When a massive star collapses into a dense neutron core, the angular momentum of the entire star is forced into a much smaller structure. Like an ice skater who pulls her arms inward when spinning, this causes that star's rotation to speed up tremendously (up to almost 1,000 times per second).

Neutron stars have strong magnetic fields, and we can detect the radiation emitted from the poles (which point up and down along one axis, rather like the North and South poles of the Earth). But because the magnetic poles of a neutron star are not aligned with its axis of rotation, as the star spins the radiation will spin in and out of our field of view (pointing toward and then away from us). This causes the neutron star to pulse on and off in a regular, periodic sequence, and gives it the name pulsar (short for pulsating star).

Can we detect pulsations from every single nearby pulsar in the Milky Way? What happens if the magnetic field of a nearby pulsar traces out a circle on the sky which does not pass over the Earth? From our point of view, this object is a neutron star, but not necessarily a pulsar. If the pole never points in our direction, then the narrowly focused beam of radio waves will not be detectable here on Earth.

Will a pulsar pulse on and off at the same rate (frequency) if observed from two different locations? As long as each location lies along the path that the magnetic pole traces out over the sky, a radio signal will be detected from the pulsar once per cycle, once for each time that the pulsar makes a full rotation and the radio beam can be detected.