Developments Through 1995
About 1,000 pulsars are now known. Almost all are within the Milky Way, but several pulsars have been found in the Magellanic Clouds, the nearest external galaxies.
Additional support for the neutron star model came in 1987 at the start of the observed outburst of Supernova 1987 in the Large Megellanic Cloud, when bursts of neutrinos were detected simultaneously at two widely separated underground observatories (in Japan and Ohio, USA). The theory of supernovae predicts that most of the gravitational energy released during the collapse of a supernova remnant to form a neutron star will be converted to neutrinos. The observed supernova bursts support this theory. The search for a pulsar ar the position of Supernova 1987 continues.
Extremely fast pulsars, which emit pulses at intervals from one to several milliseconds, were discovered in the 1980s. Several of them were found to be members of binary star systems with very short periods of revolution.
This has led to speculation that millisecond pulsars are formed by the merging of a neutron star and another star in a binary system, where the transfer of mass and angular momentum onto the neutron star" spins it up." The distances to most pulsars are uncertain. The nearest estimated distance for a pulsar, is about 280 light-years. All other pulsars seem to be considerably more distant. The Crab Nebula pulsar and the 17 pulsars that have been found in 11 globular clusters have somewhat more reliable distance estimates, but there are thousands and even tens of thousands of light-years from the solar system.
Eight of the 17 pulsars found in globular clusters are members of binary systems. Thirteen pulsars are now known to be members of binary systems. Estimates of pulsar (neutron star) masses from their orbits so far indicate masses from 1.3 to 1.6 solar masses for neutron stars. Pulsars in very close binary systems are being studied in an effort to detect relativistic effects in their strong gravitational fields, which can be used to check the predictions of the general theory of relativity. The discovery of binary pulsars has increased efforts to detect the gravitational waves predicted by this theory. Finally, the three most reliably established extrasolar planets have been discovered orbiting the pulsar-neutron star PSR 1257+12.