 In 1913, Hengnarr Hertzsprung and Henry Russell began mapping these star temperatures against their luminosity. Note that the horizontal axis is mapping temperatures in the decreasing direction. If we begin with the stars we use to illustrate black body radiation, Aldebran, Arcturus, Capella, our own Sun, Sirius A, and Spica, and throw in a few others like Sirius B, Wolf 359, Polaris, and Vega, we get a graph that looks like this. With this small sample it looks like any combination of temperature and luminosity is possible. But Hertzsprung and Russell meticulously plotted all the stars with known distance and luminosities, and they got this. Here we see that most stars fall on the diagonal line from the upper left hot blue luminous stars down to the lower right, cooler dimmer red stars. But there is also a grouping of stars well below the main line, and two groupings of stars well above the main line. This is the Hertzsprung-Russell diagram, or HR diagram for short. That is one of the most important tools in understanding stars ever devised. It tells us a great deal about the life, death, and age of stars. And more importantly, for our purposes, it can tell us how far away stars are. But in order for the HR diagram to do this, we need to know more about what makes stars shine, and we need to know more about the full spectrum of light we receive from these stars.