Britannica
Sources used in this answer
How do astronomers determine the ages of open clusters?
Created from Britannica articles
using AI.
AI can make mistakes, so verify using Britannica articles.
Star cluster
Centre of star cluster 47 Tucanae (NGC 104), showing the colours of various stars.
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.
Astronomers determine the ages of open clusters by comparing the observed properties of their member stars with theoretical models of stellar evolution. Because all stars within a cluster are assumed to have formed at approximately the same time and possess similar chemical compositions, any differences in their current states are primarily due to their different masses.1
The primary tool for this analysis is the colour-magnitude diagram, which plots the stars' temperatures against their luminosities.21
- The Turnoff Point: As a cluster ages, its most massive stars—which evolve the fastest—exhaust the hydrogen in their cores and move away from the main sequence to become giants or supergiants. The specific point on the main sequence where stars begin to "turn off" serves as a reliable indicator of the cluster's age; the lower the mass of the stars at this turnoff point, the older the cluster.321
- Evolutionary Comparison: By fitting the cluster's observed colour-magnitude diagram to theoretical models, astronomers can determine how long it has been since the cluster formed. Younger clusters still contain bright, massive blue stars, while older clusters lack these stars and may show a more evolved population.134