Crab Nebula

Gizmodo has the story of the crab nebula, which was a star that went NOVA in 1054 AD and noticed by Chinese astronomers

At its center is a tiny, dense mass made up mostly of neutrons, rotating very rapidly and emitting regular pulses of radiation across the whole electromagnetic spectrum. Pulsars like this one are what’s left behind when a star explodes. The material that doesn’t get ejected by the explosion collapses on itself. If there’s enough mass, the collapse forms a black hole, but smaller stars end up as pulsars.
At the center of the Crab Nebula, the Crab Pulsar is as massive as our Sun, but it’s only about 30 kilometers in diameter. It rotates about 30 times a second, and atronomers have used its pulses of radiation to study the Sun’s corona and the atmosphere of Saturn’s moon Titan, based on how radio and X-ray waves passed through or were blocked.