When astronomers first started to observe stars in telescope, they divided into color classes, white, yellow, red and deep red. Later is was realized that things made more sense if stars were categorized by surface temperature. So we now have O, B, A, F, G, K, and M stars. This classification system is called the Harvard system. This classification based on temperature is actually derived from Vines law, regarding blackbody radiation. Hotter objects like O, and B stars, are blue, and cooler objects like K, and M stars, are red. Also hotter stars tend to be larger, and burn brighter, with additional heat, resulting from the fact that so much more fuel being burned. All of these data, regarding temperature, luminosity, can be represented on Hertz from Russell diagram. The majority of stars, including our own Sun, fall on this continuous curve, which astronauts call main sequence stars.