Hertzsprung-Russell Diagram

The Hertzsprung-Russell diagram is said to be the most important in all astronomy. It is a graph obtained by plotting the luminosities versus the temperatures of stars. As shown in the diagram, the luminosities are measured along the vertical axis and the surface temperature or spectral type along the horizontal axis. Every star for which the luminosity and temperature are known can be represented in this graph. Notice that the stars are not scattered in a random fashion over the diagram, but are grouped in three main regions.

This tells us that there is a precise relationship between the temperature and luminosity of stars. Most stars are located along the main sequence that runs diagonally from the hot & bright stars in the upper left to the cool & dim stars in the lower right. Our Sun is near the middle of the main sequence. There is a second major grouping of stars in the upper right-hand corner of the diagram. These stars are bright & cool and called red giants. Betelgeuse, Antares, and Aldebaran are red giants. A third group of stars, white-dwarfs, can be found in the lower left-hand corner, These stars are hot and dim.

The various life stages of a star plotted on the Hertzsprung-Russell diagram is shown above. The reason almost all stars can be found only in one of the three major groups indicated above is that these represent the life stages of longest duration: main sequence or Hydrogen burning, red giant or helium burning, and cooling-off white-dwarf stage. The transition between these stages is relatively rapid and we have fewer examples available of stars in these transitions, although all stars must pass through them.

Copyright (c) 1997 Michael Erlewine