A star system called Algol has been intriguing scientists for more than 300 years. Initially, they were fascinated by its variable brightness — which was explained by the finding that it consists of two stars, the orbits of which eclipse each other when viewed from Earth. More recently, scientists have been keen to use Algol to learn more about the structure of stars' magnetic fields, particularly as Algol's is roughly 1,000 times stronger than that of the Sun. On page 207, Bill Peterson at the University of Iowa in Iowa City and his colleagues provide detailed images of Algol's magnetic field, the first such images of a star other than the Sun. He tells Nature more.
How did you study Algol?
At around 30 parsecs away, Algol is one of the closest, brightest, most active star systems we can observe. To study its magnetic field structure, we made observations using a global network of instruments spread across Earth's Northern Hemisphere.
How does Algol's magnetic field compare with that of the Sun?
Electrons are drawn to, and follow, a star's magnetic field. On the Sun, we've observed ephemeral loops of enhanced electron emission that often pop out because of heightened magnetic activity at the Sun's poles. In Algol, one of the stars is more magnetically active than the other. We show that it has a single giant loop of excited electrons that stretches from its north to its south pole. We think this ramped up level of magnetic activity occurs because the less active star's fast, three-day orbit effectively spins up the other star's magnetic field.
What was the biggest challenge you faced?
We measure very weak natural radio signals from space and turn those into radioimages. The most difficult part is superimposing the radioimage onto the optical image of the star. Because we had high-resolution radio data, we were able to track orbital movements more precisely than has been possible before, which helped us to determine with confidence the physical position and structure of the magnetic field. This in turn provides a new way to accurately determine the position of a star from its radioemissions.
Will future arrays offer even greater resolution of Algol?
The Atacama Large Millimeter Array, which is under construction in Chile, will achieve higher resolution of radioemissions at higher frequencies, which will be good for looking at the evolution of galaxies. But Algol's radioemissions are not as detectable at higher frequencies, so it will be hard to achieve higher resolution — at least in the near future.
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Abstractions. Nature 463, 134 (2010). https://doi.org/10.1038/7278134b