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You can read a star’s age by its spin

It turns out, like us, stars slow down in their old age.

That observation is the key to a new method astronomers are using to accurately tell the age of cool stars — stars that match the size of, or are smaller than, our own sun. In a study published Monday in the journal Nature, a U.S. team claims it can estimate the age of these stars to within 10 percent by measuring how fast the star is spinning.

“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. Soren Meibom, lead author of the paper and astronomer with the Harvard-Smithsonian Center for Astrophysics, told the BBC.

But how does one measure a star’s spin?

Astronomers tracked the spin of 30 stars in a 2.5 billion-year-old cluster by following specific sun spots across their surfaces using images captured by the Kepler space telescope. That cluster of stars completed a spin about once every 18 days. The team compared that rotational period to a 2011 observation of a younger, one billion-year-old cluster — which spin about once every 10 days — and our own 4.6 billion-year-old sun, which spins about once every 26 days — showing that the celestial bodies did, in fact, appear to spin more slowly as they aged.

“Such ages will enable us to study how astrophysical phenomena involving cool stars evolve over time,” the study concludes, “and will therefore be important to a wide range of research from the Galactic scale down to the scale of individual stars and their companions.”

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