Issue 4.01

The Astrophysics Spectator

January 10, 2007

Happy new year. I begin this year with a page on radio pulsars, spinning neutron stars that generate radio waves. This page discusses the basic mechanism that causes a solitary neutron star to appear as a pulsing radio beacon.

Neutrons stars first appeared to astronomers as pulsars. They were discovered in 1967, and they were predicted to exist a short time before their discovery—a rare example of an astrophysical prediction preceding a discovery. The pulsar is the most common form taken by observed neutron stars. Despite living no more than 100 million years, hundreds of these stars are observed in our Galaxy. They appear to be born once every 50 years in the galaxy, so the number of active pulsars in the Galaxy should number roughly a million.

Pulsars are the rapidly-spinning product of a supernova explosion. Each is born with an extremely strong magnetic field and a high rotation rate. In effect, they are dynamos, spinning magnets that convert their rotational energy into electric current that generates radio waves.

For a theorist, pulsars provide an environment for exotic physics. The electric fields generated at the surface of a pulsar can accelerate particles to close to the speed of light, and it can cause the spontaneous creation of electrons and positrons. The magnetic fields in pulsars are so strong that the electrons in these fields move in quantized energy levels, much as they do in the electric field of an atom. For the observer, the pulsar provides a tool for probing the environment between us and the pulsar, and a mechanism for testing general relativity.

Next Issue: The next issue of The Astrophysics Spectator will appear in two weeks on January 24.

Stars

Radio Pulsars. Radio pulsars are neutron stars that convert their rotational energy into radio waves. A pulsar is effectively a giant rotating magnet. When a magnet rotates, it generates an electric field that can drive an electric current. As the current travel along the lines of magnetic field in the pulsar's magnetosphere, it generates electromagnetic radiation. While most radio pulsars are around 10 million years old, the best known radio pulsar is less than 1,000 years old: the Crab pulsar, which was created in a supernova seen on Earth in 1054 AD. (continue)