Ages of Stars 
The two most prominent star clusters in the night sky are in good view on autumn evenings. The V-shaped Hyades cluster outlines the face of Taurus, the bull, while the smaller Pleiades cluster, which is shaped like a dipper, represents the bull's shoulder. They're in the east by around 10 o'clock tonight, with the Pleiades above the Hyades.
These and other star clusters are great laboratories for studying how stars change as they age. And they're also great laboratories for determining the ages of all stars.
That's because all the stars in a cluster were born at the same time, from the same giant cloud of gas and dust.
The stars are born with different masses -- from a fraction to many times the mass of the Sun. Heavier stars burn through their nuclear fuel faster than lighter ones, so they burn brighter and hotter. They also age more quickly -- and die sooner. So to determine the age of a cluster, astronomers look at the most-massive class of stars that remains. From models of how stars evolve, they know how long it takes those stars to die out. So those stars tell astronomers how old the cluster is.
From studying these and other clusters, astronomers have discovered that there's a relationship between a star's age and its mass, brightness, and temperature. So they can use measurements of those three parameters to estimate the ages of all stars -- whether they're members of clusters or not.
Script by Damond Benningfield, Copyright 2010