An Etymological Dictionary of Astronomy and Astrophysics

The point on the → Hertzsprung-Russell diagram of a star cluster at which stars begin to leave the → main sequence and move toward the → red giant branch. The main-sequence turnoff is a measure of age. In general, the older a star cluster, the
fainter the main-sequence turnoff. Same as → turnoff point.

See also: → main sequence; → turnoff.