## Hertzsprungrussell Diagram

A basic link between luminosities and temperatures of stars was discovered early in the twentieth century by two independent astronomers, Henry N. Russell (1877-1957) of the U.S. and Ejnar Hertzsprung (1893-1967) of Denmark. The Hertzsprung-Russell (H-R) diagram is a plot of luminosity versus temperature. Astronomers use the H-R diagram widely to check their theories (Figure 3.16).

Figure 3.16. Hertzsprung-Russell (H-R) diagram for many stars. Temperature increases from right to left. Luminosity increases from bottom to top.

Every dot on an H-R diagram represents a star whose temperature (spectral class) is read on the horizontal axis and whose luminosity (absolute magnitude) is read on the vertical axis.

Significantly, when a few thousand stars are chosen randomly and plotted on an H-R diagram, they fall into definite regions. This pattern indicates that a meaningful connection exists between a star's luminosity and its temperature. Otherwise, the dots would be scattered randomly all over the graph.

About 90 percent of the stars lie along a band called the main sequence, which runs from the upper left (hot, very luminous blue giants) across the diagram to the lower right (cool, faint dwarfs). Red dwarfs are the most common type of nearby star.

Most of the other 10 percent of stars fall into the upper right region (cool, bright giants and supergiants) or in the lower left corner (hot, low-luminosity white dwarfs).

Identify the location of the following stars indicated on the H-R diagram in Figure 3.17. Each star's absolute magnitude is given in parentheses. Refer to Figure 3.8 for temperature and spectral class. (a) Rigel (-6.6)_; (b)

Figure 3.17. An incomplete H-R diagram for selected stars.
Figure 3.18. Masses of some typical main sequence stars. (M„= mass of Sun.)