Astronomers have proved that they can accurately tell the age of a star from how fast it's spinning.
An American team measured the spin speed of stars that are more than one billion years old - and it matched what they predicted.
The method is for 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.
Finding the age of stars is important in astronomy - a bit like dating a fossil in the study of evolution.
But because the size, brightness and temperature of these stars stays about the same during most of its life, figuring out the age of a star can be tricky.