Hertzsprung-Russell Diagram

A Hertzsprung-Russell diagram, or H-R diagram, is a graph of stellar temperatures (plotted on the horizontal axis) and luminosities, or brightnesses (plotted on the vertical axis). H-R diagrams are valuable because they reveal important information about the stars plotted on them. After constructing an H-R diagram for a group of stars, an astronomer can make estimates of many important stellar properties including diameter, mass, age, and evolutionary state. Our understanding of the processes at work in the stars depends on knowing these parameters, so H-R diagrams have been essential tools in twentieth-century astronomical research.

Figure 1. The spectral classes and corresponding surface temperatures are given at bottom, while the luminosities are given at left. (The luminosities are in solar units, meaning that "1" equals the luminosity of the Sun, while "10" means ten times the luminosity of the Sun, and so forth.) Clearly the stars are not randomly distributed on this graph. They fall in several well-defined areas, with most stars on a narrow strip running from upper left to lower right. This graph, the H-R diagram, was a fundamental advance in astronomy. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Figure 1 shows all the important features of the H-R diagram. The stars fall into several relatively narrow strips which W. W. Morgan, another famous classifier of stellar spectra, called luminosity classes. Luminosity classes are denoted by Roman numerals.