What is a double-shell burning red giant star?

A double-shell burning red giant star is a late-stage stellar object that has exhausted hydrogen in its core and is undergoing nuclear fusion in two separate shells: one surrounding the core where helium is fusing into carbon, and the other where hydrogen is fusing into helium. This structure results from the star's expansion and cooling as it evolves off the main sequence. Such stars are characterized by their large size, high luminosity, and distinctive reddish hue due to their cooler surface temperatures. Eventually, they will shed their outer layers, leading to the formation of a planetary nebula and leaving behind a white dwarf core.