A massive star dies in an explosion called a supernova. Only the collapsed core remains. If the core is very dense it becomes a neutron star which rotates rapidly, sending out beams of energy. If these beams reach Earth they are picked up as pulses, and the body is called a pulsar. A supernova also occurs if a white dwarf star in a binary pair blows up when material from the other star falls on it.
On 23 February, 1987 a brilliant new star seemed to blaze in the Large Magellanic Cloud. It was easily visible to the naked eye. In fact, it was not a new star, but an existing star (called Sanduleak -69°202) that had exploded as a supernova.
The glowing cloud called the Crab SNR was caused by a supernova first recorded by Chinese astronomers in ad 1054. When the supernova exploded it blasted a great cloud of gas into space. Astronomers call such a cloud a supernova remnant (SNR). Inside the Crab SNR is a pulsar, flashing on and off 30 times a second.
Astronomer Bell Burnell discovered the first pulsar when working as a research student at the Cambridge radio observatory. On 6 August, 1967, she picked up an unusual radio signal: pulses repeating every 1.337 seconds. Astronomers later identified the pulses as coming from a rapidly rotating neutron star.