The pulsar as a laboratory
The fact that the pulses sent in our direction by a pulsar are highly regular proves exceedingly useful in the case of binary star systems in which a pulsar orbits another neutron star. The orbital motion influences the arrival time of the pulses - for instance, if its orbit has taken the pulsar a bit further away from Earth, its pulses will take slightly longer to reach us; in addition, there is a relativistic time delay whenever a pulse passes in close proximity to the companion neutron star. All in all, astronomers can obtain from the arrival times both crucial orbital data as well as the masses of the two neutron stars with high accuracy. The important role played by relativistic effects makes such systems ideal cosmic laboratories for testing general relativity - in situations with gravitational fields that are much stronger than those in the solar system.