Ch 07-02 Spectral Classes of Stars

• Spectral classes
  Stars have different absorption lines in their spectra.
  
  
  
  Earlier classification, based on the hydrogen line strength, is from Class A to Class P.
  Later, a re-ordering looks more sensible, which is O B A F G K M.
  
  
  
  
  
  The Saha equation describes the degree of ionization, which is mainly determined by the temperature.
  Temperature ⇔ color index ⇔ spectral classes
  
  
  
  
  
   
  (Brown dwarfs are stars of masses in the range from 13 to 80 M_J.)
   
• Line broadening
   
  • Natural broadening
    △E△t ~ h, △ν△t ~ 1
    Typically, △t ~ 10^-8sec, △ν ~ 10⁸Hz
    For λ = 10000Å, △ν/ν ~ 10^-7 and △λ ~ 10^-3Å
     
  • Doppler broadening
    △λ/λ ~ β
    For thermal motion, v ~ 10⁶cm/s for T ~ 10⁴K
    For rotation,            v ~ 10⁵cm/s for the Sun
     
  • Collisional broadening (pressure broadening)
    Life time (therefore uncertainty in energy) affected by collisions
    Mainly depending on density