The H-R Diagram is a graph that uses two stellar properties, such as luminosity versus surface temperature, as its axes. Individual stars are positioned on the graph according to their properties (Absolute Magnitude, and Spectral type correspond to luminosity and temperature). The resulting graph reveals relationships between a stars luminosity and temperature which in turn allows us to determine derived properties such as the age of the star, its evolutionary phase, its radius, etc. Also called the Hertzprung-Russell diagram after the astronomers who first developed this graph.