3
Hydrogen Shell Burning
•
•Fusion stops in core when hydrogen runs out
–Star has core of He, but T too low for fusion there
•Heat loss makes star contract, T goes up in interior
•Before T in core reaches He ignition point --
•Hydrogen above He core begins “shell burning”
•
•Shell burning changes the rules for structure
–Still need dense core to allow high T, P for fusion
–But high T on outside of core puts too much energy into outer parts of star
–Outer parts responds to input energy by
expanding and cooling
–
•Star in effect separates into two parts
–Hot dense core where energy generation takes place
–Extended envelope which shields and insulates core
From our text:  Horizons, by Seeds