Hertzsprung-Russell Diagram

Basics of the HR diagram

• The vertical position represents the star's luminosity.
• The horizontal position represents the star's surface temperature.

Sometimes the labels are a little different:

• The vertical position represents the star's luminosity.
  • This could be the luminosity in watts.
  • More commonly it is in units of the Sun's luminosity.
  • In either case, a ``ratio scale'' is used.
  • Absolute magnitude is also commonly used.
• The horizontal position represents the star's surface temperature.
  • Sometimes this is labelled in by the temperature in Kelvins.
    • Highest temperatures go to the left. (It's traditional.)
    • Normally the temperature is given using a ``ratio scale.''
  • Sometimes the stars spectral class (OBAFGKM) is used.
  • One could also use a measure of color as seen through filters.

  Other uses of scatter plots

  The Hertzsprung-Russell diagram us an example of a scatter plot. Such plots are useful outside of astronomy too.

  What one sees in the HR diagram

  When data on the nearest stars to us or stars in a cluster are plotted in a HR diagram, here is what one sees.

  Brightness, size, and the HR diagram

  We expect a relation between temperature and luminosity. How does that show up in the HR diagram?

  Mass and the HR diagram

  There is a relation between mass and luminosity.

  Conclusion

  Based on this evidence, we conclude
  • Stars spend most of their lives as main sequence stars.
  • During its lifetime, the surface temperature and luminosity stays pretty much constant.
    • Something else could happen in the star birth process.
    • Something else could happen in the star death process.
  • The star's mass determines what the temperature and luminosity is during the star's main sequence lifetime.
    • More mass -> hotter.
    • More mass -> more luminous.
    • Also, more mass -> bigger.
  
  Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA soper@bovine.uoregon.edu