In the year 1054, Chinese astronomers saw what they called a “guest star” appear in the constellation now known as Taurus the Bull. The star quickly became so bright it was visible in the daytime. After a month it slowly faded, taking almost two years to vanish from sight. Hundreds of years later, when modern astronomers turned their telescopes to the location of the guest star, they found a cloud of gas about 11 ly in diameter expanding at 1500 km/s. Projecting the expansion back in time they concluded that it mast have begun about nine centuries ago, just when the Taurus guest star made its visit. 
While the guest star was visible from all northern hemisphere even in the daytime(!), no European records exist. It was the Dark Ages in Europe.
In the center of the gas cloud is a neutron star, the extremely dense remains of the core of the exploded star. When the star exploded the core collapsed, packing 1.4 to 2.0 Sun’s masses into a ball just 20 kilometers across (for comparison, the Sun is nearly 1.5 million km wide).
The Crab Nebula through Planetarium On The Go telescopes. Join us and see for yourself, through the eyepiece of a telescope, what is out there!
Here is the Hubble Space Telescope’s view of the Nebula. Credit: ESA/Hubble & NASA
Because the Supernova Remnant is close at a distance of 6,500 ly and relatively young, it’s possible to actually see the debris from the explosion physically expanding over time! That is what Detlef Hartmann did by observing Crab Nebula for a decade and made this amazing video.
The expansion of the Crab Nebula. Credit: Detlef Hartmann
The neutron star at the center spins rapidly and has a fierce magnetic field. Subatomic particles are accelerated to high speeds and blast away from the poles of the star. These beams sweep around as the star spins, and if a beam passes over us we see a blip of light, a pulse for every rotation. That’s why we call these objects Pulsars. Below is a composite image in the X-ray, visible and infrared part of the spectrum. You can actually see the beams.
In the center of the Crab Nebula, the Crab Pulsar is blasting out beams of radiation and subatomic particles. Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA-JPL-Caltech
 “Stars and Galaxies” 9th Edition, Michael Seeds, Dana Backman, p. 288, (numbers revised)