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

Star Formation

Mass –Luminosity

Main Sequence Masses

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

Mass-Lifetime

Main Sequence Lifetimes

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

Clusters

1 Solar Mass Evolution

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

Approach with time scales

Approach for different Masses

Vertical & Horizontal tracks

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

MS Structure

1 Solar Mass Evolution 5.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

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

Evolution to Red Giant

Red Giant

1 Solar Mass Evolution

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 ?

Helium to Carbon Fusion

Core and Shell Fusion

Post Main Sequence HR

Asymptotic Giants

Descent to White Dwarf

Cooling Embers

Mass-Radius for Degenerate Stars

Mass-Radius for White Dwarfs

Multiple Shell Burning

Iron catastrophe

Timescales to Supernova

Heavy element synthesis

Cosmic Abundance

Open Cluster

Globular Cluster

Cluster HR Diagram

Cluster Turn-off