We can estimate the age of the galaxy by finding the oldest stars in the galaxy. Although it is difficult to estimate the age of an individual star chosen at random, it is much easier to find the age of a cluster of stars. All stars in a cluster form at about the same time.
Clusters form stars of all masses.
The massive hot blue stars are the first to exhaust their fuel supply and leave the main sequence. As the cluster ages, more and more massive stars leave the main sequence leaving only the cooler low-mass stars. Ordering stars by their temperature, which places them in order of their mass, will show which stars will be the next to leave the main sequence. These stars are said to lie at the turnoff point . The age of the cluster is equal to the main-sequence lifetime of a star at the turnoff point.