From the spectral lines of light one can infer the chemical elements in the stars. Each chemical element emits its own specific "fingerprint" on spectral lines or filters out spectral lines from the light spectrum. With the new spectrograph CIRPASS the astronomers searched specifically for certain lines.
In particular, they were interested in the so-called Paschen beta line of hydrogen, which is emitted by young hot stars. The electron of the hydrogen atom can assume a variety of energy states. It can be thought of as the electron can orbit around the atomic nucleus on different orbits in different "heights". When jumping onto another track, a characteristic light radiation is emitted.
By comparing it to a particular spectral line absorbed by carbon monoxide found in the cold atmospheres of ancient giant stars, astronomers were able to deduce the age structure of the stars. displayAxel Tilleman