Turnoff Point
The point in a where the subgiant branch deviates from the .
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Turnoff Point
The point on the H-R diagram where a cluster's stars turn off the main sequence and move toward the red giant region revealing the approximate age of the cluster.
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At 10 billion years, the turnoff point has reached solar-mass stars, of spectral type G2.
Interestingly, if you study the diagram above you see a group of hot stars that appear to be on the main sequence above the turnoff point. These are in fact known as blue stragglers.
(a) Stars (in a cluster) which fall close to the cluster's extrapolated main sequence but which occur a few magnitudes above its turnoff point. [H76] ...
Stars slightly more massive than the turnoff point have already evolved away from main sequence.
When we plot the stars of a cluster in the H-R diagram, we see the more massive stars, which are stars in the upper left, have already evolved off the main sequence. And stars at the turnoff point are just about to die.
The more massive stars evolve faster and leave the main sequence. The point on the main sequence where stars have just left (defining the end of the main sequence for that cluster) is the main sequence turnoff point.
 See also: Mass, Clusters, Star, Cluster, Giant
  
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