Globular clusters in our galaxy provide another interesting, useful cosmic measurement tool. They are giant collections of up to a million stars that form a halo around the center of the Milky Way -- outside our galaxy's core. All the stars in a globular clusters are essentially the same distance from us, and all their stars formed at the same time.
Photo. The G1 globular cluster is located in the M31, or Andromeda, galaxy -- about two light-years away from the Milky Way. The G1 is made up of stars about 300,000 years old, comparable to clusters in the Milky Way; it is the largest cluster in the Local Group of two dozen galaxies near the Milky Way. Courtesy Space Telescope Science Institute, Michael Rich, Kenneth Mighell, and James D. Neill/Columbia University, Wendy Freedman/Carnegie Observatories, NASA.
Stars are born, live out their lives, and eventually die when they run out of nuclear fuel. After their initial period of formation, their adolescence, stars enter their adult lives, a period when they are stable. The force of gravity pulling the gas in the star inward is balanced with the pressure of the heat in its core pushing the gas outward. Scientists say such stars are main sequence stars.
Some stars are very massive...and very hot...and burn themselves out after only a few million years.
Other stars, like our Sun, are of middle size, medium temperature, and live ten billion years.
Still others are smaller and cooler, and may last a hundred billion years.
As stars age and begin to die, they start to leave the main sequence. Observers study which kind of stars in globular clusters are beginning to evolve off the main sequence, into giants and supergiants. What type of stars are leaving the main sequence -- and how long they usually live -- tells us how old the cluster is.
Our observations of globular clusters and calculations about the main sequence indicate that the universe is at least 14 billion years old.