Stars are so hot right now. Annie Jump Cannon, Henriette Leavitt, Antonia Maury, Florence Cushman, Cecilia Payne and others began cataloging and manually classifying stars in the late 1800's - over 350,000 of them. During that time, two astronomers simultaneously discovered a pattern in all that data. Between 1911 and 1913, Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russel plotted the stars recorded by Cannon and her team, and placed them on a diagram - the axes of which were the spectral classes devised by Cannon on one side, and their luminosity on the other. The diagram beautiful illustrated that in the randomness of the stars observed in the universe there is a clear pattern into which all stars fall. The diagram helps bring understanding to the temperatures and colors, brightness, sizes, and even the ages and lifecycles of the stars. Over 100 years later the diagram still holds true, and is a tool used in science today.