FIGURE 3.7 The Hertzsprung-Russell (H-R) diagram. Developed independently by the two astronomers for whom it is named, the H-R diagram shows absolute magnitude versus spectral type. Since the spectral type is a surrogate for temperature, hot blue stars are found on the left of the diagram and cool red stars on the right. The Sun is a star of intermediate temperature (G spectral type) with an absolute magnitude of approximately 5. The group of stars running from upper left to lower right is called the main sequence and is where most stars are found on the diagram. A simple consideration of the Stefan Boltzman law suggests that a star above the main sequence at the same spectral type (temperature) must be large and one lower is smaller. This led to the designation of the stars in the upper right as red giants and those in the lower left as white dwarfs. Later work showed that the position in the H-R diagram was a product of stellar mass and age. The magnitude scale is a logarithmic scale of luminosity. Stars whose absolute magnitude differs by 5 differ in intrinsic luminosity by a factor of 100. SOURCE: Unsöld, 1969.

which is simply the brightness of a star if it were placed at a standard distance of 10 parsecs. Examining the visible spectrum of a star and classifying it according to the observed distribution of features seen in its spectrum determines the spectral type of the star. Second, as ideas of modern quantum mechanics explained the reason for the existence of different spectral features in stellar spectra, it was realized that the star’s spectral type was a surrogate for temperature, so that the horizontal axis of the H-R diagram ran from hot blue stars on the left to cool red stars on the right. When spectral type and absolute magnitude were plotted, most stars were found to fall on a single broad band, called the main sequence, with some hot dim stars found below the main sequence and some cool bright stars found above. In one diagram, therefore, the concept of red giant and white dwarf stars was given meaning.

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