Transcript Slide 1
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 • • • • • • • 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? • • • • 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 + 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