The age of a giant star is one of its most difficult traits to measure, but a new method tells all.

Astronomers have developed a technique to measure how old a star is by analysing a chemical ‘fingerprint’ of its starlight.

Scientists can’t uncover a star’s age through direct observations. Instead, researchers typically infer this attribute by comparing observations of stars with the results of models that predict how these bodies evolve through their lifetimes. Some stars, for instance, eventually bloat into giants.

Giada Casali at the University of Florence in Italy and her colleagues have found an alternative method. They analysed data gathered by telescopes, including the Sloan Foundation Telescope in Sunspot, New Mexico, and calculated stars’ ratios of carbon to nitrogen. These elements are commonly produced in a star’s core and rise to its surface as it swells into a giant.

The team inferred an accurate age for the members of 36 ‘clusters’ — each containing stars that are roughly the same age — using the conventional method of comparing the characteristics of these bodies to model predictions. The scientists then used the results to calibrate the relationship between stellar age and carbon-to-nitrogen ratio, generating a ‘chemical clock’ that could be used to estimate how old any giant star is.