Download H Exhaustion - University of Arizona

H Exhaustion
After core H exhaustion core must contract and heat
before He burning can begin
Shell around contracting core heated to H burning T
H Exhaustion
Convective core - H depleted out to r = rconvective core
T much lower where H still present - must contract in
order to heat to H ignition temperature
H Exhaustion
Convective core - H depleted out to r = rconvective core
T much lower where H still present - must contract in
order to heat to H ignition temperature
Whole star contracts on Kelvin-Helmholz (thermal)
timescale - star briefly increases in luminosity and moves
blueward
H Exhaustion
No convective core - H depleted only at r = 0+
Smooth transition to shell burning - no K-H jag
H shell burning
• Shell must support envelope against gravity of inert
core. All L from small r so burning takes place at
higher temperatures in shells
• Thermodynamic gradients outside shell very steep excess L goes into expanding star to flatten gradients
- star moves to red
H shell burning
• Shell must support envelope against gravity of inert
core. All L from small r so burning takes place at
higher temperatures in shells
• Thermodynamic gradients outside shell very steep excess L goes into expanding star to flatten gradients
- star moves to red
• Low mass stars (~2M) have degenerate cores &
produce enough L in shell to support envelope move to RGB on H burning timescales
• Sun will spend ~4 Gyr moving from core H
exhaustion to RGB - 40% of total H consumption and
lifetime
H shell burning
• Shell must support envelope against gravity of inert
core. All L from small r so burning takes place at
higher temperatures in shells
• Thermodynamic gradients outside shell very steep excess L goes into expanding star to flatten gradients
- star moves to red
• Higher mass stars (~2M) have non-degenerate
cores -don’t produce enough L in shell to support
envelope - move to RGB on K-H timescale
• Time for crossing can be as little as 103-104 yr - stars
not observed in this part of HR diagram - Hertzsprung
Gap