PSR B1913 plus 16

PSR B1913+16 (also known as PSR J1915+1606, PSR 1913+16, and the Hulse–Taylor binary after its discoverers) is a pulsar (a radiating neutron star) which together with another neutron star is in orbit around a common center of mass, thus forming a binary star system. PSR 1913+16 was the first binary pulsar to be discovered. It was discovered by Russell Alan Hulse and Joseph Hooton Taylor, Jr., of the University of Massachusetts Amherst in 1974. Their discovery of the system and analysis of it earned them the 1993 Nobel Prize in Physics "for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation."

Using the Arecibo 305m antenna, Hulse and Taylor detected pulsed radio emissions and thus identified the source as a pulsar, a rapidly rotating, highly magnetized neutron star. The neutron star rotates on its axis 17 times per second; thus the pulse period is 59 milliseconds.

After timing the radio pulses for some time, Hulse and Taylor noticed that there was a systematic variation in the arrival time of the pulses. Sometimes, the pulses were received a little sooner than expected; sometimes, later than expected. These variations changed in a smooth and repetitive manner, with a period of 7.75 hours. They realized that such behavior is predicted if the pulsar were in a binary orbit with another star, later confirmed to be another neutron star. Pulses from the companion neutron star have not been detected, but this might only be the result of an unfavorable viewing angle.

The pulses from the pulsar arrive 3 seconds earlier at some times relative to others, showing that the pulsar’s orbit is 3 light-seconds across, approximately two-thirds of the diameter of the Sun. Since this is a binary system, the masses of the two neutron stars can be determined, and they are each around 1–3 times the mass of the Sun. Observations have shown that the pulsar’s orbit is gradually contracting, possibly an evidence for the emission of energy in the form of gravitational waves, as described by Einstein’s theory of general relativity, causing the pulsar to reach periastron slightly early. Also, periastron advances 4° per year in longitude due to the gravitational field (thus the pulsar’s periastron moves as far in a day as Mercury’s moves in a century).