The Crab Nebula is a supernova remnant. The core of the star survived the explosion as a very compact, dense neutron star. This neutron star (pulsar) is visible in the colour photo above. Near the center of the nebula are two bright stars that are only just resolved. The top left star is just a field star in front of the nebula. The bottom right star (a little fainter than the field star) is the Crab Nebula pulsar.
In 1758 Charles Messier was looking for Halley's comet on its first predicted return. On the 28th August 1758 he mistook this nebula for the comet. He soon realized his mistake and decided to create a list of objects to avoid when comet hunting. Hence this nebula is the first object in his list (M1). It is therefore rather surprising that M1 was again confused with Halley's comet on its second predicted return in 1835! But why is it called the "Crab Nebula"? Lord Rosse made a drawing of M1 in 1844, and apparently his drawing did indeed look a bit like a crab.
We now know that this nebula was created by a supernovae (an exploding star) and we even know when - the supernovae was observed by Chinese astronomers on the 4th July 1054 AD. It was visible in day light for 23 days and it stayed visible in the night sky for 653 nights.
The progenitor star (the star that exploded) had a mass of between 8 and 12 solar masses. If it had been less than 8 it's death would have been less violent - it would have become a planetary nebula. If it had been more than 12 the nebula would have had a different chemical composition. When the star's nuclear fuel finally ran out, the star started to collapse. This collapse releases a vast amount of gravitational energy which resulted in a catastrophic rise in the star's temperature. The outer regions of the star then exploded and a huge amount of energy was released. For a few days a supernova is often as bright as it's parent galaxy.
In this type of supernovae the core of the progenitor star survives as a neutron star. This neutron star is only 28-30 km in diameter but has a mass of between 1.4 and 2 solar masses. This must be the densest material known - it is even more dense than an atomic nucleus! The progenitor star was rotating. As the star collapsed, the rotation speeds up - its similar to a spinning ice skater with out stretch arms. As they bring in their arms, they spin faster. The neutron star therefore rotates very rapidly - it's 'day' is just 33 milliseconds long. The neutron star emits a beam of light which we observe as a pulse as it sweeps past us - just like the beam from a light house. The neutron star is therefore also known as a pulsar. This pulse can be observed across the entire electromagnetic spectrum, from X-rays to radio waves. In visible light the pulsar is of 16th apparent magnitude. Given the large distance of 6,300 light years this equates to the same luminosity as our sun! And it emits even more energy as X-Rays and Radio waves.
The outer portion of the progenitor star formed the nebula. It has now spread over a volume some 10 light years in diameter and is currently expanding at about 1,800 km/sec. The nebula emits light in two different ways. The diffuse region is caused by synchrotron radiation. This is created when electrons traveling at up to half the speed of light are forced to follow a curved path by an extremely strong magnetic field. The filaments are regions of denser gas mainly made up of ionized hydrogen and helium, along with carbon, oxygen, nitrogen, neon, sulphur and iron. These filaments absorb energy from the synchrotron ultra violet radiation and re-emit it as emission lines. The hydrogen alpha emission line is particularly strong, so the filaments emit strongly in the red. The total energy emitted by the nebula is equivalent to about 100,000 times the luminosity of the sun!
The nebula's entire energy source is the rotating neutron star at it's center. Rotational energy is transfered to the nebula via the extremely strong magnetic field. This also produces a braking effect on the neutron star. It's rotation rate is very gradually slowing down.
The nebula is expanding so quickly that when early photographs of the nebula are compared with recent ones the difference in size is relatively easy to see. It is expanding at a rate of about 0.2 arc seconds per year. But this implies that the expansion started in the year 1180 which is 126 years too late! This shows that the rate of expansion has increased since the explosion.