How does a colour-magnitude diagram show the age of a star cluster?
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.
A colour-magnitude diagram determines the age of a star cluster by identifying the "turnoff point"—the specific location where stars begin to evolve away from the main sequence.12
Because stars spend most of their lives on the main sequence, their position on the diagram is dictated by their mass; more massive stars burn through their core hydrogen much faster than less massive ones. As a cluster ages, its most massive stars exhaust their fuel first and move off the main sequence toward the region of giants and supergiants.12
By observing which stars have left the main sequence, astronomers can calculate the cluster's age:
* Young clusters: Still contain massive, luminous stars on the upper end of the main sequence.2
* Older clusters: The turnoff point shifts downward as progressively less massive stars exhaust their hydrogen and evolve away.12
In very old clusters, such as M67, even the brighter main-sequence stars have disappeared, leaving only lower-mass stars on the main sequence.2