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 Hep-p chain

Deuterium

Gamma-rays

neutrino

electron

positron

P.S. No gamma rays produced in the p-p reaction itself

The Atomic and Sub-Atomic Zoo

• 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 ZonesCore, Radiative, Convective

How long with the Sun last?

• 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 + 2He2 + 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 SequenceRG – Red Giant AGB – Asymptotic Giant BranchPne – Planetary NebulaeWD – 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:

12C6 + 16O8 28Si14

28Si14 + 28Si14 56Ni28 56Fe26

• 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 + (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