The Hertzsprung-Russell Diagram

The Hertzsprung-Russell diagram , also referred to as the H-R Diagram, is a graph of stellar absolute magnitude or luminosity versus their spectral type or photosphere temperature. The H-R Diagram was created circa 1910 by Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russell . Plotting stellar luminosity versus color (temperature) allowed Hertzsprung Russell to see patterns in stars, from groupings of stars to how stars change over time, what is called stellar evolution.Looking at the H-R Diagram, you will first note that numerous stars are plotted. Imagine millions of stars, that is what makes up the diagram itself.

A graphic, which shows the location of different star groups on the Hertzsprung-Russell diagram. To the above left, this simplified H-R Diagram shows how stars group according to temperature (Spectral Class) and Luminosity (Absolute Magnitude). Upper stars are bright, whereas stars towards the bottom, or X-Axis, are dim stars. Stars on the left are hot stars; to the right are cool stars. Based on this, groupings of stars become noticeable..
Image courtesy of Florida State College at Jacksonville.
Hertzsprung-Russell diagram. A plot of luminosity (absolute magnitude) against the colour of the stars ranging from the high-temperature blue-white stars on the left side of the diagram to the low temperature red stars on the right side. Converted to png and compressed with pngcrush.
CC BY-SA 2.5 | Image courtesy of Richard Powell.

The graph also shows the colors of the plotted stars — note the stars on the left of the graph are more blue-white, towards the center yellowish, and on the right red to almost a purple color. That indicates the stellar temperature.

As you go higher on the graph, up the Y-axis, the stars are brighter — the stellar luminosity. This can be plotted as the L sun with the L sun = 1. It can also be plotted as Absolute Magnitude, M.