The Interior of Stars II

1. The Interior of Stars II • Stellar Energy Sources (CONTINUED…) • Energy Transport and Thermodynamics • Stellar Model Building • The Main Sequence

2. Coulomb Repulsion and Quantum Tunneling • Range of strong nuclear force is approximately 1fm=10-15m • Coulomb repulsion barrier until d=1fm…Vc=3.43 Mev between protons • Classically the kinetic energy of proton must exceed this barrier potential • Quantum mechanical tunneling effectively lowers coulomb barrier. • Proton must approach approximately within one Debroglie wavelength of the target • http://en.wikipedia.org/wiki/Quantum_tunneling

3. Coulomb Repulsion and Quantum Tunneling • Classically proton would need to climb full coulomb repulsion barrier to get close enough (~1fm) for strong nuclear potential to become effective. • Quantum Mechanical barrier penetration allows the proton to tunnel “close enough” • Lowers temperature at which fusion occurs

4. Nuclear Reaction Rates and the Gamow Peak • Nuclear reaction rates depend upon: • Distribution of kinetic energies for a given temperature • Number density in given energy range • Probability for tunneling at that energy • Must integrate over all energies to get total rate • Maxwell-Boltzmann distribution gives the number density in the energy range E to E+dE • Cross-section: Number of reactions per target nucleus per unit time • Number of incident particles contained in the cylinder • Number of incident particles/volume • Number of reactions per target nucleus /time interval dt having energies between E and E+dE is

5. Nuclear Reaction Rates and the Gamow Peak • Total number of reactions per unit volume per unit time • Need to know form of (E). Use idea of DeBroglie Wavelength • Cross-section also proportional to tunneling probability • Combining all of these for the cross-section • The reaction rate integral becomes • Resulting in the “Gamow Peak” at

6. Nuclear Reaction RatesOther Efffects • Resonance • If energy of incoming particle “matches” differences in energy levelsof target nucleus…cross-section goes through a narrow resonance peak • Electron Screening • Ionized electrons produce a “sea of negative charge” that screens electric charge of nuclei effectively reducing Coulomb barrier (10% to 50% enhancement in helium rates!!!)

7. Nuclear Reaction RatesOther Efffects and Stellar Luminosity • Power Laws • Reaction rates can be represented as a power law such as • Can then formulate the amount of energy liberated per kilogram of material per second as (in units of W kg-1) • Can then show the dependence of energy production on temperature and density • The Luminosity gradient equation • To determine the luminosity of a star must consider the energy generated • The contribution to the luminosity from an infinitesimal mass can be expressed as: • Where  is the total energy released per kilogram per second by all nuclear reactions and gravity nucleargravity note that gravity can be negative!!! • Interior Luminosity, Lr, for a spherically symmetric star satisfies:

8. Stellar Nucleosynthesis and Conservation Laws • Nucleosynthesis • What are the exact sequence of steps by which one element is converted into another? • Conservation Laws of physics must be satisfied • Electric charge • Number of Nucleons • Number of leptons • …Antimatter

9. Proton-Proton Chains • Proton-Proton Fusion chains

10. Proton-Proton Chain I • Each step has its own rate • Slowest step involves the decay of a proton into a neutron via the Weak nuclear force

11. p-p chain stage 1

12. p-p chain stage 2

13. p-p chain stage 3

14. p-p chain stage 4

15. Berrylium Proton-Proton Chains • PP II (Proton-Proton II Chain) • Possible reactions of He-3 and He-4 That produce Beryllium • Via Lithium or Boron • Neutrino Energy Distribution is distinct from PP I chain… • Observations of Solar Neutrino Energy Distributions/rates • Tests • Solar Model • Basic Particle Physics!!!! • PP III (Proton-Proton III Chain)

16. Solar Neutrino Generation • Neutrinos allow the interior of the sun to be viewed “directly” but… • Solar Neutrino Problem too few solar neutrinos observed. Standard solar model predicted greater neutrino flux that that was observed… • Resolution:…Neutrino Oscillations. Neutrinos change “flavor” and become undetectable on their flight from the Sun.

17. Solar Neutrino Generation • Too few SNUs!!! (solar neutrino unit) • Solar Model predicts SNU rates with GREAT CONFIDENCE!!! • Particle Physics is incomplete!! Neutrino Oscillations

18. CNO Cycle • CNO energy production • Strong temperature dependence • If star begins to collapse density and temperature increase…This results in higher energy production • Collapse balanced by energy production

19. Triple Alpha Process of Helium Burning A process to burn Helium… Very strong temperature dependence!!!!

20. Carbon and Oxygen Burning • For massive stars that can generate sufficiently high temperatures and densities …Carbon and Oxygen can be “burned…” • T>6 x 108 K --> C burning • T>109 K --> O burning

21. Curve of Binding Energy • Fusion is an exothermic process until Iron

22. Overview: Equations of Stellar Structure • Pressure • Mass • Luminosity • Temperature • http://abyss.uoregon.edu/~js/ast121/lectures/lec22.html HYDROSTATIC EQUILIBRIUM GEOMETRY/ DEFINITION OF DENSITY NUCLEAR PHYSICS THERMODYNAMICS (ENERGY TRANSPORT)

