One of the best ways to test the theories of stellar evolution is to look at star clusters of different ages. Sometimes when stars form, they form in big groups which all have about the same age, are made of the same composition, and have a variety of masses. As the cluster ages, the stars will leave the main sequence according to their mass - highest mass stars first. For clusters with very high mass stars the low mass stars may not even make it to the main sequence before the first stars die. By studying the HR diagram of a cluster (a task made easier by the fact that all stars are at the same distance) we can see if our predictions of how a star evolves as a funtion of mass are borne out. By studying clusters of different ages, we can fill in the whole picture.

There are 2 basic types of star clusters: open and globular. Below are some of the comparative characteristics of each:
Open clusters
    no regular shape
    relatively young (few billion years or less)
    main sequence can extend to high masses (depending on age)
    composition similar to Sun
    contains few hundred to a few thousand stars
    found in galactic plane (disk)
Globular clusters
    spherical in shape
    old stars (several billion to 12 billion years; almost age of Universe)
    main sequence cut off at low mass stars
    few heavy elements compared to Sun
    hundred thousand to million stars
    distributed in galactic halo (spherical distribution much bigger than disk)

Because all the cluster stars are at the same distance, we can find that distance by "main sequence fitting". This just means that since we know the intrinsic luminosity of main sequence stars (by local calibration), we can shift the cluster apparent brightnesses to the intrinsic main sequence luminosities by adjusting the assumed distance. The cluster age can then be found by asking how bright or hot the main sequence extends to. For a given age, all the stars whose lifetime is less than that age will have already left the main sequence. So the age of the cluster is the lifetime of stars just at the "main sequence turnoff" today. The speed with which stars evolve after they leave the main sequence will also determine how many we see at each subsequent stage; if stars pass through a given phase quickly then we will not catch many of them in the act right now. By putting all the cluster stars on an HR diagram, we can learn a lot about how their physical characteristics are determined by mass and age. We can even test the theory for stars of different compositions by looking at clusters with different compositions. For example, stars with few metals are predicted to make a "horizontal branch" in the HR diagram during the helium burning phase. This extends at a certain luminosity to rather blue colors; stars like the Sun will stay fairly close to the red giant branch. We can see that this prediction is true through observations.