Neutron Star

Many neutron stars look like the drawing in Figure 1. The neutron star's rotation axis is inclined with respect to its magnetic axis. (The same situation prevails on Earth.) The neutron star's rapid rotation and intense magnetic field cause radiation to be emitted in narrow beams from the magnetic poles. If the Earth happens to be in line with one of these beams, we will see a flash every time the neutron star rotates and carries the beam past us.

This is precisely what we see in the Crab Nebula. The rate of the flashes indicates the neutron star is spinning 30 times per second. (Compare that with the Sun's rotation period of about one month.) If a pulsar's beam never sweeps past our line of sight, however, we will never see it.

Some neutron stars are members of binary systems, in which they orbit another star. The intense gravity of the neutron star drags material off its companion. The material forms an accretion disk as it approaches the neutron star. In the process it becomes so hot that it begins to emit large amounts of x rays. Only a gravitational field as strong as one predicted to be associated with a neutron star could heat the infalling gas this much.

Even more spectacular are neutron stars that combine the above phenomena: the binary pulsars. In these systems we see pulses of x rays, as the powerful beams of high-energy radiation sweep past us.

Neutron stars have also provided the first direct evidence of planets outside the solar system. The pulsar PSR 1257+12 rotates 161 times per second-but its pulses are not evenly spaced, as are the ones from the Crab. Observations made with the giant Arecibo radio telescope showed that some pulses arrive slightly too soon, others just a bit too late. This means something is tugging the pulsar back and forth slightly. Careful measurements showed that a pair of planets is responsible.