Ch 07-03 Hertzsprung-Russell Diagram

• Hertzsprung-Russell diagram (H-R diagram)
  
  
   
  L = 4πd²F = 4πR²σT⁴
  
  
   
  
  The radii of some bright, nearby stars have been measured with optical/infrared interferometry, e.g., by CHARA.
  Examples are: Altair (2 R_⊙, 17 ly), Vega (2.6 R_⊙, 25 ly), Regulus A (4 R_⊙, 78 ly), Achernar A (10 R_⊙, 144 ly), Betelgeuse (1000 R_⊙, 427 ly).
   
• Luminosity classification
  
  
  
  
  
  The luminosity class is based on line widths. It provides another dimension in the H-R diagram.
   
• Spectroscopic parallax
  e.g., Spica, B1V, m_v = +1
  => M_v = -3
  => d = 63pc
  The uncertainty can usually be quite large, say, >10%.
   
• Stellar density in the H-R diagram