The Hertzsprung-Russell Diagram

They say that a picture is worth a thousand words. In science, a graph can be worth a thousand words or more. One of the central graphs in astronomy is the Hertzsprung-Russell diagram, or HRD for the spelling-challenged. The HRD plots some measure of the luminosity or power of a star against some measure of its temperature. The observed HRD plots absolute magnitude (usually visual) against the spectral type, which is a measure of the star's "surface" temperature. The absolute magnitude is the apparent magnitude that the star would have if it was at a distance of 10 parsecs, or 32.6 light-years. For historical reasons, temperature increases to the left, not to the right, and brighter magnitudes are more negative· not more positive. Theoreticians plot the logarithm of luminosity against something called effective temperature, explained below. No wonder students get confused.