Located about 28,000 light-years from Earth, this swarm of stars, known as M80, is one of the densest globular star clusters in the Milky Way. The cluster contains hundreds of thousands of stars, all held together by their mutual gravitational attraction. In addition to being beautiful, clusters such as this one tell us much about stellar evolution and provide a means of measuring the ages of stars. All the stars in a cluster like this one were formed at the same time and so are of the same age. They are among the oldest stars in our galaxy. Yet they cover a range in terms of mass and size. The more massive stars in the cluster burn their nuclear fuel more quickly and evolve into red giants and ultimately white dwarfs. The less massive stars have longer lives and many are still burning their original hydrogen fuel in their central cores, much like our own Sun does.

Hubble is able to resolve individual stars in globular clusters, covering an unprecedented range of mass, size, and degree of evolution, and to accurately measure their brightness and colors. By comparing these accurate measurements to theoretical models describing how stars evolve, Hubble can measure the age of the entire cluster. A lot of work remains to be done. But Hubble's accurate measurements, coupled with recent improvements in the measurement of the clusters' distances, has led to revised estimates of the ages of the oldest stars in our galaxy — about 13-14 billion years.