How can main-sequence fitting be used to measure a star cluster’s distance?
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.
Main-sequence fitting is a technique used to determine the distance to a star cluster by leveraging the fact that all stars within a cluster are at approximately the same distance from Earth.1
Because the stars are at a common distance, the effect of distance on their brightness is uniform for every member of the cluster. To measure this distance, astronomers construct a colour-magnitude diagram for the cluster, which plots the stars' observed brightness (apparent magnitude) against their colour (a proxy for surface temperature).12
The process involves comparing the observed position of the cluster's main sequence on this diagram with a standard main sequence of known absolute magnitude. By shifting the observed main sequence to align with the standard one, astronomers can determine the difference between the apparent and absolute magnitudes. This difference, often adjusted for interstellar reddening caused by intervening matter, allows for the calculation of the cluster's distance.1