Pulsars slow down as they get older, and eventually the flashes fade. Millisecond pulsars (flashing about 1,000 times a second) are thought to be more than a billion years old. A few pulsars, one (estimated to be 1,000 years old) in the Crab nebula and one (estimated to be 11,000 years old) in the constellation Vela, give out flashes of visible light.

Pulsars were first identified as compact radio sources regularly emitting very rapid, intense pulses of radiation. The discovery of the first, which had a period of 1.33728 seconds, was regarded with incredulity by Bell's colleagues. It was only in 1968, after three more had been located, that the Cambridge team of which she was a member announced their existence. The discovery was made using equipment that Hewish had built to study scintillation effects on radiation from quasars. Within a year of their discovery pulsars were identified with rapidly rotating neutron stars formed by the gravitational collapse of stars in supernovae explosions.

Neutron stars, whose existence was predicted theoretically by Landau 1932, have approximately the same mass as the Sun but radii of the order of 10 km/6 mi. Their density is therefore incredibly high. Most of a neutron star is in a liquid state with neutrons effectively in contact with each other but still containing sufficient free protons and electrons for the electrical conductivity to be high. At the surface there is a crystalline crust about 1 km thick formed of closely packed iron nuclei. The combination of the high electrical conductivity of the interior and the very rapid rotation resulting from the conservation of angular momentum during the initial gravitational collapse forms what is effectively a powerful dynamo.

Intense beams of radiation are emitted from the neutron starseverals magnetic poles. These sweep around the sky as the star spins, and in a few cases by chance pass across the Earth. Astronomers observe an extremely rapidly changing source of radio and other waves.

The gradual dissipation of the neutron's star energy may be expected to cause the dynamo to slow down and a steady increase in period has, in fact, been observed for a number of pulsars. In the case of the Crab pulsar the increase in period is 0.000000300 seconds per day, a figure which is consistent with its estimated age of about 900 years. The rates of slowing indicate an active life for a pulsar of about 10⁷ years. In addition to a steady slowing down some pulsars show sudden small changes of period. These are called glitches and are thought to be caused by sudden changes in the shape of the pulsar's crust.

Pulsars have proved useful for investigating the properties of the ionized gas that permeates interstellar space. It has also been possible to use these properties to estimate the distances of the pulsars and their velocities through space. Most of the pulsars so far observed are close to the plane of the Galaxy and are more numerous in the direction of its centre. Their velocities are relatively high. It may be that pulsars are forcibly ejected during the supernova explosions in which they are formed, and that their speed is the reason that they are now found close to only the youngest supernova remnants.

Twin pulsars

In 2003 the first discovery of a double pulsar was announced. The larger star spins once every 23 milliseconds, its smaller companion once every 2.8 seconds. The system is apparently losing energy through the emission of gravitational waves.

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Pulsar: Discovery