Astronomers studying the double star system Algol at radio wavelengths were surprised to find a giant magnetic loop stretching out from one of the stars, the first time a magnetic structure has been seen around a star other than our Sun.

The double star system comprises a star roughly three times more massive than our Sun with a less massive companion orbiting it at a distance of just 5.8 million miles (around six percent of the Earth-Sun distance). The observations were made using 13 different telescopes including the National Science Foundation’s Very Long Baseline Array, Very Large Array, Robert C. Byrd Green Bank Telescope and the Effelsberg radio telescope in Germany. Combined, the telescopes offered a highly sensitive array that provided the resolving power needed to detect very faint radio waves.

“We imaged the coronal loop using a global array of radio telescopes,” says Robert Mutel of the University of Iowa College of Liberal Arts and Sciences Department of Physics and Astronomy . “We also carefully compared radio and optical coordinates, so we know where the radio source was located with respect to the star. Earlier attempts to image stellar coronal loops in visible light resulted in fuzzy blobs, but we used a global array of radio telescopes to make a series of images over a six-month period. High resolution radio interferometry allows us to image features which would otherwise be undetectable.”

This is the first time that astronomers have seen a feature like this in the magnetic field of any star other than the Sun. The newly-discovered magnetic loop emerges from the poles of the less-massive star and stretches outward in the direction of the primary star. As the secondary star orbits its companion, the side with the magnetic loop constantly faces the more-massive star.

The astronomers say that the coronal loop at Algol is similar to those seen at the Sun, but with a magnetic field some 1,000 times more powerful, and that its unexpectedly large size is likely attributed to tidal effects distorting the loop and stretching it.

Further study of the Algol's coronal loops could lead to discoveries of similar magnetic features in other double star systems as well as teach us about processes occurring on our own Sun.“We really need to understand our Sun,” says Mutel. “The Sun is close to us and can be studied, but it is only one star. By studying other stars, we will be able to put its behavior into a broader context.”

Coronal loops at the Sun are associated with sunspots, which in turn are associated with space weather – the constant stream of charged particles flowing outward from the Sun that can influence everything from communications systems and satellites, to the health of astronauts working in space. “Perhaps we can work toward predictions of space weather. Maybe we can better understand the physics of space weather through a study of coronal loops,” adds Mutel.

Algol is located 93 light years from Earth and is visible to the naked eye in the constellation Perseus. It is a popular subject for amateur astronomers since the two stars regularly pass in front of each other, causing a notable change in brightness over a period of less then three days.

The scientists discuss their findings in the 14 January edition of the journal Nature.