Older stars should slow down, but new observations show they have just as much spring in their crotch as their younger cousins. Astronomers suspect that complex interactions with the star’s magnetic field could be to blame.
All the stars are turning. And all stars have magnetic fields. As they get older, they should slowly spin through a process called magnetic breaking. Occasionally, a star’s magnetic field can throw material away (as in the case of a coronal mass ejection), diverting angular momentum from the star and slowing it down.
To test this idea, astronomers had used star spot observations to monitor the spin rate of distant stars. These results had already indicated that stars might spin faster than expected, but the technique was limited to younger stars because older stars have 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 performance to measure the sound waves tumbling around inside it. When stars spin, the vibrational frequencies can split into different frequencies. The main advantage of this technique is that it can be used on stars of all ages.
The team found that older stars spun much faster than models for magnetic breaking had predicted. This implies that the situation is much more complex than we assumed.
The lead author of the paper, Dr. Oliver Hall, said: “Although we have suspected for some time that older stars are spinning faster than the theories of the magnetic brake predict, these new asteroseismic data are the most compelling to have shown this ‘weakened magnetic brake’ to be actually the case. Models based on young stars suggest that the change in a star’s spin is consistent throughout its lifetime, which is different from what we see in these new data. “
Astronomers need to develop more complex models to take into account the new observations. On the future of our own sun, co-author Dr. Guy Davies: “These new discoveries show that we still have a lot to learn about the future of our own sun and other stars. This work helps put into perspective whether or not we can expect decreased solar activity and harmful space weather in the future. To answer these questions, we need better models of solar rotation. This work is an important step in improving the models and providing the data required for testing. “