factors affecting fusion rate density –since protons are closer together, the mean free path...
TRANSCRIPT
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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
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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
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Star Formation
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Mass –Luminosity
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Main Sequence Masses
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Lifetime
High mass stars have more hydrogen to fuse
BUT
They fuse this hydrogen much faster
SO
They run out sooner
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Mass-Lifetime
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Main Sequence Lifetimes
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Cluster Formation
Red green and yellow dots represent post main sequence stars
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Clusters
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1 Solar Mass Evolution
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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
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Approach with time scales
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Approach for different Masses
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Vertical & Horizontal tracks
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Protostar TracksProtostars 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
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MS Structure
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1 Solar Mass Evolution5. Vertical track
6. Horizontal trrack
7. Main Sequence
8. Red Giant Branch
9. Helium Flash
10. Horizontal Branch
11. Asymptotic Giant Branch
12. Planetary Nebula
13. White Dwarf
14. Brown Dwarf
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Core Degeneracy
Heat energy goes into nuclei but density is controlled by the electrons
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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
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Evolution to Red Giant
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Red Giant
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1 Solar Mass Evolution
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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
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• In a non-degenerate core…1. More reactions Higher Temp2. Higher Temp Higher Pressure3. Higher Pressure Expansion4. Expansion Lower Temp5. 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
Safety Valve ?
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Helium to Carbon Fusion
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Core and Shell Fusion
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Post Main Sequence HR
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Asymptotic Giants
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Descent to White Dwarf
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Cooling Embers
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Mass-Radius for Degenerate Stars
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Mass-Radius for White Dwarfs
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Multiple Shell Burning
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Iron catastrophe
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Timescales to Supernova
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Heavy element synthesis
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Cosmic Abundance
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Open Cluster
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Globular Cluster
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Cluster HR Diagram
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Cluster Turn-off