Astronomy HyperText Book: Stellar Clusters

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Much Still Under Development

Stellar Clusters are very useful laboratories for empirically studying stellar evolution. This is because:

All cluster stars are at the same distance
All cluster stars have the same physical age
As the cluster ages, different phases of stellar evolution are represented
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.

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