Life of a Star

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Department
Astronomy & Astrophysics
Course
AST201H1
Professor
Michael Reid
Semester
Winter

Description
March  5,  2013   Lecture  14:  Life  of  a  Star   Low  Mass  Star’s  Life  Stages   • Helium  fusion  turns  on  very  suddenly  which  is  called  helium  flash.   o For  the  brief  time,  the  Sun  will  be  alive  again  by  fusing  helium  into   carbon,  but  the  re-­‐live  is  very  short  and  moves  to  the  horizontal   branch  of  H-­‐R  diagram   o Helium  fusion  is  via  triple-­‐alpha  process   o After  about  100  millions  years  of  helium  fusion,  helium  in  the  core   will  burn  out.  When  there  is  a  carbon,  the  Sun  is  officially  dead.   (carbon  ash  in  the  center)     § Low  mass  stars  will  never  get  hot  enough  to  fuse  carbon  atoms   together   • Horizontal  Branch  –  same  luminosity  for  all  spectral  classes   • Carbon  core  starts  to  shrink  as  it  is  inactive  (no  pressure  in  the  core  to  keep   the  balance)    -­‐  electrons  get  closer  together  by  the  gravity  à  electron   degeneracy  pressure  to  support  the  stars  against  gravity   o White  Dwarf  =  Humongous  amount  of  materials  are  dense  together   • Helium  shell  burning  generates  enough  amount  of  energy  to  blow  the  outer   layers  of  the  Sun  off  into  space,  forming  planetary  nebula  –  nothing  to  do   with  planets!   o White  Dwarf-­‐  what  is  left  for  the  Star  is  just  its  white-­‐hot  carbon-­‐rich   core  
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