Space
How Do We Know How Old Stars Are? The Science of Stellar Ages
Determining a star's age is one of astronomy's most difficult problems — stars carry no birth certificates. Astronomers use stellar evolution models, colour-magnitude diagrams of star clusters, radioactive isotope ratios, asteroseismology (listening to stars vibrate), and gyrochronology (how stars spin down with age). The oldest stars in the Milky Way are 13.5 billion years old — almost as old as the universe itself.