A pulsar is a rotating neutron star with a strong magnetic field that constantly emits regular, short pulses of electromagnetic energy. We detect pulses of energy here on Earth because the rotation of the star periodically beams radiation from the star's magnetic poles in our direction. Pulsars were discovered in 1967 by Jocelyn Bell, an English graduate student in radio astronomy, and her advisor, Antony Hewish, received the 1974 Nobel Prize in Physics for his role in the discovery.

Pulsars were discovered during a fruitful time in the history of radio astronomy. The 1960's saw the development and planning of several large-scale radio telescopes around the world, as well as the discovery of new types of radio-bright astronomical objects, particularly the radio-loud quasars and active galaxies. It was during a survey of the sky for these quasars that Jocelyn Bell, a graduate student at University of Cambridge, first discovered pulsars. Bell found an object that changed in radio brightness not in days or hours, but in seconds. Because the speed of light is finite, this meant that the object in question had to be very, very small, since light can't travel very far in a few seconds. Later, objects would be found that pulsed not in seconds, but in milliseconds. Furthermore, these objects literally pulsed like clockwork -- variations in period were almost immeasurably small. Early on, it was speculated that these pulsed signals might be the signature of intelligent civilizations, and the objects were fancifully given the designation "LGM" -- little green men. Later, they were found to be wholly natural in origin, and were dubbed "pulsars".

Shortly after their discovery, it was realized that pulsars were the first reliable detection of neutron stars, previously hypothetical objects created by the collapse of a degenerate star more massive than the Chandrasekhar limit of 1.4 solar masses. The pulses of energy come from the interaction of charged particles in the circumstellar environment with the incredibly strong, compressed magnetic fields of these stars; the rotation of these stars turn their magnetic poles into cosmic lighthouses, sweeping the sky with radio waves.

It is now known that pulsars are not just radio-bright objects. Since they emit synchrotron radiation, if the conditions are right they can also emit pulsed light in the infrared, optical, and even X-ray and gamma-ray energy regimes. For example, the pulsing X-ray source Geminga is visible only at high energies, and not in radio light. Magnetars are a new and exciting class of objects believed to be pulsars with extreme magnetic fields, a thousand times stronger than those of "ordinary" pulsars. They emit bursts of gamma-ray radiation, and are now known as "soft gamma repeaters" or "SGRs". The existence of millisecond pulsars and pulsars with decreasing periods suggests that some pulsars are accreting matter, and "spinning up" as matter falling onto them applies a torque that makes them spin faster. Many neutron stars are also known to exist in binary systems (see Scorpius X-1 for example) and are not known to be pulsars. While all neutron stars probably have a magnetic field (even a weak one), pulsars seem to be those that have strong fields whose magnetic poles happen to sweep across our line of sight as they rotate. There is a wealth of interesting physics to be explored in these extreme objects, including the behavior of matter at extreme densities, fundamental particle physics, supernova physics, and the physics of supernova remnants and the interstellar medium.

Currently (July 5, 2004), the ATNF Pulsar Database(1) lists 1488 pulsars distributed around the sky.

Postscript: the Nobel Prize

The discovery of neutron stars was yet another triumph for astrophysical theory, which had also successfully predicted the existence not only of white dwarfs but of the Chandrasekhar limit as well, and the discovery was ultimately deemed worthy of a Nobel prize in physics. The 1974 Nobel committee awarded the prize solely to Bell's advisor, Antony Hewish of University of Cambridge. This caused some controversy, and a few scientists (notably Fred Hoyle) said Bell had been denied proper credit for what was largely her work -- she built the telescope that was used in the discovery and analyzed the data. In a 2004 editorial, Bell suggested that Hewish received credit since, at the time, the popular myth was that research was "carried out by distinguished men leading teams of unrecognized minions..."(2) Though Bell did not (and apparently does not still) begrudge Hewish the Nobel Prize, she has stressed the need for cultural changes in the sciences, particularly in the role of women. Posterity has since acknowledged Bell's primary role in the discovery of pulsars and she has been awarded several other prestigious prizes in physics and astronomy.

(1) http://www.atnf.csiro.au/research/pulsar/psrcat/
(2) Bell-Burnell, S. Jocelyn, 2004, "So Few Pulsars, So Few Females," Science vol. 304, num. 5670, p. 489

Some historical information taken from the "Pulsars" special issue of Science, vol. 304, no. 5670 (23 April 2004).
I also made use of the NASA ADS abstract service to research this writeup: http://adsabs.harvard.edu.

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