Astronomers make magnetic map of a star.

Storing and unleashing vast amounts of energy, guiding the motion of chunks of matter hurled into space, and keeping the lid on cauldrons of roiling gas, magnetic fields profoundly influence the destiny of stars. The sun is close enough to Earth for astronomers to track its magnetic structure by monitoring sunspots, the dark markings that house intense fields. Other stars, however, lie too far away for researchers to have gleaned much about their fields.

Now, scientists have for the first time mapped in detail the magnetic field of a star other than the sun. Relying on the sharp eye of a continentwide array of radio telescopes, astronomers measured the average magnetic field near the surface of the elderly star TX Camelopardalis (TX Cam), which resides 1,000 light-years from Earth.

The field appears to be 5 to 10 times stronger than the sun's, they report in the June 1 Astrophysical Journal Letters. It also appears to have a remarkably well ordered structure that resembles the bar magnet pattern--exemplified by magnetic lines of force looping from the north pole to the south pole--exhibited by Earth and the sun.

Such a pattern isn't necessarily surprising, notes Athol J. Kemball. He and Philip J. Diamond, both of the National Radio Astronomy Observatory in Socorro, N.M., weren't sure what to expect when they began studying the star with the Very Long Baseline Array (VLBA), a network of telescopes stretching from Hawaii to the Virgin Islands.

Because researchers can't measure the magnetic field of a distant star directly, they use the polarization of radio waves as a tracer. TX Cam proved tailor-made for this approach. Kemball and Diamond had already shown that silicon monoxide gas in the star's cool, bloated atmosphere exhibits maser emission, the radio wavelength equivalent of the bright light of a laser. The polarization of radio waves from this kind of maser is exquisitely sensitive to magnetic fields. In addition, the maser resides low in the atmosphere, providing a probe of magnetic structure close to the star's surface.

The VLBA, which can discern features one-thousandth the size of those recorded by the Hubble Space Telescope in visible light, reveals that most of the maser emission forms a shell around the star. This suggests that the magnetic field resembles that of a bay magnet.

Disruptions in that pattern, says Kemball, may indicate regions of intense magnetic activity, possibly places where mass is expelled. TX Cam is known to shed the equivalent of one-third of Earth's mass each year, far more than the sun expels.

Calling the radio technique "a tour de force," Moshe Elitzur of the University of Kentucky in Lexington says it provides "the only hope to see what's going on at the surface of a star."