Stars Spin Huge Magnetic Loop
Artist's conception of the Algol binary star system with radio image superimposed on the grid.
Credit: Peterson et al., NRAO/AUI/NSF

Astronomers have detected a giant magnetic loop sweeping out from a pair of binary stars in the Milky Way.

"This is the first time we've seen a feature like this in the magnetic field of any star other than the sun," said William Peterson of the University of Iowa.

The stellar pair, called Algol, includes a star about three times more massive than the sun along with a less-massive companion. The two lie about 93 light-years away from Earth. They have been known since ancient times as "The Demon Star" because they appear as one object that blinks on and off ? a phenomenon caused when one star passes in front of the other.

Within the binary, the smaller star orbits the larger at a distance of 5.8 million miles, only about 6 percent of the distance between Earth and the sun.

The newly discovered magnetic loop emerges from the poles of the mini star and stretches outward in the direction of the primary star. As the little star orbits, one side ? the side with the magnetic loop ? constantly faces its companion, just as the same side of our moon always faces the Earth.

The researchers discovered the magnetic loop by making extremely detailed images of the system using a network of large radio telescopes called the High Sensitivity Array. This collection includes the National Science Foundation's Very Long Baseline Array (which includes radio dishes across the United States), the Very Large Array in New Mexico, the Robert C. Byrd Green Bank Telescope in West Virginia, and the Effelsberg radio telescope in Germany. When used as a single observing system, the network offers very high resolution and sensitivity to detect distant radio waves.

Algol, in the constellation Perseus, is visible to the naked eye and well-known to amateur astronomers. As seen from Earth, the two stars regularly pass in front of each other, causing a notable change in brightness. The pair completes a cycle of such eclipses in less than three days. The variability in brightness was discovered by an Italian astronomer in 1667, and was finally understood as an eclipsing binary in 1889.

The newly discovered magnetic loop helps explain phenomena seen in earlier observations of the Algol system at X-ray and radio wavelengths, the scientists said. In addition, they now think there
may be similar magnetic features in other double-star systems.

Peterson and team reported their findings in the Jan. 14 issue of the journal Nature.