The Crab Nebula is a supernova remnant in the constellation of Taurus. The now-current name is due to William Parsons, 3rd Earl of Rosse, who observed the object in 1840 using a 36-inch telescope and produced a drawing that looked somewhat like a crab.
What is the Crab Nebula?
Corresponding to a bright supernova recorded by Chinese astronomers in 1054, the nebula was observed later by English astronomer John Bevis in 1731. The nebula was the first astronomical object identified with a historical supernova explosion.
At an apparent magnitude of 8.4, comparable to that of Saturn’s moon Titan, it is not visible to the naked eye but can be made out using binoculars under favourable conditions. The nebula lies in the Perseus Arm of the Milky Way galaxy, at a distance of about 2.0 kiloparsecs (6,500 ly) from Earth. It has a diameter of 3.4 parsecs (11 ly), corresponding to an apparent diameter of some 7 arcminutes, and is expanding at a rate of about 1,500 kilometres per second (930 mi/s), or 0.5% of the speed of light.
In the 1960s, because of the prediction and discovery of pulsars, the Crab Nebula again became a major centre of interest. It was then that Franco Pacini predicted the existence of the Crab Pulsar for the first time, which would explain the brightness of the cloud. The star was observed shortly afterwards in 1968. The discovery of the Crab pulsar, and the knowledge of its exact age (almost to the day) allows for the verification of basic physical properties of these objects, such as characteristic age and spin-down luminosity, the orders of magnitude involved (notably the strength of the magnetic field), along with various aspects related to the dynamics of the remnant.
The role of this supernova to the scientific understanding of supernova remnants was crucial, as no other historical supernova created a pulsar whose precise age we can know for certain. The only possible exception to this rule would be SN 1181 whose supposed remnant, 3C 58, is home to a pulsar, but its identification using Chinese observations from 1181 is sometimes contested.
Image: NASA, ESA
Text: Wikipedia contributors. “Crab Nebula.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 6 Aug. 2017. Web.9 Aug. 2017.