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The HR Diagram

Learn about the HR Diagram and what it reveals about stellar properties such as brightness, size, temperature, and luminosity. Discover how mass affects a star's temperature and luminosity, and how stars evolve throughout their main sequence lifetime.

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The HR Diagram

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  1. The HR Diagram L ~ R2T4

  2. What one sees in the HR diagram

  3. What one sees in the HR diagram

  4. What one sees in the HR diagram

  5. Brightness, size, and the HR diagram • We know two things about objects that are approximately blackbodies • Hotter things are brighter. • Energy radiated per unit time per unit area is proportional to T4, • so bigger T means more energy radiated • Bigger things are brighter. • Energy radiated per unit time per unit area is proportional to 4πR2, • so bigger surface area means more energy radiated. • Putting this together, we have

  6. Temperature, size and luminosity

  7. Temperature, size and luminosity

  8. Mass, luminosity, and the HR diagram Our conclusion: Different stars have different masses. Among all stars with the same mass, almost all have the same properties. The most massive stars are the most luminous.

  9. 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.

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