What does the main-sequence turnoff point reveal about a star cluster’s age?
Most of the brightest stars are older yellow stars, but a few young blue stars are also visible. This picture is a composite of three images taken by the Hubble Space Telescope.
The main-sequence turnoff point is a critical indicator used by astronomers to estimate the age of a star cluster.
Stars spend the majority of their lives on the main sequence, burning hydrogen into helium in their cores. Because a star's rate of evolution is directly proportional to its mass, more massive stars consume their fuel much more rapidly than less massive ones. Consequently, as a cluster ages, its most massive stars exhaust their hydrogen first and evolve away from the main sequence toward the region of giants and supergiants.12
The turnoff point represents the specific location on a Hertzsprung-Russell diagram where stars are currently exhausting their core hydrogen and departing the main sequence. By identifying the mass of the stars at this turnoff point, astronomers can determine how long the cluster has been evolving. If the turnoff point occurs at a high luminosity, the cluster is relatively young; if it occurs at a lower luminosity, the cluster is older, as only lower-mass stars remain on the main sequence.13