THE PULSAR PHENOMENON

The discovery 25 years ago of the remarkable objects which came to be known as pulsars, and their identification as neutron stars, fulfilled a prediction made more than 30 years earlier. Over 550 pulsars are now known, almost all detected at radio frequencies. Their pulse periods range from 1.5 ms to several seconds. Most pulsars are single neutron stars but, in an important subset, the pulsar is in a binary orbit with a companion star. Observations have revealed a wealth of detail about the structure and evolution of pulsars and the pulse-emission process, giving new insight into the behaviour of matter in the presence of extreme gravitational and electromagnetic fields. Pulsars have unique properties which make them nearly ideal probes for a wide range of physical studies. Those observational results which are most relevant to these applications are summarized in this paper.