Star Keeps Magnetic Lock on Its Big Brother

Mutual attraction. The binary Algol stars share a powerful, permanent magnetic field.

W. Peterson et al./NRAO/AUI/NSF

Taking advantage of an unusual pair of nearby stars, astronomers have for the first time captured images of a magnetic field generated by a star other than our sun. Studying that field should help researchers gain a much better understanding of the internal dynamics that produce stellar magnetic fields, which in our sun's case can influence everything from climate to satellite orbits to telecommunications.

Astronomers have indirectly detected magnetic activity associated with other stars. But because of the great distances and the limitations of telescopes, no one had been able to observe another star's magnetism directly. It's important to do so, however, because without other stars for comparison, astronomers can't determine whether the sun's magnetism is normal.

In today's issue of Nature, a team of astronomers reports a breakthrough. They collected images of the magnetic field generated by one of the stars in the Algol system, located about 93 light-years away. The two stars in the system--one about three times more massive than the sun and the other a little less massive--are so close to each other that one orbit takes only 3 days. The smaller star is the source of the magnetic field, and even though that field is about 1000 times stronger than the sun's, imaging it still required two arrays of radiotelescopes, plus two extra dishes, to detect the signals generated by magnetic fields. One array stretches from Hawaii to the Virgin Islands, whereas the other occupies an expanse of the New Mexico desert, and the individual dishes sit in West Virginia and Germany.

The resulting images show a giant magnetic loop extending from the north and south poles of the smaller star all the way to its larger partner, located about 9 million kilometers away. The loop persisted for all 6 months of observations, instead of flaring up and quieting down, like our sun's field. It's a mystery why the loop seems to be permanent, says astrophysicist and lead author William Peterson of the University of Iowa in Iowa City. He and colleagues plan to use the arrays to look for magnetic loops on other stars and investigate them with computer simulations.

The data on the Algol magnetic field are "quite beautiful," says astrophysicist Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. They're bound to "improve our ability to predict solar magnetic storms, which impact satellites orbiting the Earth." And extending the observations to other binary stars "is an important step toward understanding all magnetic structures in astrophysics, he says.

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