Measuring the ages of stars

The procedure for assigning an age to a cluster consists of

1) Measure brightnesses and colors of many stars in the cluster (ideally one would take a spectrum of each star in the cluster but this usually takes too long)

2) Correct measurements for any dimming caused by interstellar material

3) Plot an H-R diagram

4) Use models of stellar evolution to find the main sequence turn off and then the age of the cluster.

open_agea.jpg (45624 bytes) A series of clusters is plotted on this H-R diagram, from very young (h & X Persei) to very old (M67). (this illustration and following ones from U. Tenn,
hrgloba.jpg (31237 bytes) hrperseia.jpg (29845 bytes)
An old cluster And a young one.
animation of cluster H-R diagrams vs. age A few stages in the evolution of a cluster as they would appear on the H-R diagram. This figure extends from the lowest mass to the highest mass stars. (adapted by G. Rieke, from T. Herter,

Our understanding of stellar evolution is pieced together from relating stars to each other in different clusters of different ages. We make use of the Russel-Vogt Theorem, that stars are basically simple and their properties depend only on mass, age, and composition