30 to 100-kG magnetic fields in the cores of red giant stars

A red giant star is an evolved low- or intermediate-mass star that has exhausted its central hydrogen content, leaving a helium core and a hydrogen-burning shell. Oscillations of stars can be observed as periodic dimmings and brightenings in the optical light curves. In red giant stars, non-radial acoustic waves couple to gravity waves and give rise to mixed modes, which behave as pressure (p) modes in the envelope and gravity (g) modes in the core. These modes were previously used to measure the internal rotation of red giants, leading to the conclusion that purely hydrodynamical processes of angular momentum transport from the core are too inefficient. Magnetic fields could produce the additional required transport. However, due to the lack of direct measurements of magnetic fields in stellar interiors, very little is currently known about their properties. Asteroseismology can provide direct detection of magnetic fields because, like rotation, the fields induce shifts in the oscillation mode frequencies. Here we report the measurement of magnetic fields in the cores of three red giant stars observed with the Kepler satellite. The fields induce shifts that break the symmetry of dipole mode multiplets. We thus measure field strengths ranging from ~30 to ~100 kG in the vicinity of the hydrogen-burning shell and place constraints on the field tolopolgy.