Red Stars, known also as Red Giants, are a type of star found throughout the universe. They represent the late stages of a star's evolution. As red stars put out different wavelengths of light than Yellow Stars (Red is a lower-energy wavelength than yellow), Kryptonians and Daxamites will rapidly lose their powers under red stellar radiation. The Photonucleic Effect does not take place under a Red Star.
Krypton's sun, Rao, was a red star. Lexor also orbited a red sun.