Archive of Astronomy Questions and Answers

How do astronomers figure out the ages of stars?

It is impossible to simply LOOK at a star and know its age. Instead, what you have to do is ask the following question. Given the mass and chemical composition of the star, how long did it take for it to reach the current configuration of luminosity and temperature? To get the answer, you have to know how a collection of matter as massive as the star change through time under the influence of gravity. What thermonuclear reactions are occurring in the core of the star to support its weight under gravity? All of these things can now be computed from detailed models that describe, with the help of our knowledge of a number of fundamental laws of physics, just how the interior of a star must change as its nuclear fuel is burned, and gravity gains the upper hand. The result is a 'track' or history for such a star on the so-called H-R diagram. This diagram represents a star by its current luminosity and temperature. What you now try to do, by knowing the stars mass and chemical composition, is to iterate your stellar evolution model until one of the evolution tracks intercepts the properties of the star, or collection of stars in a cluster, for which you want an age estimate. The above figure ( from an introductory astronomy course at Prof. Mike Bolte) shows how the age of a star cluster can be estimated by identifying where stars on the Main Sequence have begun to turn-off onto the so-called 'Giant Branch'. The mass of the star is related to the age of this turnoff by the relationship between mass and age, summarixed in the table below:

To get the best estimates, you must know precisely the luminosity, mass, temperature and chemical composition of the star. All of these are subject to observational uncertainty. This may be one of the problems we may be having with the current dilemma of globular cluster stars seeming to be older than the universe which I have discussed in several questions in the Cosmology archive.