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Stellar Fuel, Nuclear Energy
and Elements
• How do stars shine?
E = mc2
• How did matter come into being?
Big bang  stellar nucleosynthesis
• How did different elements form?
Stars  Supernovae
• What is thermonuclear fusion ?
Synthesis of lighter atoms into heavier
ones at high temperature-density
Nuclear Fusion: H  He
p-p chain
neutrino
Deuterium
positron
Gamma-rays
electron
P.S. No gamma rays produced in the p-p reaction itself
The Atomic and Sub-Atomic Zoo
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Atom  protons, electrons neutrons
Atomic number (#protons)
Atomic weight (#protons+neutrons)
Hydrogen  1H1
Deuterium  1H2
Same element, different nuclei  isotopes
Nuclear reactions  energy
Deuterium (Heavy Hydrogen) + Hydrogen
 Light Helium + gamma-rays (energy)
Final Product H-fusion : Ordinary He + Energy
For each layer:
Weight + Pressure Above = Pressure Below
Density and Temperature vs. Radius of Sun
Percentage Mass and Luminosity vs. Radius of Sun
Structure of the Sun: Three Zones
Core, Radiative, Convective
How long with the Sun last?
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What is its current state?
What is its mass ?
How much does it burn?
How old is it?
Answer: Section 9.3
• And then what?
Future: Sun The Red Giant
• When the Sun can no longer burn
Hydgrogen in the core
• Core becomes helium dominated
• Star expands; H-burning in outer shell
• Triple-alpha nuclear reaction
• Three helium nuclei  carbon
• 4He2 + 4He2 + 4He2  12C6 + 2g
 4He2 + 12C6  16O8
• Helium burning  Carbon/Oxygen core
Stellar Evolution – HR Diagram
Low Mass Stars
MS  RG  AGB  Pne  WD
High Mass Stars
MS  Cepheids / Supernovae
MS – Main Sequence
RG – Red Giant
AGB – Asymptotic Giant Branch
Pne – Planetary Nebulae
WD – White Dwarf
Sne – Supernovae
Nucleosynthesis and Stellar Evolution of
low mass stars
• Red giants continue to eject outer layers
and evolve along the Asymptotic Giant
Branch (AGB)
• AGB stars are left with the stellar core
surrounded by a relatively thin sphere of
hot gas which looks like planetary disk,
and called Planetary Nebulae (PNe)
(nothing to do with planets per se)
• PNe cores continue to cool and become
White Dwarfs (94% stars end up as WDs)
Nucleosynthesis in High Mass Stars
• Nuclear fusion continues beyond C/O
• For example:
12C + 16O
28Si

6
8
14
28Si
28Si
56Ni
56Fe
+
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14
14
28
26
• Radioactive Ni  Fe
• Fusion beyond iron is endothermic; does
not produce energy; stars out of fuel;
gravity wins and……………….
The Supernova Onion
The End
• If the WD mass is more than 1.4 times
more massive than the Sun, it undergoes
a gravitational collapse into a Neutron
Star
• 1.44 M(Sun)  Chandrashekhar Limit
• Electrons fall into nuclei (protons)
e- + p+  no + n (neutrino)
• Gravitational collapse may continue;
massive stars end up as neutron stars or
black holes after supernova explosion
Cosmic Abundances
• Not yet known accurately, even in the Sun
• To wit: C, N, O abundances revised
downwards by 30-50% in the last decade
• What is the Sun made of?
• Cosmic abundances relative to the Sun