Supernovae, neutron stars and pulsars
Imagine two stars that dance around each other in a game of gravitational tug-of-war. One is a high-mass star, and the other a low-mass star. Heavy stars live shorter lives than less heavy stars and the high-mass star's life therefore soon - after a few million years - ends violently as a supernova. When stars explode as supernovae their central parts collapse and very often leave a neutron star behind. This is a peculiar body so dense that it consists of the densest form of matter known to exist. The electrons and protons that form the normal matter in the exploding star are squeezed into neutrons and exotic subatomic particles. The particles are packed so tightly together that a sugar cube sized piece of neutron star would weigh 200 billion kg (about the weight of a million passenger planes).
This bizarre class of gravitationally collapsed, burned out stars preserve the original star's rotational energy, but due to their small size - about the same a fair-sized city, like Paris - their rotation is very fast. At the poles of neutron stars radiation streams out in two narrow beams sweeping across the skies like the beam from a lighthouse. If the Earth happens to lie along the line of sight we observe what is known as a pulsar.
Origin of millisecond pulsars - Spinning up a neutron star
In 1982 the first example of a rapidly blinking neutron star, or 'millisecond' pulsar, was found with the mighty 300-metre Arecibo radio telescope in Puerto Rico. Since then more than 90 millisecond pulsars have been found and the well-established `recycling scenario' has been proposed for the acceleration process: When the pulsing neutron stars get old, the companion low-mass star ages gracefully and expands as it becomes a red giant. At some point it overflows its gravitational confines and the pulsar starts to accrete parts of its gas thereby spinning it up to breakneck speed over the next tens of millions of years. A millisecond neutron star is born.