Stellar Fuel, Nuclear Energy and Elements 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 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  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

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? 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 4 He He He 2  12 C    He C 6  16 O 8 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: 12 C O 8  28 Si Si Si 14  56 Ni 28  56 Fe 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 +  n o +  (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