A red giant is a type of star that has reached a late stage in its evolution. After exhausting the hydrogen fuel in its core, the star expands and cools, giving it a reddish appearance. This phase occurs after a star like our Sun has spent billions of years in the main sequence stage. During the red giant phase, the star can undergo nuclear fusion of helium and other heavier elements. Eventually, it may shed its outer layers, creating a beautiful planetary nebula, while the core remains as a dense object known as a white dwarf.