Astronomers can measure how fast stars spin by observing “starquakes”—seismic tremors that are the equivalent of earthquakes on our planet. Yet these observations have posed a puzzle because many stars seem to be spinning slower than they should be. In a new study, researchers modeled how a magnetic field could grow in the internal layers of a star, dragging its rotation down.

Many stars' cores contract at some point, especially toward the ends of their lives when they have ceased fusing hydrogen in their centers. Usually this contraction would speed up a star's spin, just as figure skaters will twirl faster when they pull their arms in. Concentrating more mass in a smaller space will force an object to speed up to preserve angular momentum.

Graphic visualizes the analogy of a spinning star core increasing speed as it gets smaller to a figure skater twirling faster as she pulls her arms in.
Credit: Lucy Reading-Ikkanda

But the actual spin rate of many stars is slower than theory predicts, particularly in old stars.

Graphic shows a star’s actual and predicted spin rates with the core spinning slower in the actual scenario.
Credit: Lucy Reading-Ikkanda

In a new numerical model, researchers found that a small, random magnetic field inside the radiative layer of a star could be amplified by the plasma's flow. Once strong enough, this magnetic field spurs turbulence in the star's plasma, which in turn strengthens the magnetic field, which boosts the turbulence, and so on.

Graphic highlights the radiative zone of a star near the edge of the core.
Credit: Lucy Reading-Ikkanda

This magnetic force exerts a powerful torque on the star's plasma, slowing its spin. “It causes a braking effect,” says Florence Marcotte, a scientist at Côte d'Azur University in France, who co-authored the study published in Science.

Graphic uses color coding to indicate magnetic field strength and angular velocity fluctuations in the radiative zone before and after the onset of turbulence.
Credit: Lucy Reading-Ikkanda; Source: “Spin-Down by Dynamo Action in Simulated Radiative Stellar Layers,” by Ludovic Petitdemange, Florence Marcotte and Christophe Gissinger, in Science, Vol. 379; January 20, 2023 (reference)

This mechanism is compatible with observations of the spin rates of neutron stars and white dwarfs. It could possibly occur within the sun's radiative zone as well.

Graphic shows newly calculated slower spin rate with magnetic lines in play.
Credit: Lucy Reading-Ikkanda