By observing and surveying 30 cool solar-type stars in the 2.5 Billion-year-old cluster NGC 6819 an international research team has created an analytical procedure for accurately determining the ages of stars with knowledge of their masses and rotation periods. 

 This "gyrochronology",  a neologism of co-author Sydney Barnes, has been shown to work over a wide age range, significantly improving the accuracy of stellar age determination.

"The relationship between mass, rotation rate and age of the observed stars is now defined well enough that by measuring the first two parameters, the third, the star's age, can be determined with only 10 percent uncertainty," said Barnes.

The speed of rotation of a star decreases over time, and also depends on the mass of the star; heavy stars rotate faster than smaller, lighter ones, as a rule. While some aspects of this basic behavior have been known to astronomers for a while, this work has seized an opportunity to test and clarify the precision and accuracy of the method.

The newly released study firmly provides the intimate relationship between mass, rotation rate, and age of the cool star using coeval (same age) cluster stars so that the ages of even non-cluster (so-called ``field'') stars can be determined. The determination of the rotation rates was carried out by observing changes in brightness caused by star spots on the surface of the observed stars rotating into and out of view. At this age, a typical star changes its brightness by much less than 1 percent. Such precise observations were only possible using NASA's Kepler Space telescope.

This most accurate determination of the age of stars is important in understanding how various astronomical phenomena evolve over time. For example, knowledge of stellar ages can be helpful in targeting the search for life outside our solar system, because it has taken billions of years for the development of the complexity of life on Earth. Planets orbiting stars with ages similar to the sun are therefore seen as particularly promising objects of study.

Published in Nature.