Stellar evolution is the process by which a star
undergoes a sequence of radical changes during its lifetime. Depending
on the mass of the star, this lifetime ranges from only a few million
years (for the most massive) to trillions of years (for the least
massive, which is considerably more than the age of the universe).
Stellar evolution is not studied by observing the life of a single
star, as most stellar changes occur too slowly to be detected, even
over many centuries. Instead, astrophysicists come to understand how stars evolve by observing numerous stars at the various points in their life, and by simulating stellar structure with computer models.