Presentation on theme: "Factors affecting Fusion Rate Density –Since protons are closer together, the mean free path between collisions will be smaller Temperature –At higher."— Presentation transcript:
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
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
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
In a non-degenerate core… 1.More reactions Higher Temp 2.Higher TempHigher Pressure 3.Higher PressureExpansion 4.ExpansionLower Temp 5.Lower TempLess 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 ?