GEOL3045: Planetary Geology
Lysa Chizmadia
11 Jan 2007
The Big Bang & Nucleosynthesis
Introduction
 Parts of an atom:
 Protons (p+): +’ly charged particles in nucleus,
mass ~ 1 atomic mass
 Neutrons (n0): 0 charged particles in nucleus,
mass ~ 1 atomic mass
 Electrons (e-): -’ly charged particles in nucleus, mass
= 5.45 x 10-4 atomic masses [= 1/ 1836 of p+ mass]
 Definition of an isotope:
 Different # of n0s
 Same element, different atomic mass
 E.g. Oxygen



16O
= 8 p+s and 8 n0s
17O = 8 p+s and 9 n0s
18O = 8 p+s and 10 n0s
The Big Bang
 Initially universe
 Only contains photons (), p+s, n0s, e-s and e+s
 Due to high , particles collide
 Pair production
 Annihilation
 Proton - Neutron Conversion
Images from: http://aether.lbl.gov/www/tour/elements/early/early_a.html
Nucleosynthesis (H & He)
 Due to  T, particles don’t stick
 With  T, particles stick to form elements
 2 Pathways to form H & He:
Pathway #1
Pathway #2
Images from: http://aether.lbl.gov/www/tour/elements/early/early_a.html
Nucleosynthesis (Li & Be)




6Li
= 4He + 2H
7Li = 4He + 3H
7Be = 3He + 4He
8Be = 4He + 4He
 Finally, cools enough for atoms to capture
e- to be neutral
Images from: http://aether.lbl.gov/www/tour/elements/early/early_a.html
Big Bang Nucleosynthesis
Produced during Big Bang
Image from: http://aether.lbl.gov/www/tour/elements/element.html
 So where do the heavier elements originate?
Stellar Nucleosynthesis
 Initially ~75% H & ~25% He
 Not homogeneously distributed
 Places of  , become clumps
 Clumps collapse into nebulae
 Nebula collapses into star
  T, P and 
Images from: http://aether.lbl.gov/www/tour/elements/stellar/stellar_a.html
Stellar Nucleosynthesis
 Inside star’s core:




P &  very high
Nuclear fusion
H-burning: H  He
Mass > 1.5 M,
= Sunlight
 He-burning: He  C
Images from: http://en.wikipedia.org/wiki/Main_sequence and
http://aether.lbl.gov/www/tour/elements/stellar/stellar_a.html
Stellar Evolution
 Hertzsprung-Russell diagram
 Mass > 4 M
 H  He  C  Ne 
O  Si  Fe
Images from: http://en.wikipedia.org/wiki/Main_sequence
Nucleosynthesis Reactions
 Mass > 4 M,
 C-burning

12C

16O
 Ne-burning:
+   16O + 4He
20Ne + 4He  24Mg + 
20Ne
+ 16O  28Si + 4He
 31P + 1H
 31S + n0
 31P + 1H
+ 12C  20Ne + 4He
 23Na + 1H
 24Mg + n0
 Mass > 8 M,


 O-burning
 30S + 21H
 30P + 22H
 Mass = 8-11 M,
 Si-burning
 16O  20Ne 
24Mg  28Si  32S 
36Ar  40Ca  44Ti 
48Cr  52Fe  56Ni
 56Ni decays to 56Co  56Fe

12C
 t1/2 = 6 days & 77 days
What about elements > Ni?
Nucleosynthesis
Produced during Big Bang
Stellar Nucleosynthesis
Image from: http://aether.lbl.gov/www/tour/elements/element.html
 So where do the heavier elements originate?
Fe has highest binding E
 All elements up to Fe release energy (E)
when formed (exothermic)
 Heavier elements need more energy
added (endothermic)
 How to add more
E to form heavier
elements? Image from: http://en.wikipedia.org/wiki/Silicon_burning_process
Supernovae
 With burning of heavier elements, star
continues to collapse
 Fe breaks down to 4He, n0 and p+
 Finally no more compression
possible
 Outer layers bounce off core
 Supernova occurs
 Releases neutrons
Images from: http://aether.lbl.gov/www/tour/elements/stellar/stellar_a.html
Supernovae Processes
 R-process: rapid neutron capture
 Occurs over seconds
 S-process: slow neutron capture
 Occurs over 1000s of years
 P-process: knocks out neutrons
 Results in proton-rich isotopes
 rP-process: rapid proton capture
 Occurs over seconds
 Cannot progress > Te
Image from: http://en.wikipedia.org/wiki/S-process
Chart of the Nuclides
 Red = stable
 Blue = extremely
short half lives
 For more info:

http://en.wikipedia.org/wiki/Isotope_table_
%28complete%29
Image from: http://en.wikipedia.org/wiki/Main_sequence
Summary
 1) Only H, He, Li & Be produced during Big
Bang
 Initially only energy & sub-atomic particles existed
 With  T, particles collided to form atoms
 2) Elements up to Fe produced by stellar
nucleosynthesis
 Larger star produce heavier elements
 3) All elements heavier than Fe produced in
supernovae
 Neutron capture
 Proton capture