In 1758, Charles Messier found this nebula while hunting for comets. When he
noticed that, unlike a comet, it didn't move, he decided to create his now famous
catalog (from his point of view a catalog of things that shouldn't be
mistaken for comets).
The supernova the created the Crab exploded on about July 4 1054 AD.
It was seen and recorded by Chinese astronomers and perhaps others.
The gas is expanding so fast that we can see actual changes by comparing new
photographs with those taken early in this century. Notice how much more compact
M 1 is than the Vela SNR which is 10 times older.
At the center of the Crab is a pulsar, the neutron star that remains from the
original star. Neutron stars are fantastic objects -- about the size of the
Earth but with the mass of the Sun and rapidly rotating (the Crab pulsar rotates
30 times per second, some pulsars ten times faster).