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Factors affecting Fusion Rate

Factors affecting Fusion Rate. Density Since protons are closer together, the mean free path between collisions will be smaller Temperature At higher temperatures a larger proportion of protons are moving fast enough to overcome the Coulomb Barrier

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Factors affecting Fusion Rate

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  1. Factors affecting Fusion Rate • Density • Since protons are closer together, the mean free path between collisions will be smaller • Temperature • At higher temperatures a larger proportion of protons are moving fast enough to overcome the Coulomb Barrier • Faster protons take less time to cover the distance between collisions

  2. The Effect of Mass • Higher mass condensing into the star means • More hydrogen fuel to fuse • Higher pressure leading to higher density and temperature in the centre of the core • Much higher nuclear energy generation rates • So higher mass stars have much higher luminosities • e.g. a 10 Solar Mass Star generates 10,000 times more luminosity than the Sun

  3. Star Formation

  4. Mass –Luminosity

  5. Main Sequence Masses

  6. Lifetime High mass stars have more hydrogen to fuse BUT They fuse this hydrogen much faster SO They run out sooner

  7. Mass-Lifetime

  8. Main Sequence Lifetimes

  9. Cluster Formation Red green and yellow dots represent post main sequence stars

  10. Clusters

  11. 1 Solar Mass Evolution

  12. Approach to Main sequence Core is always contracting and heating up, in vertical track heat is transported by convection so the increase in core temperature doesn’t show on the photosphere

  13. Approach with time scales

  14. Approach for different Masses

  15. Vertical & Horizontal tracks

  16. Protostar Tracks Protostars are always getting smaller and hotter • Vertical Track <> Convective envelope • Convection requires larger temperature difference between the core and the photosphere • Although the core is getting hotter , the temperature of the photosphere stay relatively constant • Luminosity falls because the star is shrinking • Horizontal Track <> Radiative Envelope • Radiation results in smaller temperature difference between the core and the photosphere • Photosphere temperature rises but contraction results in luminosity staying nearly constant

  17. MS Structure

  18. 1 Solar Mass Evolution • Vertical track • Horizontal trrack • Main Sequence • Red Giant Branch • Helium Flash • Horizontal Branch • Asymptotic Giant Branch • Planetary Nebula • White Dwarf • Brown Dwarf

  19. Core Degeneracy Heat energy goes into nuclei but density is controlled by the electrons

  20. Degenerate Gas If you push the electrons closer together, the energy levels all get farther apart , in a non degenerate gas some electrons will jump to lower energy levels

  21. Evolution to Red Giant

  22. Red Giant

  23. 1 Solar Mass Evolution

  24. Helium Flash • Ash dumped from shell raises core Temp to 100 Million K, • Helium to Carbon Fusion • Degenerate gas has no safety valve, He fusion proceeds explosively • Most of the energy goes into making the core expand, lifting degeneracy

  25. Safety Valve ? • In a non-degenerate core… • More reactions Higher Temp • Higher Temp Higher Pressure • Higher Pressure Expansion • Expansion Lower Temp • Lower Temp Less Reactions • In a degenerate core step 2 doesn’t happen because heat energy goes to lifting the degeneracy rather than raising the pressure

  26. Helium to Carbon Fusion

  27. Core and Shell Fusion

  28. Post Main Sequence HR

  29. Asymptotic Giants

  30. Descent to White Dwarf

  31. Cooling Embers

  32. Mass-Radius for Degenerate Stars

  33. Mass-Radius for White Dwarfs

  34. Multiple Shell Burning

  35. Iron catastrophe

  36. Timescales to Supernova

  37. Heavy element synthesis

  38. Cosmic Abundance

  39. Open Cluster

  40. Globular Cluster

  41. Cluster HR Diagram

  42. Cluster Turn-off

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