Ch 11-01 Evolution of Low-Mass Stars

• Asymptotic giant branch (AGB stars)
  
  
  
  
• Significant convection and mass-loss of AGB stars
  Carbon stars; C/O dispersed into ISM by strong stellar winds
  Helium shell flash, thermal pulses; mass loss by ejection, ~ 40%
  
  
  
  
• Planetary nebulae
  
  
  
  
• White dwarfs
  
  
  
  M ~ M_⊙, R ~ 10⁹cm, T_surface ~ 10^4~5K, ρ_average ~ 10⁶g/cm³
  Pressure of electron degeneracy against gravity
   
• Chandrasekhar limit ~ 1.4 M_⊙
  The maximal mass of a white dwarf
  
  
  
  
• Evolution of low-mass stars
  
  
  
  * Stars of mass (when in the main sequence) between about 0.4 M_⊙ and 8 M_⊙, like our Sun, evolve in the way we describe above, i.e., ending up with a white dwarf consisting mainly of carbon and oxygen.
  
  * Stars of mass (when in the main sequence) between about 0.08 M_⊙ and 0.4 M_⊙ (red dwarfs) do not have the stage of hydrogen shell burning, because of strong convection, which stirs up the whole star. These stars are red dwarfs consisting mainly of helium.
  
  * Some model computations suggest that stars of mass (when in the main sequence) between about 4 M_⊙ and 8 M_⊙ can have carbon burning. Such stars become white dwarfs consisting mainly of O, Ne, Na, and Mg.