<P> 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 . </P> <P> 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 . </P> <P> Various properties of stars can also be used to determine their age . For example, the Eta Carinae system is emitting large quantities of gas and dust . These enormous outbursts can be used to infer that the star system is nearing the end of its life, and will explode as a supernova within a relatively short period of astronomical time . Very large stars like UY Scuti, the largest star known, together with Mu Cephei, Betelgeuse and VY Canis Majoris all have radii larger than that of the average orbital radius of Jupiter in the Solar System, thus showing that they are in extremely late evolutionary stages . Betelgeuse in particular is expected to die in a supernova explosion within the next million years . </P>

How can we tell the age of a star