History of the HR diagram 

Ashes to ashes, dust to dust - that proverb reminds us that all human beings are given life and all die, with stories in between. Stars too have life cycles, and like human beings they don't all have the same life story.  

Stars are a long way away and even in a powerful telescope they never look like much more than tiny points of light. How can astronomers learn enough to ever tell the life stories of stars? How can they really say they know about the birth, adolescence, middle age and death of these pinpricks in the sky? Understanding the nature and history of stars was not easy and it came slowly to humanity. Sometimes progress came with new measurement, techniques and sometimes it came by simply finding a new way to look at the data. 

By the late 1800s, astronomers had tools that allowed them to make quantitative measurements of stellar properties: apparent magnitude (how bright a star appears to us on Earth), spectral type (related to temperature), distance, radial velocity, etc. Several groups of astronomers worked to compile big catalogs of these measurements drawing all the information taken so far into a single resource. A big book of numbers is, of course, not science. At first, all the catalogues could provide was a confusing mess of data with no clear way to extract any insights. 

In the early 1900s, two astronomers -- Ejnar Hertzsprung in Denmark and Henry Norris Russell in America -- realized that there was a way to bring order to the problem (notice that science has always been an international endeavor). They used distance measurements to convert the APPARENT magnitude of each star into its ABSOLUTE magnitude. Apparent magnitude is a measure of how bright we on Earth perceive a star to be. Of course objects look dimmer the farther away they are. That is why astronomers like to use absolute magnitude when they know a star's distance since it is true measure of a star's intrinsic luminosity, its total energy output per second. Using the big catalogs, both men discovered a simple relationship between the absolute magnitude of stars and their spectral type or color (Why? Because the relationship between color and temperature came from the blackbody formula). Taken as a whole, bluish stars tended to be luminous and reddish stars tended to be faint. On a graph of absolute magnitude (luminosity) versus spectral type (temperature) most of the stars fell in a diagonal stripe - a pattern that became known as the MAIN SEQUENCE. A few stars, however didn't follow the pattern, falling instead in a clump in the upper-right of the diagram. The existence of the main sequence, and these outliers that avoided it, soon provided astronomers with the key insights they needed to transform thier data into a profound understanding of how stars age.  

As time went on, this graph became known as a "Hertzsprung-Russell" (or HR) diagram and came to be THE tool for understanding stellar history. Astronomy was changed forever.