A pulsar is a neutron star that spins rapidly, beaming out regular pulses of radio waves — rather like an invisible cosmic lighthouse.
The first pulsar was detected by a Cambridge astronomer called Jocelyn Bell Burnell in 1967.
At first astronomers thought the regular pulses might be signals from aliens, and pulsars were jokingly called LGMs (short for Little Green Men).
Most pulsars send their radio pulse about once a second. The slowest pulse only every four seconds and the fastest every 1.6 milliseconds.
The pulse rate of a pulsar slows down as it gets older.
The Crab pulsar slows by a millionth each day.
More than 650 pulsars are now known, but there may be 100,000 active in our galaxy.
Pulsars probably result from a supernova explosion — that is why most are found in the flat disc of the Milky Way, where supernovae occur.
Pulsars are not found in the same place as supernovae because they form after the debris from the explosion has spread into space.
We know they come from tiny neutron stars often less than 10 km across, because they pulse so fast.
The Crab nebula contains a pulsar also known as NP0532. It is the youngest pulsar yet discovered and it probably formed after the supernova explosion seen in the Crab nebula in AD 1054. It has a rotation period of 76 0.0331 seconds, but it is gradually slowing down.