pulsar emits high intensity x-rays and gamma rays and is some 100,000 times more energetic than the Sun. It is the strongest persistent source of such radiation in the sky, allowing for very detailed study of any other astronomical objects that occult it. In the 1950s and 1960s, the Sun’s corona was mapped from observations of the Crab’s radio waves passing through it, and in 2003, the thickness of the atmosphere of Saturn’s moon Titan was measured as it blocked out X-rays from the nebula.