Introductory Astronomy: Pulsars

This image shows the Crab Nebula, a supernova remnant. A pulsar lies at the center of the nebula; it flashes light toward Earth 30 times every second.

Pulsars were first discovered in 1967. Due to their periodic nature, they were first called LGMs (for Little Green Men, since it was proposed that they were actually signals from extraterrestrials). It was determined, though, that the some of the signals were actually originating from regions where supernovae had occurred (like the Crab and Veil nebulae).

It turns out that pulsars are rotating neutron stars, the stellar remnants of high mass stars. As the star collapses into a neutron star, it begins to spin faster and its magnetic field increases. As the field increases, high energy radiation is emitted along the magnetic poles. If these poles are not aligned in the same direction as the rotation axis (the line about which the star spins), then the magnetic pole is swept across space over time.

The concept of pulsars can be explained using the lighthouse model, as shown below.

Just as a lighthouse sweeps light across an area over time, so too does a pulsar. If the emission from the pulsar happens to cross our line of sight, we register this as a "pulse" of radiation originating from the star. When the magnetic pole is not pointing towards us, we do not see the emission. Because the pulsar spins at a uniform rate, the pulses appear periodic, that is, a pulse is seen at regular intervals. The periods of pulsars range from 33 milliseconds to 3.7 seconds.

Pulsars are slowing down over time; this is due to the loss of rotation energy through the radiation pulses. The slowing of pulsars is thought to account for the variety of pulsation periods we observe for different pulsars.