A red giant is a type of star that has expanded and cooled after exhausting the hydrogen fuel in its core. As it ages, the star's outer layers swell, giving it a reddish appearance. This phase occurs in stars like our Sun, which will eventually become a red giant in several billion years.
During the red giant phase, the star undergoes nuclear fusion of helium and other heavier elements. This process can lead to the formation of elements like carbon and oxygen. Eventually, red giants may shed their outer layers, creating beautiful nebulae, while the core remains as a dense object known as a white dwarf.