31 Aquilae is an aging star that has puffed up into a subgiant. It once resembled the Sun, but has since cooled and grown larger. It rotates very slowly — one full turn takes many days.

The star's magnetic field acts like a brake: it grips the surrounding gas and slows the rotation. In young stars, this brake is strong, but it weakens in middle age. For 31 Aquilae, the brake has kicked back in. This is evident from the way the star is losing speed. Using NASA's TESS satellite, astronomers measured its rotation and saw that the spin is slowing down.

The magnetic field of a star is tied to its internal motion. Inside the star, hot gas rises and falls like boiling water in a pot. This motion generates the magnetic field. In 31 Aquilae, the field is strong and doesn't change with time. That means the gas inside is moving differently than in the Sun. The Sun's field flips every 11 years, but 31 Aquilae's field is frozen.

The researchers also modeled how the star rotated in the past. It turns out the star has crossed a threshold beyond which the magnetic brake switches back on. This threshold depends on the Rossby number — the ratio of the star's rotation period to the time it takes for gas inside to rise. When the Rossby number gets large, the brake engages.

Now the team wants to check whether other old stars behave this way. If so, it could help us understand how stars like the Sun will age. Perhaps the Sun, too, will one day regain a strong magnetic brake.