A red giant is a star in a late stage of stellar evolution that has exhausted the hydrogen fuel in its core. This core exhaustion triggers the fusion of hydrogen in a shell surrounding the helium core, causing the star's outer layers to expand dramatically and cool down, resulting in a much larger apparent size and a reddish hue. The luminosity increase and radius expansion can encompass volumes comparable to those of certain planetary systems. This evolutionary phase marks a significant transition, as the star continues to burn through its fuel reserves, eventually leading to the eventual fate dictated by its initial mass.