The oldest globular clusters contain only stars less massive than 0.7 solar masses. Upper right shows that the core of the cluster is composed of many, many stars. Hubble Space Telescope image of the Globular Cluster M15. 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.

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 10 million years old, then all of its hydrogen burning stars will be less massive than 10 solar masses. Conversely, a star that is half as massive as the Sun burns slowly enough for its fuel to last more than 20 billion 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. A star that is twice as massive as the Sun will burn through its fuel supply in only 800 million years. A star like the Sun has enough fuel in its core to burn at its current brightness for approximately 9 billion years. High mass stars are much brighter than low mass stars they rapidly burn through their supply of hydrogen fuel. The life of a star depends upon its mass. This way of estimating the age of the universe is based on our understanding of the life cycles of stars. Since all of the stars in a globular cluster formed at roughly the same time, they can serve as cosmic clocks. Globular Clusters are a dense collection of roughly a million stars. One way astronomers estimate the age of the universe by studying globular clusters. Astronomers estimate the age of the universe in two ways: (a) by looking for the oldest stars and (b) by measuring the rate of expansion of the universe and extrapolating back to the Big Bang. We do not know the exact age of the universe, but we believe that it is around 13 billion years - give or take a few billion. How old is the universe? StarChild Question of the Month for December 2000 Question:

How old is the earth