HD 166620 is a star currently in a Maunder Minimum — a prolonged period of suppressed magnetic activity analogous to the Sun’s quiet interval from 1645 to 1715. Its chromospheric activity has been declining for decades, placing it among the handful of known Maunder Minimum analog stars.

Spectropolarimetric observations reveal why the activity is suppressed: the large-scale magnetic field is weak. The measured dipole field strength is approximately 1.1 Gauss — substantially below what stars of similar age and rotation rate normally exhibit.

This measurement matters because it connects to a broader puzzle about stellar spin-down. Stars lose angular momentum through magnetized stellar winds: the magnetic field channels the wind, which carries away rotation. Standard models predict that magnetic braking weakens smoothly with age. But observations show a transition — some stars appear to stop spinning down, as if their magnetic brakes failed.

The leading hypothesis is that the large-scale magnetic field itself weakens at this transition, reducing the torque applied by the wind. HD 166620 provides the first direct empirical evidence: a star that has crossed the transition and whose field is measurably weaker than expected.

The star is not merely quiet — it is magnetically diminished. The activity suppression is not a surface phenomenon (fewer spots, less chromospheric emission) masking a normal underlying field. The field itself has weakened. The brake has not merely been released; the mechanism that generates the braking force has changed.