The existence of a third magnetic pole is suggested by the range of signals emitted by the pulsar at the centre of the Crab Nebula, 6,300 light years from Earth. The suggestion is at odds with previous theories, which hold that pulsars, like other bodies, have two poles, north and south.

The Crab Nebula, which is the remains of an exploded star, has at its centre a pulsar, or neutron star, which is the super-compressed remains of a giant star. It rotates 30 times a second and emits strong pulses of radiation ranging from gamma rays to radio waves.

Strong magnetic forces are thought to focus the radiation into two narrow beams, in much the same way as a lighthouse emits light, which can be received by observatories.

The radiation beams emitted from the north and south poles of the pulsar should, according to current theory, be identical. But observations from the Arecibo Observatory in Puerto Rico have identified extra blasts of energy that appear to emanate from another area of the pulsar. These emissions between the predicted signals have led scientists to speculate that they may have identified a third magnetic pole.

“This is a cool result,” said Jean Eilek, from New Mexico Tech University, Socorro, New Mexico. “The fact that the ‘left hand’ and the ‘right hand’ of the pulsar — or the north and south magnetic poles — don’t know what each other is doing is very striking. It knocks just about every existing theory of pulsar radio emission for a loop. Maybe we’ve discovered an unknown, unexpected third magnetic pole somewhere else in the star. These strange emission features are not showing up in other pulsars. Maybe the magnetic field is not as simple as we think.”

The radio signals from the Crab Nebula last for periods as short as four-tenths of a nanosecond — a billionth of a second — but can be as powerful as 10 per cent of the energy emitted by the Sun.

Tim Hankins, acting director of the observatory and one of the researchers who identified the radio pulses, said that the frequency and duration of the signals were completely unexpected. Professor Hankins said: “We never see the strange frequency structure in the main pulse and we never see the really short blasts in the interpulse . . . this is the first time seeing this in a pulsar.”

He and Professor Eilek presented the findings at a conference of the American Astronomical Society in Seattle.

By observing the behaviour of radio waves, scientists mapped the Sun’s corona, and by measuring X-rays that were blocked, they were able to calculate the thickness of the atmosphere of Titan, Saturn’s moon.

John Bevis, an English doctor and astronomer, observed the Crab Nebula in 1731, followed by Charles Messier, a French astronomer, in 1758. It is said to have received its name after the Earl of Rosse said that his drawing of it looked like a crab.

Analysis in the early 20th century revealed that it was still expanding. By measuring the speed, scientists were able to work out that the original star had exploded about 900 years ago.

The pulsar at the centre of the nebula was discovered soon after the first pulsar was identified in 1967, which initially prompted suggestions the radiation beams were a signal from another lifeform.

The Crab Nebula

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