A low or medium mass star begins its life on the main sequence and, as it ages, transforms into a Red Giant and later a White Dwarf, as it depletes its hydrogen and undergoes changes in its core and structure.
As a low or medium mass star ages and depletes its hydrogen supply, it undergoes a transformation. Initially, during the main-sequence phase, the star fuses hydrogen into helium. As it exhausts its hydrogen, the inner core contracts and the outer layers expand, causing the star to evolve from the main sequence to a Red Giant.
This stage is characterized by an increased size, a cooler surface resulting in a reddish appearance, and higher luminosity. The progression does not stop there; after shedding its outer layers and expelling a planetary nebula, the core that remains will cool and compact into a White Dwarf.
This stellar evolution pathway is highlighted in the Hertzsprung-Russell (H-R) diagram, tracing the life of a star like our Sun. These changes in chemical composition, interior structure, energy generation, and surface temperature move the star off the main sequence and through subsequent phases of its stellar life cycle.