Pulsars are rapidly-spinning neutron stars (see the Life Cycle of Stars for more on neutron stars) that have very strong magnetic fields. Jets of charged particles are ejected from the magnetic poles of the star. This material is accelerated, producing beams of radiation (light) of all wavelengths from the magnetic poles.
The "north" and "south" poles of the magnetic field may not coincide with the star's rotational poles, as shown in the image below. This means that the beams of light sweep around as the star rotates.
We see a pulsar, then, when one of its beams of radiation crosses our line-of-sight. In this way, a pulsar is like a lighthouse. The light from a lighthouse appears to be "pulsing" because it only crosses our line-of-sight once each time it spins. Similarly, a pulsar "pulses" because we see bright flashes every time the star spins. This is illustrated below.
A typical pulsar has a period of a several tenths of a second to a few seconds. However, there is a class of pulsars, millisecond pulsars, that show pulses every few milliseconds. The fastest pulsar currently known is called PSR B1937+21, which has a period of 1.56 ms. That means that PSR B1937+21 spins nearly 650 times per second!
Gamma-ray image of the Geminga pulsar (above and left of center) and the Crab pulsar (below and right of center). (Image from NASA, Compton Gamma Ray Observatory.)
Geminga was long a mystery of high-energy astrophysics. At first, it was only seen as a steady gamma-ray source, with counterparts in other wavelengths elusive. Then, in 1992, the ROSAT X-ray observatory discovered X-ray pulses and gamma-ray pulses were found shortly thereafter with the Compton Gamma Ray Observatory (CGRO).
Traditionally, pulsars have been the arena of radio astronomers, with pulses discovered in other wavelengths seen to pulse in concert with the radio pulses. However, Geminga is the only confirmed pulsar that does not show pulses in the radio band.
In 1054 A. D., Chinese astronomers recorded the explosion of a star with a mass of about ten times the mass of our Sun. It was visible in the daytime sky for about a month, and in the evening sky for nearly a year. The small star left behind is a neutron star, spinning about 33 times per second. The remarkable star is one of few that pulses in all observable wavelengths, from radio to gamma ray. The Crab was one of the first X-ray sources found in the early 1960s and was also one of the first two pulsars known to pulse in gamma rays (the Vela pulsar was the other).
The Crab nebula, which is the remnant of the supernova explosion that created the Crab pulsar, is such a steady X-ray source that it is often used as a calibration source for X-ray telescopes.
