This Demonstration presents this simple case by imagining that the various spectral classes of stars, OBAFGKM, are laid out on a shiny table next to each other for comparison. If you change the temperature of a star, its color and radius (measured in terms of the Sun's radius, ) also change. The hottest stars, the O and B stars, are always blue. At the other end of the spectral class range, the color changes become more evident.