Transcript: Certain rare stars in the sky with either red or blue colors are extremely luminous, up to a million times the luminosity of the Sun. Application of the Stephan-Boltzmann Law shows that their sizes must be in the range of ten to a thousand times the size of the Sun. These exceptional stars are called supergiant stars. Some are hot and blue, and others are cool and red. Although in each case the color only refers to the outer nebulous atmosphere of the star, the centers are much hotter.