Transcript: The properties of stars in a star cluster as measured in the HR diagram change with time, and this can be a chronometer for measuring the age of groups of stars. The main sequence for a young star cluster is fully populated all the way up to the most massive, most luminous, and hottest stars. Remember that the main sequence runs from high luminosity and high temperature and high mass down to low luminosity, low temperature, and low mass. After ten to the seven years stars more than a thousand times the luminosity of the Sun have left the main sequence. After ten to the eight years stars more than a hundred times the luminosity of the Sun have left the main sequence. After ten to the nine years stars more than five times the Sun’s luminosity have left the main sequence, and after ten to the ten or ten billion years a star like the Sun is leaving the main sequence. Stars much less massive than the Sun have not had long enough in the age of the universe to exhaust their hydrogen, and so the main sequence is always populated in any star cluster for very low mass stars. This evolving point at which the most massive most luminous and hottest star exist on the main sequence is called the main sequence turnoff point.