A schematic representation of this is shown below.
CM diagram Young Cluster In
this case we see the Main Sequence fully
represented. The blue dots represent massive stars with short
main sequence lifetimes. This stars would have masses greater than
2 solar masses. The yellow dots represent stars with masses between
1 and 2 solar masses and the red dots represent stars with masses
less than 1 solar mass. Since the cluster is very young, no
Red Giants have appeared.
CM diagram 10 Million Years Old
Now we are seeing the disappearance of the most massive main sequence
stars as their main sequence lifetimes is less than 10 million years.
These stars have evolved to become the first generation of Red Giants .
CM diagram 1 Billion years Old
Now we see that all the blue stars have evolved off the main sequence
and that some of the Red Giants have evolved to become
White Dwarfs . As the main sequence lifeftime of a 2 solar mass
star is about one billion years, then there are no stars still on the
main sequence with masses higher than this. Thus, identifying the
most massive star which is still on the main sequence is an effective
means of determining the age of a stellar cluster.
CM digram 10 Billion years Old
All of the yellow stars have no disappeared. The Giant Branch is now well populated, Overall, the cluster colors
would be very red since all the stars are either red giants or
low mass mainsequence stars. The number of white
dwarfs has increased quite bit. In the next few billion years
the cluster will continue to evolve and slowly fade as all its
former members end up as white dwarfs, neutron stars or black holes.