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8 - The Lives of Stars

Published online by Cambridge University Press:  05 February 2013

Kenneth R. Lang
Affiliation:
Tufts University, Massachusetts
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Summary

Main-Sequence and Giant Stars

The Hertzsprung–Russell Diagram

Once the luminosity of stars was obtained from their brightness and measurements of their distance, astronomers were able to show that most stars exhibit a systematic decrease in luminosity as one progresses through the spectral sequence O, B, A, F, G, K, M. (These spectral types are described in Section 7.4.) This progression is exactly what we would expect because the spectral sequence also denotes a scale of decreasing stellar temperatures, and the luminosity of a radiating body depends strongly on temperature.

The luminosity drop is illustrated in the famous Hertzsprung–Russell (H–R) diagram of luminosity or absolute magnitude plotted against the spectral class or effective temperature. The diagram's name derives from the Danish astronomer Ejnar Hertzsprung, who first plotted such diagrams for the Pleiades and Hyades star clusters, and the American astronomer Henry Norris Russell, who next published early versions of this diagram for both cluster and noncluster stars (Fig. 8.1).

Most stars, including the Sun, lie on the main sequence that extends diagonally from the upper left to the lower right, or from the high-luminosity, high-temperature blue stars to the low-luminosity, low-temperature red stars. The stars on the main sequence are the most common type in the Milky Way, constituting about 90 percent of its stars.

The Stefan–Boltzmann law describes the general characteristics of the H–R diagram. This expression indicates that for a fixed radius, the luminosity of a star increases with the fourth power of the temperature; therefore, hotter stars are more luminous. That is exactly what happens along the main sequence, for although the radius varies by a relatively small amount along the main sequence, the luminosity variation is due mainly to a change in temperature.

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Publisher: Cambridge University Press
Print publication year: 2013

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  • The Lives of Stars
  • Kenneth R. Lang, Tufts University, Massachusetts
  • Book: The Life and Death of Stars
  • Online publication: 05 February 2013
  • Chapter DOI: https://doi.org/10.1017/CBO9781139061025.009
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  • The Lives of Stars
  • Kenneth R. Lang, Tufts University, Massachusetts
  • Book: The Life and Death of Stars
  • Online publication: 05 February 2013
  • Chapter DOI: https://doi.org/10.1017/CBO9781139061025.009
Available formats
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  • The Lives of Stars
  • Kenneth R. Lang, Tufts University, Massachusetts
  • Book: The Life and Death of Stars
  • Online publication: 05 February 2013
  • Chapter DOI: https://doi.org/10.1017/CBO9781139061025.009
Available formats
×