One way astronomers estimate the age of the universe by studying globular clusters. Globular Clusters are a dense collection of roughly a million stars. Since all of the stars in a globular cluster formed at roughly the same time, they can serve as cosmic clocks. This way of estimating the age of the universe is based on our understanding of the life cycles of stars.
The life of a star depends upon its mass. High mass stars are much brighter than low mass stars; they rapidly burn through their supply of hydrogen fuel. A star like the Sun has enough fuel in its core to burn at its current brightness for approximately 9 billion years.
A star that is twice as massive as the Sun will burn through its fuel supply in only 800 million years. A 10 solar mass star, a star that is 10 times more massive than the Sun, burns nearly a thousand times brighter and has only a 20 million year fuel supply.
Conversely, a star that is half as massive as the Sun burns slowly enough for its fuel to last more than 20 billion years. So if a globular cluster is more than 10 million years old, then all of its hydrogen burning stars will be less massive than 10 solar masses.
This implies that no individual hydrogen-burning star will be more than 1, 000 times brighter than the Sun. If a globular cluster is more than 2 billion years old, then there will be no hydrogen-burning stars more massive than 2 solar masses.
