What do theoretical stellar models called isochrones map on the H-R diagram?

Theoretical positions of stars of specific initial mass and composition over time.

Isochrones are essential theoretical tools utilized in stellar evolution studies, frequently visualized as tracks plotted upon the Hertzsprung-Russell (H-R) diagram. An isochrone represents a set of points corresponding to stars that share an identical initial mass and chemical composition, showing where they would all be located on the H-R diagram at one specific, shared age. By observing a star's current measurable color (temperature) and brightness (luminosity), astronomers can compare this position against various calculated isochrones. The specific isochrone that intersects the star's current measured location provides an estimate of how long it has taken that star to evolve to its present state. This technique is particularly valuable for dating stars that have already evolved significantly past the main sequence phase.