What are globular clusters and how are they used to estimate the Milky Way’s age?

Globular clusters are massive, spherical systems of stars that orbit the Milky Way Galaxy in its halo. They are among the oldest objects in the galaxy, containing up to a million stars each, and are characterized by a deficiency of heavy elements compared to stars in the galactic disk.1234

Astronomers use these clusters to estimate the age of the Milky Way because they are believed to have formed during the galaxy’s earliest stages. To determine their age, scientists plot the stars of a cluster on a Hertzsprung-Russell diagram (or colour-magnitude diagram). Because massive stars evolve and leave the main sequence more rapidly than smaller ones, the point at which stars "turn off" the main sequence serves as a reliable indicator of the cluster's age. By identifying the oldest globular clusters, researchers can establish a minimum age for the galaxy, which is currently estimated to be approximately 12.5 billion years or older.12354