The Crab nebula is a supernova remnant found in the Taurus constellation. The only remnant of the source star is the Crab Pulsar in the middle which spins every 30 seconds or so an emits gamma rays and x-rays.
The radiation emitted by this awesome supernova remnant is often used to calculate more intersolar bodies. For example, in the 1950's and 1960's the sun's corona was mapped by the Crab Nebula's radio waves passing through it and more recently the thickness of the atmosphere of Titan, one of Saturn's confirmed moons, was studied using the Crab Nebula's radio waves as well.
The Crab Nebula is 6,300 light years away and has an apparent magnitude of only +8.4.
The supernova was recorded by astronomers in 1054 A.D. Below is a sample of what they may have seen: