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Combined X-Ray and Optical Images of the Crab Nebula

Combined X-Ray and Optical Images of the Crab Nebula

NASA/CXC/ASU/J. Hester et al.
Quite simply, pulsars are rotating neutron stars. This rotation is what makes them pulse.

Pulsars were first discovered in July 1967 by graduate student Jocelyn Bell Burnell and her professor Antony Hewish. Burnell noticed radio sources that blink on and off at a constant frequency and was baffled by the seemingly unnatural regularity of their emissions. Because of this regularity, it was suggested (perhaps facetiously) that the origin could be intelligent extraterrestrial beings. Burnell and Hewish initially called their discovery LGM-1 for "little green men." The following year both Thomas Gold and Franco Pacini separately suggested that these pulsars (PULsating stARS) were actually rotating neutron stars. In 1974, Antony Hewish became the first astronomer to be awarded the Nobel Prize in physics. Many felt that Jocelyn Burnell had been cheated since it was she who had made the initial discovery, though while working as Dr. Hewish's Ph.D student, yet did not share in the award.

That first discovered pulsar was later dubbed CP 1919, and is now known by a number of designators including PSR 1919+21, PSR B1919+21 and PSR J1921+2153.

Although the first pulsars were observed as a source of radio wave emissions, now we observe the brightest ones at almost every wavelength of light. Pulsars are spinning neutron stars that have jets of particles moving almost at the speed of light streaming out above their magnetic poles. These jets produce very powerful beams of light. For a similar reason that "true north" and "magnetic north" are different on Earth, the magnetic and rotational axes of a pulsar are also misaligned. Therefore, the beams of light from the jets sweep around as the pulsar rotates, just as the spotlight in a lighthouse does. Like a ship in the ocean that sees only regular flashes of light, we see pulsars turn on and off as the beam sweeps over the Earth. Neutron stars for which we see such pulses are called "pulsars", or sometimes "spin-powered pulsars," indicating that the source of energy is the rotation of the neutron star.

Pulsars spin fast for the same reason ice skaters pull in their arms to spin. This is conservation of angular momentum. Pulsars are formed with a certain amount of angular momentum. As gravity causes them to shrink (and thus have a smaller radius) they must spin faster in order to conserve angular momentum.

Pulsars come in three varieties, based on the source of energy that powers the radiation (though the Fermi Space Telescope may have discovered a potential fourth class of pulsars that emit only gamma ray radiation):

  • Rotation-powered pulsars, where the loss of rotational energy of the star powers the radiation
  • Accretion-powered pulsars (accounting for most but not all X-ray pulsars), where the gravitational potential energy of accreted matter is the energy source (producing X-rays that are observable from Earth), and
  • Magnetars, where the decay of an extremely strong magnetic field powers the radiation.
The incredibly strong gravitational and magnetic fields of a pulsar make it an excellent laboratory for the study of physical processes in extreme conditions. A pulsar may be seen in gamma rays, X-rays, visible light, radio waves or other bands of radiation. There are many unanswered questions about exactly how different pulsars produce the radiation that we see. At the Marshall Space Flight Center, astronomers are attempting to answer some of these questions. One of the instruments used is the Burst and Transient Source Experiment, an instrument on board the Compton Gamma Ray Observatory, a NASA satellite.

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