Turn-off Point

Point on the main sequence where stars in a cluster have just started to move off toward the red giantGiant and highly luminous red star in the later stages of stellar evolution after it has left the main sequence. These red stars have a relatively cool surface whose core has burned most of its hydrogen. Red giants lose parts of their atmospheres and thus provide new elements into interstellar branch.

Image source: http://imgsrc.hubblesite.org/hu/db/2008/25/images/e/formats/web_print.jpg