Final answer:
When the helium fuel runs out, a low-mass star like our Sun goes through a series of changes, including core collapse, helium fusion, becoming a red giant again, shedding outer layers to form a planetary nebula, and forming a white dwarf.
Step-by-step explanation:
When the helium fuel runs out at the center of a low-mass star like our Sun, the star goes through a series of changes. First, the star's core begins to collapse under its own gravity. As the core collapses, the heat released by the shrinking of the carbon and oxygen core flows into a shell of helium just above the core. This helium is heated enough for fusion to begin, producing a new flow of energy. The star then becomes a red giant again due to the fusion of hydrogen and helium in shells around the contracting core. Finally, the star sheds its outer layers, forming a planetary nebula, and the remaining core becomes a white dwarf.