The Crab Nebula (M1, Messier 1, NGC 1952, Tauras A) is the results of a Supernova recorded in 1054. The Nebula was discovered in 1731 with an Apparent Magnitude of 8.4, invisible to the naked eye. In the center is a Pulsar (the Crab Pulsar, PSR B0531+21) with a pulse Frequency of 30/second, which is rare as a pulsar that has been identified optically.
The Crab Nebula has been much studied, being useful in gathering data and testing models. For example, when pulsars were first discovered, it was predicted that a pulsar would be found in the Crab Nebula, and one's subsequent discovery helped confirm the model that pulsars are spinning Neutron Stars formed from supernovae. This also means Crab Pulsar is unique in that its age is known.
Supernova Remnant (SNR)