New technique to detect magnetic fields inside stars

Scientists have for the first time developed a way to determine the presence of strong magnetic fields deep inside pulsating giant stars by using a technique similar to medical ultrasound.

Magnetic fields have important consequences in all stages of stellar evolution, from a star's formation to its demise.

Now, for the first time, astrophysicists are able to determine the presence of strong magnetic fields deep inside pulsating giant stars.

Researchers used asteroseismology - a discipline similar to seismology - to track waves travelling through stars in order to determine their inner properties.

"We can now probe regions of the star that were previously hidden," said co-lead author Matteo Cantiello, from University of California Santa Barbara's Kavli Institute for Theoretical Physics (KITP).

"The technique is analogous to a medical ultrasound, which uses sound waves to image otherwise invisible parts of the human body," said Cantiello.

Cantiello's curiosity and that of his co-authors was sparked when astrophysicist Dennis Stello of the University of Sydney presented puzzling data from the Kepler satellite, a space telescope that measures stellar brightness variations with very high precision.

The data were explained by the presence of strong magnetic fields in the inner regions of these stars.

The puzzling phenomenon was observed in a group of red giants imaged by Kepler. Red giants are stars much older and larger than the Sun.

Their outer regions are characterised by turbulent motion that excites sound waves, which interact with gravity waves that travel deep into the stellar core. Magnetic fields in the core can hinder the motions produced by the gravity waves.

"Imagine the magnetic field as stiff rubber bands embedded in the stellar gas, which affect the propagation of gravity waves," said Jim Fuller, a postdoctoral fellow at the California Institute of Technology.

"If the magnetic field is strong enough, the gravity waves become trapped in the star's core. We call this the magnetic greenhouse effect," said Fuller.

The trapping occurs because the incoming wave is reflected by the magnetic field into waves with a lower degree of symmetry, which are prevented from escaping the core.

As a result, stellar surface oscillations have smaller amplitude compared to a similar star without a strong magnetic field.

"We used these observations to put a limit on - or even measure - the internal magnetic fields for these stars," Cantiello said.

"We found that red giants can possess internal magnetic fields nearly a million times stronger than a typical refrigerator magnet," Cantiello added. The findings appear in the journal Science.