Final answer:
As a high-mass star evolves off the main sequence, it follows an evolutionary track on the H-R diagram towards the red giant or supergiant stage. This main-sequence turnoff point marks the progression of a star's life as it grows older and changes in brightness and temperature.
Step-by-step explanation:
As a high-mass main-sequence star evolves off the main sequence, it follows an evolutionary track on the H-R (Hertzsprung-Russell) diagram. Clusters age and their H-R diagrams change, showing most massive stars evolving into red giants and supergiants, with these changes marked on H-R diagrams to illustrate a star’s life cycle. The path stars take on the H-R diagram, known as the main-sequence turnoff, indicates the point where stars deviate from the main sequence to become red giants, highlighting changes in luminosity and temperature as stars age.
Specifically, these evolutionary tracks describe the changes in stellar temperature and luminosity as stars exhaust their hydrogen and begin fusing heavier elements. This transition leads stars to ascend upward and to the right on the H-R diagram. Over time, smaller mass stars also follow this pattern, marking their progression towards the red giant stage as they leave the main sequence, thus continually reshaping the appearance of the diagram for star clusters of different ages.