Pulsar in Crab Nebula Has Four Poles, Astronomers Suggest

John Roach in Seattle, Washington
for National Geographic News
January 9, 2007

A pulsating star, or pulsar, in the Crab Nebula may be the first known celestial body to have more than two poles, astronomers said Monday.

Most pulsars are dipolar, meaning they have two magnetic poles: north and south. The radio waves that pulse from the poles are identical to each other.

"We've got a much more complicated magnetic field [in the Crab Nebula pulsar] than the simple dipole model," Tim Hankins, acting director of the Arecibo Observatory in Puerto Rico, said at press briefing Monday.

Hankins and colleague Jean Eilek from New Mexico Tech in Socorro studied the radio waves coming from the pulsar's poles and found they are dramatically different.

"What we think is going on is that there is another pole which is influencing and distorting the magnetic field," said Hankins, who is also an emeritus professor at New Mexico Tech.

If there is a third pole then there is presumably a fourth as well, Hankins added, since all magnetic fields are dipolar.

Hankins presented the research at the 209th meeting of the American Astronomical Society in Seattle, Washington.

Crab Nebula

Pulsars are dense, rapidly spinning neutron stars that are relics of supernova explosions—the powerful death throes of massive stars.

The Crab Nebula pulsar rotates 30 times a second and emits strong pulses of radiation that range from radio rays to gamma rays.

The pulsar's strong magnetic field concentrates most of its radiation pulses into cones that beam from the poles, like light beamed from a lighthouse.

Observers detect the pulses whenever these emission cones sweep toward Earth.

Continued on Next Page >>


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