What is a red giant star?
A red giant star is a star with a mass like the Sun that is in the last phase of its life, when Hydrogen fusion reactions in the core decrease due to the lack of fuel. With the gravitational collapse of the core, the fusion reactions now occur in a shell surrounding the core. The outer layer of the star expands enormously up to 1000 times the size of the Sun. When the Sun becomes a red giant, its volume will include the orbit of Mercury and Venus and maybe even Earth. The increased size increases the luminosity even though the outer layer cools to only 3000 K or so. The cooler outer layer causes it to be a red star. After a few more million years, the star evolves into a white dwarf-planetary nebula system.