Stars of the sky play a bit coy with their ages—an ancient star can often pass for a much younger one. That is a problem for astronomers seeking out habitable planets orbiting distant stars because a star’s age correlates with the life-forms it could support.

“We know from studying our own planet that if the star and the planet [are] about one billion years old, only the most primitive microbial life might exist,” said Søren Meibom of the Harvard-Smithsonian Center for Astrophysics at the May American Astronomical Society meeting in Boston. “Is it perhaps 4.6 billion years old? Well, all of a sudden we know we could have a planet teeming with complex and intelligent life.”

But, as Meibom put it, “stars do not have birth certificates.” And many visual attributes remain the same for most of a star’s life. One feature, though, does change: stars spin more slowly as they grow old. “And so we can use the spin rate, the rotation rate of a star, as a clock to measure its age,” Meibom said.

But first someone has to paint the numbers on the face of that clock. Researchers have already pinned down the relation between rotation and age for very young stars. So Meibom and his colleagues are measuring the rotation rates of older stars. If they can figure out the relation between age and rotation for many vintages of stars, a star’s age will be much easier to estimate. No birth certificate required.

Editor's note: This story was printed with the title, "Ageless,No Plastic Surgery Required."