At the end, the sun size star life after it depletes its energy, it collapses, and end up being a white dwarf and the outer shell becomes of a planetary nebula. As the sun size star goes through stellar evolution, it grows in size gradually through different stages.
The red giant phase is the last stage of sun size star stellar evolution. The increase in size is due to the nuclear fusion of hydrogen atoms and the presence of helium later. The evolution of sun size stars takes 10 billion years.
