turnoff pointThe point on the Hertzsprung–Russell diagram of a globular or open cluster at which stars are leaving the main sequence and entering a more evolved phase (see illustration at globular cluster). Assuming that all the stars in a cluster contract out of a single gas cloud at approximately the same time, the locations of these stars on the H–R diagram depend on the time elapsed since the initial contraction. The length of time spent on the main sequence depends on stellar mass: stars of about solar mass will remain there for maybe 1010 years while more massive, and hence more rapidly evolving stars will turn off earlier. Since position on the main sequence also depends on mass, the lower the turnoff point, the older the cluster. The turnoff point of a cluster is the most accurate means of determining its age, and hence provides astronomers with a sample of stars of known age.
turnoff point[′tərn‚ȯf ‚pȯint]
The point on a Hertzsprung-Russell diagram of a star cluster at which stars leave the main sequence and move toward the giant branch.