Stellar Evolution

This image is an artist’s impression of Sun-like stellar evolution. The star begins as a main-sequence star at the lower left, and then expands through the subgiant and giant phases at the middle, until its outer envelope is ejected to form a planetary nebula, shown at the upper right.
CC BY 4.0 | Image courtesy of European Southern Observatory (ESO).

Stellar evolution is the process by which a star experiences a sequence of drastic variations during its stellar life. Depending on the initial stellar mass, this lifetime ranges from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the age of the Universe at 13.7 billion years.