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Stellar Nucleosynthesis
Charles Hyde
2 March 2009
Nucleosynthesis in Stars
• Great triumphs of 20th century physics
– Discovery that sun, stars are mostly H
– Explanation of nuclear fusion reactions powering sun
• Nuclear Binding Energy
• Quantum mechanics
• Weak interaction ( beta decay)
– Neutrino flux from sun,
• Ray Davis, BNL (Cl detector in Homestake mine, SD)
– Neutrinos from SuperNova
• Kamiokande Water Cerenkov detector: SN1987a
Nucleosynthesis in the 21st Century
• How exactly do Supernovae explode
– Complex 3D hydrodynamics
– Role of neutrino mass and oscillation?
• See next 10 years of accelerator and reactor based neutrino
experiments
• Exact pathways for nucleosynthesis of elements
heavier than Fe
– FRIB project launched at MSU, Dec 2008
• Exact understanding of nuclear masses from
fundamental theory
– Quark mass - Higgs mechanism? FermiLab, LHC
– Proton mass, nuclear force from QCD
• Jlab, Lattice QCD, …
Nuclear Energy in Stars
• E=mc2
– Release of energy by fusion reactions
• Pp chain
• CNO chain
• Explosive nucleosynthesis
Stellar Hydrodynamics
• See Lectures, Prof. Bueltmann
– Astrophysics in a Nutshell, Section 3.1
• Temperature, Luminosity of sun from quantum black
body emission
2
P Sun  4 RSun
T4





   2 k 4 c / 60 c3  5.7 108 W / m 2 K 4

• Hydrostatic/Gravity equilibrium determines
temperature / pressure profile of sun.
• 
Only mildly effected by profile of nucleo-synthesis
(concentrated at temperature/pressure maximum at
center)
Solar Properties
• Temperature
– Surface T=5800K, kT = 0.5eV
– Center 15106 K.
• Mass 21030 kg.
• Radius R=7108 m.
• Density
– Average = 1.4 g/cm3.
– Central = 150 g/cm3.
• Total Luminosity 71033 W.
• Gravitational potential energy released in collapse
from infinity to R
– GM2/(2R) = 21041 J.
Possible Source of Solar
Luminosity
• Gravitational collapse
– Lifetime = [Potential Energy]/Luminosity
– T = [21041 J] / [71033 W] = 1 yr
• Nuclear Fusion
– Enough energy
– Slow enough to sustain sun for > 109 years
• Reaction rate depends upon subtle details of nuclear
physics
Nuclear Energy
• E=mc2.
• Atomic mass unit (u)
– 1 mole of 12C defined to have a mass of 12 g
• (This defines Avogadro’s number = 6.021023)
–
–
–
–
1 neutral 12C atom has a mass = 12u
1 u = (0.012 kg)c2/(12NA)=1.510-10 J
1eV = (1.610-19 C)(1V)= 1.610-19 J
1 u = 934106 eV
• Nuclear Wallet cards www.nndc.bnl.gov/wallet/
– M(H) = 1u+ 8.071 MeV
– M(n) = 1u+ 7.289 MeV
– Mass(AZ) = A(1u)+(AZ)
 (12C)=0
Nucleosynthesis
in final stages of
a star before
Supernovae (or
white dwarf)
Nucleosynthesis
in Supernovae
explosions
Fe, Ni,
Most stable nuclei
Unstable to
a-decay,
fission
Heats the
earth
Fusion of protons to 4He
Drives the sun
E=mc2
Nuclear Fusion Reactions
• Energy release in fusing 6 p + 6 n  12C.
– 6 (p) + 6 (n)  0 = 92,16 MeV released
– Fractional energy release ≈ 1%
• Mass conservation violated at 1% level
– Compare to chemical energy
• e+p  H: Energy Release = (13 eV)/(938 MeV) ≈ 10.
• Mass is conserved (parts per billion) in chemical reactions
• Energy Release in pp chain
– HHHH  4He
– 4 (p)- (4He) = 4(8.071 MeV)(2.424 MeV)= 30 MeV