Older Stars Rotate Faster Than Expected

Older stars should slow down, but new observations reveal that they have just as much of a spring in their step as their younger cousins. Astronomers suspect that complex interactions with the star’s magnetic field might be to blame.

All stars spin. And all stars have magnetic fields. As they age, they should slowly spin down through a process called magnetic breaking. Occasionally the magnetic field of a star can fling material away (like in the case of a coronal mass ejection), which saps angular momentum from the star and slows it down.

To test this idea, astronomers had been using observations of starspots to monitor the spin rate of distant stars. Those results had already suggested that stars may be spinning faster than expected, but the technique had been limited to younger stars, as older stars feature fewer spots.

To study stars older than the sun, a team of astronomers at the University of Birmingham turned to another technique: asteroseismology. Their results were recently published in Nature Astronomy.

Asteroseismology studies variations in a star’s output as a way to measure the sound waves crashing around inside of it. When stars spin, the frequencies of vibration can split into different frequencies. The main advantage of this technique is that it can be used on stars of any age.

The team found that older stars were spinning much faster than models of magnetic breaking have predicted. This implies that the situation is much more complex than we had assumed.

Lead author on the paper, Dr Oliver Hall, said, “Although we’ve suspected for some time that older stars rotate faster than magnetic braking theories predict, these new asteroseismic data are the most convincing yet to demonstrate that this ‘weakened magnetic braking’ is actually the case. Models based on young stars suggest that the change in a star’s spin is consistent throughout their lifetime, which is different to what we see in these new data.”

Astronomers will need to develop more sophisticated models to account for the new observations. As to the future of our own sun, co-author Dr Guy Davies said, “These new findings demonstrate that we still have a lot to learn about the future of our own Sun as well as other stars. This work helps place in perspective whether or not we can expect reduced solar activity and harmful space weather in the future. To answer these questions we need better models of solar rotation, and this work takes an important step towards improving the models and supplying the data needed to test them.“