How To Gauge the Age of a Star? It's all in the Spin

Keeping accurate time and determining age are two crucial, constant goals in science. In the 1700s the proof and construction of an elegant, precise maritime clock opened up much safer and more efficient ocean exploration and provided a way forward for more accurate mapping on Earth. Before then, mariners and astronomers alike were both, literally, at sea.

Likewise, until now, determining the age of stars has been equivalent only to saying that a person is young or old, and our guesses of someone’s age are typically off by as much as 15 percent. But by building on the work of others (as it goes in science) and carefully working out for over a decade how to construct a “clock” to measure the ages of stars, Sydney Barnes, of the Leibniz-Institut fuer Astophysik Potsdam (AIP), Germany, derived an elegant and extraordinary method he named “gyrochronology” to derive a star’s age from its spin rate and its mass.

Barnes named his method from the Greek “gyros” which equals rotation, “chronos” which means time and “logos” for study.

“We here develop an improved way of using a rotating star as a clock, set it using the sun and demonstrate that it keeps time well,” wrote Barnes in 2007 (PDF), but his work on this groundbreaking theory goes back to 2000.

Now, in a new study published in the journal Nature and announced today at the 225th American Astronomical Society meeting in Seattle, Barnes and his colleagues have measured more than 20 sun-like stars believed to have identical ages, all belonging to a single a star cluster, and by showing that gyrochronology gives an age of 2.5 billion years for all of them to within 10 percent, have essentially proven the method beyond reasonable doubt.

“In fact, the uncertainty on the gyro-age of the cluster as a whole is two percent, which means that the new clock is now more precise than the ones used to set it,” said Barnes.

The study’s team, led by Soren Meibom, of the Harvard-Smithsonian Center for Astrophysics, in Cambridge, Mass., used NASA’s Kepler Space Telescope to measure the tiny variations of starlight over days or weeks that are caused as dark spots on the surfaces of the stars are alternately revealed and hidden by the rotation.

These space telescope measurements represent the culmination of a hard slog for over a decade by Meibom, Barnes and the other co-authors using ground-based telescopes to acquire and analyze the required support observations, to develop the theoretical framework adequately and to measure and interpret other appropriate clusters for possible deviations.

Among other crucial things, this work ensures that all the stars measured are indeed cluster members and that their interpretation resides securely within a framework that works for all other current knowledge about rotating stars.

“We have found that the relationship between mass, rotation rate and age is now defined well enough by observations -- and is sufficiently supported by the theory -- that it is possible to obtain the ages of non-cluster stars to within 10 percent,” said Barnes.

In the scientific race of the 1770s to determine longitude at sea, John Harrison designed and constructed a maritime clock that pinpointed longitude to within one half of a degree. Similarly, using the mathematical clock of gyrochronology, combined with state-of-the-art measurements from the Kepler Space Telescope and hard work from ground-based ones, this science team has made a quantum leap in accurately determining ages of stars.

“Now we can look at a random cool star on the sky and use the best gyrochronology models to get its age,” said Meibom.

“This means that precise ages can be derived for large numbers of cool field stars in our galaxy,” added Barnes.

Gyrochronology ushers in a new era in astronomy and astrophysics by enabling a better understanding of the chronologies of astronomical phenomena and how our galaxy was assembled over time. It could also help in selecting which planets to target in the search for ‘exo-life.’

But, like the stars it measures, gyrochronology is interesting a priori. An outline history of how this fundamental method was derived is also an unknown but compelling story. To this author it is a modern day history rather like the scientific journey towards accurately determining longitude at sea.

“After my academic training on the East Coast, I was lucky enough to be able to go out West for postdoctoral work,” recalled Barnes. “There I spent two years mostly relearning how to do science empirically by hiking Grand Canyon and wandering among the trees of the American Southwest, including those that inspired the founders of dendrochronology.” Dendrochronology is a branch of science concerned with determining the ages of trees by studying tree rings.

“In 2000 (published in 2001) in Madison, Wisconsin, where Soren and I became friends, I was examining the rotation rates of exoplanet host stars and realized that the overwhelming majority were not defined by the presence of planets,” Barnes recalled.

“Rather, their periods, like those of all the other cool stars, changed very simply with only two variables: stellar age and stellar mass. Two additional years of work resulted in my 2003 paper naming and describing gyrochronology.”

Since no one seemed to appreciate that such ages would actually be better than others, he published another paper in 2007, proving that such ages would also be precise.

Read more at Discovery News