The researchers studied a cluster of stars with a known age
Astronomers have proved that they can accurately tell the age of a star from how fast it is spinning.
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.
Closing a gap
Establishing the age of stars is a central question in astronomy - much like dating fossils is crucial to studying evolution.
This method applies to "cool stars" - suns about the size of our own, or smaller. These are the most common stars in our galaxy and they also last for a long time.
"They act as lamp posts, lighting up even the oldest parts of our galaxy," said senior author Dr Soren Meibom from the Harvard-Smithsonian Center for Astrophysics.
Cool stars also host the vast majority of earth-like planets that we have spotted in the distance.
Most properties of a star like ours - like its size, mass, brightness and temperature - stay about the same throughout most of its life.
This makes figuring out a star's age decidedly tricky.
The solution of measuring spin was first proposed in the 1970s and was dubbed "gyrochronology" in 2003.
"A cool star spins very fast when it's young, but just like a top on a table it gets slower and slower as the star grows older," Dr Meibom said.
But it is difficult to see a star spinning. Astronomers use sun spots, travelling across the surface, and these only dim its brightness by much less than 1%.
Old stars are particularly problematic, because they have fewer and smaller spots.
Dr Meibom's team used images from the very sensitive Kepler space telescope, which has been trailing Earth around the Sun since 2009.
They managed to measure spin speeds for 30 stars in a specific cluster known to be 2.5 billion years old.
This cluster, known as NGC 6819, plugs what Dr Meibom called a "four-billion-year gap" in our knowledge of stellar spin.
