• How do stars shine?
E = mc 2
• 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

• Atom  protons, electrons neutrons
• Atomic number (#protons)
• Atomic weight (#protons+neutrons)
• Hydrogen  1 H
1
• Deuterium  1 H
2
• Same element, different nuclei  isotopes
• Nuclear reactions  energy

Deuterium (Heavy Hydrogen) + Hydrogen
 Light Helium + gamma-rays (energy)

Final Product H-fusion : Ordinary He + Energy

Stellar Structure
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

• 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?

• 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
• 4 He
2
 4
He
2
+ 4 He
2
+ 12 C
6
+ 4 He

2
16

O
8
12 C
6
+ 2 g
• Helium burning  Carbon/Oxygen core

Stellar Evolution – HR Diagram
Low Mass Stars: Protostar 
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

• 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)

• Nuclear fusion continues beyond C/O
• For example:
12 C
6
+ 16 O
8
 28 Si
14
28 Si
14
+ 28 Si
14
 56
• Radioactive Ni  Fe
Ni
28
 56 Fe
26
• Fusion beyond iron is endothermic; does not produce energy; stars out of fuel; gravity wins and……………….

• 1.44 M(Sun)  Chandrashekhar Limit
• If the WD mass is more than 1.4 times more massive than the Sun, it undergoes a gravitational collapse into a Neutron
Star
• Electrons fall into nuclei (protons) e + p +  n o + n
(neutrino)
• Gravitational collapse may continue; massive stars end up as neutron stars or black holes after supernova explosion

• Big Bang nucleosynthesis produced mainly: ~90% H, ~8% He (by number)
 primordial H, He 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