The Hertzsprung Russell Diagram
Early in the twentieth Century, astronomers had begun to collect information about the physical characteristics of stars. The instinct of every good scientist is to look for relationships in data. So it was Eijnar Hertzsprung and Henry Norris Russell, who independently did just that.
The Hertzsprung-Russell Diagram, or H-R Diagram, is a fundamental tool of stellar astronomy.
The following text is from Henry Norris Russell's 1928 text on astronomy:
The relation between the absolute magnitudes and spectra of the stars has proved of fundamental importance. The number of good determinations of parallax is now great enough to provide reliable information on this subject. The results are best exhibited graphically, by plotting the absolute magnitudes, M, against the spectral classes, as in the figure. (Such a diagram was first made by Russell in 1913.) Bright stars are represented by points near the top of the diagram, faint stars by points near the bottom, red stars at the right, and white ones at the left.
The points are by no means distributed at random. The great majority (more than five sixths of the whole) fall in a fairly narrow belt extending diagonally downward to the right, from the bright light stars of classes B and A to the faint red ones of class M. The stars in Orion's belt, Sirius, Procyon, the Sun, 61 Cygni, and Barnard's star all belong to this group, which Eddington calls the Main Sequence. ... outside the Main Sequence there are a good many bright yellow and red stars and a few faint white ones. It is certain in both cases that the separation from the main sequence is real and not due to errors of observation.
Since Russell first drew the diagram in 1913, it has come to be known as the Hertzsprung-Russell Diagram to honor Eijnar Hertzsprung who was developing it at the same time. This plot of the two important characteristics of stars has been used since to reveal a vast amount of information about the lives of stars. The diagram led to the discovery of such interesting objects as Red Giants and White Dwarfs.
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Updated February 24, 2003