23. Energy Transport and Thermodynamics • Radiation • Convection • Conduction

24. The Radiative Temperature Gradient • Radiation Pressure Gradient from ch 9 • Equating gives temperature gradient • Using We obtain an expression of the temperature gradient for Radiative transport

25. The Pressure Scale Height • If temperature gradient becomes too steep, convection can play an important role in energy transport • Convection involves mass motions:hot parcels move upward as denser parcels sink • Characteristic length scale for convection, Pressure Scale Height typically about the size of the star. • To estimate the size of the convective region, consider • We can solve for P(r) • Using equation 10.6 we obtain

27. Internal Energy and the First Law of Thermodynamics • First Law of Thermodynamics • For an Ideal Gas, the internal energy is:

28. Specific Heats To find Cp for a monatomic gas Ratio of specific heats • The change in heat of a mass element for a change in temperature is expressed as the specific heat. • Depends on whether pressure or volume is held constant… • Play with differentials … • Consider a gas in a cyclinder with a piston that moves, we have for dW • Consider • With dV=0, we have • For an ideal monatomic gas

29. The Adiabatic Gas Law • For an adiabatic process (dQ=0) for which no heat flows into or out of the mass element. The first law can be written as: • From 10.78 with constant n • Since dU=CVdT, we have • Combining • Using (10.79) and (10.80) gives • Solving this differential equation we obtain • The Adiabatic Gas Law • Using the Ideal Gas Law a second adiabatic relation may be obtained

30. Sound Speed

31. The Adiabatic Temperature Gradient • Consider a hot convective bubble of gas that rises and expands adiabatically • After it has traveled some distance it thermalizes, giving up excess heat • Loses its distinction and dissolves into the surrounding gas • Differentiating the ideal gas law (10.11) yields an expression involving the bubble’s temperature gradient (How the buble’s temperature changes with position) • Using the adiabatic relationship between temperature and density (10.82) and specific volume V=1/we have • Differentiating and re-writing, we obtain • Assuming constant combining 10.85 and 10.87 gives the • Adiabatic Temperature Gradient • Using 10.6 and the ideal gas law we obtain

32. The Adiabatic Temperature Gradient • If |dT/dr|act is just slightly greater than |dT/dr|ad deep in the interior of the star than this may be sufficient to carry nearly all of the luminosity by convection!!!! • It is often the case that either radiation or convection dominates the energy transport deep in the interior of stars • Dominant Energy Transport mechanism is determined by the temperature gradient in the deep interior • Near surface of star situation is more complicated and both transport mechanisms can be important • This expression describes how the temperature of the gas inside the bubble changes as the bubble rises and expands adiabatically • If the star’s actual temperature gradient is steeper than the adiabatic temperature gradient • The temperature gradient in that case would be said to be superadiabatic

33. A Criterion for Stellar Convection

34. A Criterion for Stellar Convection • What conditions must be met for convection to dominate over radiation in the deep interior of a star? • When will a hot bubble of gas continue to rise rather than sink back down after being displaced upward • Archimedes Principle: If the initial density of the bubble is less than its surroundings it will begin to rise • Buoyant force per unit volume exerted on a bubble immersed in a fluid of density (s) is: • Graviational force on bubble • Net force per unit volume on bubble • If after having traveled dr the density of the bubble becomes greater than the surrounding medium it will sink again and convection is prohibited. Otherwise if the density of the bubble remains less it will continue to rise and convection will result

35. Criteria for stellar convection • Express this condition in terms of temperature gradients. Assume that initially the gas is very nearly in thermal equilibrium and the densities were almost the same • Bubble expands adiabatically This condition must be satisfied for convection to occur

36. The Mixing Length Theory of Superadiabatic Convection • Pressure of bubble and surrounding gas are always equal • Ideal gas law then implies that bubble must be hotter than surrounding gas • Therfore the temperature of the surrounding gas must decrease more rapidly with radius • Is required for convection to occur. Since the temperature gradients are negative • Assuming that the bubble moves adiabatically , let • After a bubble travels dr, its temperature will exceed the surrounding gas by: • Assume that a bubble has traveled • Before dissipating. At which point it etherealizes giving up its excess heat at constant pressure. The distance l is called the mixing length. Hp is the pressure scale height and • Is a free paramter that is the ratio of the mixing length to pressure scale height. Generally assumed to be about 1. (0.5 -3)

37. The Mixing Length Theory of Superadiabatic Convection • After the bubble travels one mixing length , the excess heat flow per unit volume from the bubble to its surrounding is • Use 10.96 with dr=l . Multiplying the average velocity of the convective bubble one obtains the convective flux • Average velocity is obtained from the net force per unit volume acting on the bubble • Finally we obtain: • After further manipulations …an expression for the convective flux

38. The Mixing Length Theory of Superadiabatic Convection • To evaluate the convective flux you need to know the difference between the temperature gradients of the bubble and its surroundings… • For simplicity assume that all the flux is carried by convection • This will allow us to estimate the difference in the temperature gradients • Comparing to the adiabatic temperature gradient