Stellar age estimation

Various methods and tools are involved in stellar age estimation, an attempt to identify within reasonable degrees of confidence what the age of a star is. These methods include stellar evolutionary models, membership in a given star cluster or system, fitting the star with the standard spectral and luminosity classification system, and the presence of a protoplanetary disk, among others. Nearly all of the methods of determining age require knowledge of the mass of the star, which can be known through various methods. No individual method can provide accurate results for all types of stars.

Luminosity increase and the Hertzsprung–Russell diagram

As stars grow older, their luminosity increases at an appreciable rate. Given the mass of the star, one can use this rate of increase in luminosity in order to determine the age of the star. This method only works for calculating stellar age on the main sequence, because in advanced evolutionary stages of the star, such as the red giant stage, the standard relationship for the determination of age no longer holds. However, when one can observe a red giant star with a known mass, one can calculate the main-sequence lifetime, and thus the minimum age of star is known given that it is in an advanced stage of its evolution. As the star spends only about 1% of its total lifetime as a red giant, this is an accurate method of determining age.