Ask AI: why sequnce star grows in size to become a red gint

A main sequence star grows in size to become a red giant due to changes in the core conditions and fusion processes occurring during its stellar evolution. Main sequence stars generate energy by fusing hydrogen into helium in their core. As a star like our Sun uses up its hydrogen fuel over millions to billions of years, the helium concentration increases in the core. Since helium fusion requires higher temperatures and pressures than hydrogen, the core starts to contract under its own gravity, causing the temperature and pressure to increase even more. At the same time, the hydrogen burning starts occurring in a region surrounding the core, creating a shell of nuclear fusion. This additional energy pushes the outer layers of the star outward, causing the star to expand. As the star expands, the surface area increases and cools down, causing its color to change from yellow or white to reddish, hence the name "red giant." The size of the red giant can be considerably larger than it was during its main sequence phase, reaching up to several hundred times its original size. This phase of a star's life lasts for several million to tens of millions of years and eventually leads to other phases, such as the formation of a planetary nebula, and ultimately, the creation of a white dwarf, neutron star, or black hole, depending on the star's mass.