Star age

The age of a star tells us how long it has been shining since it formed. Knowing a star’s age helps astronomers understand its life stage, how it has changed over time, and what will happen to it in the future.

Astronomers use a combination of methods depending on the type of star to determine the star's age. For stars like the Sun, they look at chemical composition, temperature, luminosity, and how the star fits into stellar evolution models. If a star is part of a cluster, its age can be estimated more precisely by comparing it to other stars in the group. In some cases, scientists use rotation rate or changes in brightness to estimate age — younger stars spin faster and are more active.

The Sun is about 4.6 billion years old, and it’s roughly halfway through its life. Young stars may be just a few million years old and are often found in star-forming regions. Old stars, like many found in the galactic halo or globular clusters, can be over 10 billion years old.

Astronomers also classify stars into stellar populations based on age and chemical content: Population I stars (like the Sun) are young, metal-rich, and found in the spiral arms of galaxies. Population II stars are older, with fewer heavy elements, and often live in the galactic halo or bulge. Population III stars were the first stars in the universe, made almost entirely of hydrogen and helium — none have been directly observed yet.

In the Stellar Catalog, star ages are listed in billions of years (Gyr) when available.