The Hertzsprung-Russell diagram is a graph of many stars. It shows the relation between stars' luminosity (that is, how bright they are) and their temperature (how hot they are). These diagrams are not pictures or maps of the locations of the stars. Rather, Hertzsprung-Russell diagrams plot each star on a graph measuring the star's brightness versus its temperature. Hertzsprung-Russell diagrams are also called H-R diagrams or HRDs.