Also known as HR Diagram, the Hertzsprung-Russel diagram is believed to be one of the most important tools used in the study of evolution. Independently developed in the early 1920s, the HR Diagram is primarily used to plot the temperature of stars against their luminosity. It is also used to plot the color of the stars against their magnitude.
The absolute magnitude or luminosity of the stars shows an upward increase on the vertical axis as the temperature of the same decreases to the right side on the horizontal axis. It is believed that a good majority of the stars lie on a diagonal band extending from the hot stars to cool stars found in the lower left corner.
The various forms of this diagram come from the different manners in which stars can be studied and understood. Theoreticians prefer to do a direct graphing of the numerical quantities derived from calculations. Observational astronomers, on the other hand, prefer to use the same observable quantities. Cosmos further states that depending on the initial mass of a star, it may go through evolutionary stages that are dictated by the way it produces energy and its internal structures. All of these stages correspond to the temperature and luminosity changes of the star.