If the stars turning off the main sequence in the H-R diagram
of a star cluster have masses of about 15 times solar, how
old is the cluster?
cluster will be about as old as the main sequence lifetime.Can use lifetime (as fraction of
solar lifetime) = 1/M2.5 and get 1/1000 of the solar
lifetime or look up in the table in the slides.15 solar masses is about a B star which
have lifetimes of around 10 million years.