PSR B1913+16

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Obital decay of PSR B1913+16.The data points indicate the observed change in the epoch of periastron with date while the parabola illustrates the theoretically expected change in epoch according to General Relativity.

PSR B1913+16 (also known as J1915+1606) is a pulsar in a binary star system, in orbit with another star around a common center of mass. In 1974 it was discovered by Russell Alan Hulse and Joseph Hooton Taylor, Jr., of Princeton University, a discovery for which they were awarded the 1993 Nobel Prize in Physics.

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.

The pulsar and its companion both follow elliptical orbits around their common center of mass. Each star moves in its orbit according to Kepler's Laws; at all times the two stars are found on opposite sides of a plane passing through the center of mass. The period of the orbital motion is 7.75 hours, and the stars are believed to be nearly equal in mass, about 1.4 solar masses.

The minimum separation at periastron is about 1.1 solar radii; the maximum separation at apastron is 4.8 solar radii. In the case of PSR B1913+16, the orbit is inclined at about 45 degrees with respect to the plane of the sky. The orientation of periastron changes by about 4.2 degrees per year in direction of the orbital motion (relativistic precession of periastron). In January 1975 it was oriented such that periastron occurred perpendicular to our line of sight.

The orbit has evolved since the binary system was initially discovered, in precise agreement with the loss of energy due to gravitational waves predicted by Einstein's General Theory of Relativity.

The rate of decrease of orbital period is 0.0000765 seconds per year, the rate of decrease of semimajor axis is 3.5 meters per year, and the calculated lifetime to final inspiral is 300,000,000 years.^[2]

• Mass of companion 1.387 MSun
• Orbital period 7.751939106 hr
• Eccentricity 0.617131
• Semimajor axis 1,950,100 km
• Periastron separation 746,600 km
• Apastron separation 3,153,600 km
• Orbital velocity of stars at periastron (relative to center of mass) 450 km/sec
• Orbital velocity of stars at apastron (relative to center of mass) 110 km/sec

[edit] Star of Bethlehem

It was theorized that this pulsar was the Star of Bethlehem. About it, the famous inventor of the satellite and sci-fi novelist, Arthur C. Clarke said,

How romantic, if even now, we can hear the dying voice of a star, which heralded the Christian era.^[3]

[edit] References

• J. M. Weisberg and J. H. Taylor, Relativistic Binary Pulsar B1913+16: Thirty Years of Observations and Analysis, July 2004.