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H Exhaustion

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 = r convective core

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H Exhaustion

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  1. 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

  2. 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

  3. 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

  4. H Exhaustion No convective core - H depleted only at r = 0+ Smooth transition to shell burning - no K-H jag

  5. 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

  6. 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

  7. 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

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