Spinning stars speedier than expected

From planets to galaxies, asteroids to black holes, everything in the universe moves and spins, largely thanks to the good old conservation of angular momentum.

Stars are born spinning too, but as they age, they begin to slow down. Astronomers theorise that this is due to a process called “magnetic braking”, where solar winds are caught by the star’s magnetic field and rob it of angular momentum.

Now, a new study led by the UK’s University of Birmingham shows that old stars aren’t slowing down as quickly as the magnetic braking theory predicts.

This confirms previous observations made back in 2016, which studied the spinning of stars by tracking the movement of dark spots across their surface. But this new paper – published in Nature Astronomy – uses a different method called asteroseismology.

Seismology may be a more familiar field: it’s the study of seismic waves (vibrations) through the Earth’s crust, used to predict and understand earthquakes. Asteroseismology uses a similar principle to study the sound waves that move through the internal structure of stars.

These waves cause oscillations of certain frequencies, which are visible on the surface of the star as vibrations. As the stars spin, the frequencies change slightly – imagine listening to the sirens of two ambulances change as they drive around a roundabout.

By observing how the surface vibrations vary over time, the research team could calculate the star’s rate of rotation – as well as other properties like its mass and age.

“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,” says lead author Oliver Hall from the University of Birmingham.

“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.”

The team is now working on understanding how a star’s magnetic field interacts with its rotation, which may be key to solving this inconsistency.

This kind of research could also help astronomers understand how our Sun will evolve over the next few billion years.

“This work helps place in perspective whether or not we can expect reduced solar activity and harmful space weather in the future,” concludes co-author Guy Davies, also from the University of Birmingham.

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