Measuring the ages of stars

• Very difficult (essentially impossible) for stars on the main sequence
• Star clusters can be used to estimate ages by taking advantage of stellar evolution. As a cluster ages, its main sequence turn off moves to later spectral types (cooler, less massive, longer-lived 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.

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, http://csep10.phys.utk.edu/astr162/lect/hr/hr.html)

An old cluster	And a young one.

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, http://instruct1.cit.cornell.edu/courses/astro101/astro101.htm)

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