dipper@utastro.UUCP (Debbie Byrd) (10/25/86)
A tool for understanding how stars evolve -- after this. October 25 The H-R Diagram Today is the birthday of the American astronomer Henry Norris Russell. In 1913, he introduced a powerful new astronomical tool -- which wasn't an instrument -- it was a diagram. This diagram, or graph, is used to illustrate both the brightnesses and temperatures of a lot of different stars. The brightness of each star is plotted on one axis of the graph, and each star's temperature on the other axis. The sun, for instance, is medium bright and medium hot -- so its place is as a dot at about the center of the graph. When they're all down on paper, the stars represented on the graph fall into well defined patterns. Most dots, including the one representing the sun, fall along a diagonal band which stretches from the upper left of the graph to the lower right -- a band called the main sequence. These main-sequence stars are ordinary stars, which as expected get brighter as they get hotter. But other groupings also appear on the graph that weren't expected -- in the upper right-hand corner and lower left-hand corner. Russell realized that the different areas on the graph represent different stages in the lives of stars. Main sequence stars are in the prime of their lives, but the other groups for the most part contain very old stars. Being able to look at stars in such an organized way has helped astronomers learn how stars evolve. Unknown to Russell, a similar diagram was plotted two years earlier by the Danish astronomer Ejnar Hertzsprung. Today it's called the Hertzsprung-Russell Diagram -- or simply the H-R Diagram. Script by Bill Edrington and Deborah Byrd. (c) Copyright 1985, 1986 McDonald Observatory, University of Texas at Austin