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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
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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
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
Lifetime High mass stars have more hydrogen to fuse BUT They fuse this hydrogen much faster SO They run out sooner
Cluster Formation Red green and yellow dots represent post main sequence stars
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
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
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
Core Degeneracy Heat energy goes into nuclei but density is controlled by the electrons
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
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
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
