An important parameter to measure is the mass of a neutron star. This can best be determined in the same manner as for ordinary stars, by the observation of a binary system that contains a neutron star.

To get the masses unambiguously we really need an eclipsing, double-lined binary, in which both components give us radial velocity measurements. Neutron stars have not as yet revealed any spectroscopic lines, but the pulses due to their rotation are just as useful! In fact no spectroseopic measurements would be necessary at all if we could find two pulsars orbiting each other. Unfortunately such a system has not yet been found, although a binary system with one pulsar has been discovered. The binary pulsar PSR 1913 +16 consists of a pulsar with a 59.03ms period in orbit round an invisible companion. This unseen companion cannot be much larger than a white dwarf, and could indeed be another neutron star or even a black hole.

The high eccentricity of the orbit (0.615), the short peri0(j (0.323 days), and high accuracy to which pulse times can b obtained may still allow us to determine the masses of the two components. The last two of those factors mean that Doppler effects, predicted by the theory of special relativity are measurable These lead to a determination of the inclination of the orbit relative to the plane of the sky. APSIDAL MOTION, a gradual shift in the position of periastron, is 4° per year. There are three possible causes for this: first, tidal distortion of the unseen companion, second, rotation of this star and third, general relativistic precession similar to the precession of 43 seconds of arc per century of the perihelion of the planet Mercury. The first two may be ignored if the unseen companion is as compact as a neutron star or black hole. The rate of apsidal motion then yields the sum of the masses of the stars. This, together with the inclination angle and the data deter¬mined from the radial velocities, gives the individual masses of the components. Preliminary analyses suggest that they are each about 1.4 solar masses, but this does depend upon the assumption that the apsidal motion is solely due to effects of relativity.

A precise clock (the pulsar) in a close simple orbit around a companion provides us with a beautiful laboratory for performing experiments. In such a star system, gravitational theories can be put to the test. Most theories predict a similar rate for apsidal advance, but those predicting the emission of gravitational waves, such as Einstein’s theory of general relativity undergo at least one further test. The emission of gravitational waves results in a reduction in the separation of the two stars. The associated change in binary period should be detectable by about 1986.

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