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No astronomer is old enough to have actually *seen* a star evolve!
However,
when we plot a single stellar cluster, where all the stars have the same
age, then we get a glimpse of how stars evolve.
The H-R Diagram to the right demonstrates how the ``turn off'' mass can be used to
determine the age of a stellar cluster.
Studying clusters in this way also gives us a glimpse of how single stars evolve.
The cluster members leave something of a ``bread crumb''
trail, showing us how stars will appear (in luminosity and temperature) when
they leave the main sequence.
How a star will evolves is dependent, almost exclusively, on just ONE stellar characteristic:
THE MASS OF THE STAR.