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Download H Exhaustion - University of Arizona
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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
