We know that stars slow down over time, but until recently there was little data to support exact calculations.

For the first time, a US team has now measured the spin speed of stars that are more than one billion years old - and it matches what they predicted.

The finding resolves a long-standing challenge, allowing astronomers to estimate a star's age to within 10%.

The work was presented in Seattle at a meeting of the American Astronomical Society and also appears in the journal Nature.

(BBC News)