In 1974, astronomer Joe Taylor discovered the first radio pulsar in a binary system (called PSR 1913+16). Because this discovery turned out to be so important (see below), Joe Taylor and his student were awarded the Nobel Prize in 1993. Since astronomers can measure the pulse period (59 milliseconds) and pulse arrival times with great accuracy, we know the orbit of this system and the masses of the two stars more accurately than in any other binary system. Both components are neutron stars with masses very close to 1.4 solar masses, the Chandrasekhar limit mass for a white dwarf star. The two neutron stars orbit each other with a period of 7.75 hours. Click here for pictures and more details about the orbit.

This binary pulsar provides the first definite evidence of gravitational radiation. As the two neutron stars orbit each other, they emit gravity waves, a phenomenon predicted by Einstein's general theory of relativity but never observed directly. Scientists at Colorado and NASA have designed a satellite experiment LISA to detect such radiation, but LISA won't be completed for several years. But we can see the effect of gravitational radiation on the binary pulsar system. The energy that is lost through the emission of these waves causes the two neutron stars to slowly spiral in toward each other, and that causes a decrease in the orbital period that radio astronomers can measure accurately. In fact, the orbit shrinks by about 3.5 meters per year. At that rate, the two neutron stars will merge after another 300 million years.

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