Astronomers have enlisted a continent-wide array of radio telescopes to map the magnetic field of a bloated red giant star. This first magnetic map of another star, to be published in next month's issue of Astrophysical Journal Letters, suggests that at least some stars have magnetic fields like those of Earth and the sun. And the details of the map--about 50 times smaller than the finest features visible to the Hubble Space Telescope--could help theorists model how red giant stars shed gas and dust into interstellar space.
Astronomers can't observe magnetic fields directly. The stellar cartographers--Athol Kemball and Philip Diamond of the National Radio Astronomy Observatory in Socorro, New Mexico--relied on radio waves, which are polarized by magnetic fields. Normally, the radio waves from the star they observed--TX Camelopardalis (TX Cam), which lies about 1000 light-years away--would be too faint to see. But TX Cam is swathed in clouds of excited silicon monoxide molecules. These clouds act as an amplifying medium for radio waves, in much the same way that a laser amplifies light.
To pick up the signals and map their polarization, Kemball and Diamond turned to the Very Long Baseline Array (VLBA), a network of 10 identical radio telescopes that stretch across the United States. By combining signals from its far-flung dishes, the VLBA mimics a single antenna the size of the continent, picking up details that would elude smaller telescopes.
The VLBA's map of radio polarization across TX Cam is "strongly suggestive of order in the magnetic field," says Kemball, although he cautions that the map shows only two dimensions and doesn't pin down the field's north and south poles. But Kemball says the field's most likely structure is similar to that of Earth, with magnetic field lines looping around the planet parallel to lines of longitude. In several spots, the picture is more complicated: Field lines are twisted and kinked, much as they are around flares on the sun. The magnetic kinks could affect how gas and dust escape from the star, which has a 2-year-long cycle of expansion and contraction, says Kemball.
He and Diamond hope to monitor TX Cam every 2 weeks to watch for changes in the field lines to see whether this giant star has a magnetic cycle. Meanwhile, astrophysicists are delighted by their first clear pictures of a star's field. The resolution, says Mark Reid of the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, "is truly astounding."