The term stellar evolution refers to a star's changing stellar structure over its own lifetimes including how long each general phase lasts. The term is generally used to refer to how individual stars live and die rather than to the differences between stars born at different periods, the latter of which might be expected from the term "evolution". Models of stellar evolution of main sequence stars take into account the change in the constituents of stars as fusion reduces the hydrogen, i.e., gradually changing stellar structure models. Such evolution models can be checked somewhat through star counts and the resulting H-R diagrams, to identify the counts of stars in different phases and see if the counts are consistent with the stellar evolution models and what's known of the galaxy's star formation.
Among the general phases in the life of a main sequence star according to well-established models: