stellar age determination

Stellar age determination is, at best, estimated. One method which aims at the age of a specific star uses stellar rotation: stars are presumed to rotate due to angular momentum in the molecular clouds from which they form. They slowly lose this rotation as stellar wind carries away angular momentum, so a particular rotation rate is characteristic of a particular age. This method of estimation is called gyrochronology.

A number of factors give some overall notion of the star's age:

• Some particulars of the star imply a certain relative age: T-Tauri stars, protoplanetary disks, giant stars.
• Stellar mass determination: massive stars have a shorter lifetime, thus necessarily are relatively young stars, e.g., millions of years old rather than billions.
• Spectrography for metallicity: over time, the metallicity of molecular clouds and stars has increased as nucleosynthesis within stars and supernovae has formed elements beyond hydrogen and helium. Stars whose spectra indicate no metals are presumed to be old enough to be born in a time of lower metallicity, and vice versa.

Determining the ages of one or more neighboring stars can helps determine the age their other neighbors as well; Stellar clusters (e.g., globular clusters, open clusters), stellar association, and moving groups are assumed to have formed stars over a relatively short period (i.e., they are coeval), thus the ages of some stars in the cluster (e.g., massive stars, necessarily young) implies a similar age for stars in the cluster that will have a longer lifetime.