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Stellar Fuel, Nuclear Energy and Elements

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

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Stellar Fuel, Nuclear Energy and Elements

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  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

  2. The Atomic and Sub-Atomic Zoo • Atom  protons, electrons neutrons • Atomic number (#protons) • Atomic weight (#protons+neutrons) • Hydrogen  1H1 • Deuterium  1H2 (heavy hydrogen) • Same element, different nuclei  isotopes • Nuclear reactions  energy

  3. Nuclear Fusion: H  Hep-p chain (T > 15 million K) neutrino Deuterium positron Gamma-rays electron P.S. No gamma rays produced in the p-p reaction itself

  4. Deuterium (Heavy Hydrogen) + Hydrogen  Light Helium-3 + gamma-rays (energy)

  5. Final Product: He-3  Ordinary He-4 + Energy Helium nucleus is called alpha (a)-particle

  6. Future: Sun The Red Giant • When MS star exhausts H in the core and becomes a Red Giant • Core becomes helium dominated H  He; contracts and heats • H-burning in outer shell; envelope expands and cools; • Helium Flash  helium burning: 3 He  C (triple-a nuclear fusion) • 4He2 + 4He2 + 4He2  12C6 + 2g • 4He2 + 12C6  16O8 • Helium burning  Carbon/Oxygen core

  7. Solar-type star

  8. Main Sequence Lifetime of Solar-type Star

  9. Evolution beyond the Red Giant • L does not increase at the onset of the He-flash itself since the central region of the core is quite opaque • The H-burning shell is slowly extinguished and L decreases, even as the star shrinks and temperature rises; the star moves leftward along a nearly Horizontal Branch on the H-R diagram • Luminosity rises again as the energy from the He-burning core of the RG rises to the surface • The star then resumes its climb up the H-R diagram along a second vertical branch – the Asymptotic Giant Branch (AGB)

  10. Low-Mass Stellar Evolution

  11. Evolution Beyond the AGB Phase He-burning via the triple-a fusion to carbon is highly temperature sensitive (T > 100 million K) The AGB star is unstable; radiation pressure from the interior push away the envelope – hot core separates from the envelope Hot core is mainly C-O (products of triple-alpha) Hot core is very luminous initially, but rapidly cools through a Planetary Nebula (PN) phase (NO relation to planets!) The PN C-O core surrounded by the brightly lit ejected envelope appears as a ‘ring’ The PN core cools and collapses to White Dwarf

  12. Central Star and Spherical Ejected Shell

  13. Cat’s Eye Planetary Nebula

  14. 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……………….

  15. The Supernova Onion

  16. High-Mass Stellar Death • 1.44 M(Sun)  Chandrashekhar Limit • If the WD mass is more than 1.44 times more massive than the Sun, it undergoes a gravitational Fe-core collapse into a Neutron Star • 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

  17. Pulsating Variable Luminosity Stars:Instability Strip on the HR Diagram Cepheid stars are “Standard Candles” Cepheids used to establish the cosmological distance scale

  18. Period-Luminosity Relation of Variable Stars: Apparent magnitude m vs. Period (days)

  19. Longer the period, more luminous the Cepheid star; Determine absolute luminosity M from period; Distance d from: m-M = 5 log d – 5

  20. Summary of Stellar Evolution

  21. Stellar Evolution – HR Diagram Low Mass Stars: Proto-star  MS  RG  AGB  Pne  WD High Mass Stars: MS  Variable Cepheids/ Supernovae/Black Holes MS – Main Sequence RG – Red Giant AGB – Asymptotic Giant Branch Pne – Planetary Nebulae WD – White Dwarf Sne – Supernovae

  22. Cosmic Abundances • 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

  23. Three Pillars of Big Bang Theory • Hubble’s Law  Redshift of galaxies • 2.73 K Cosmic Microwave Background  Remnant radiation from the Big Bang • Primordial and fixed ratio of H or D to He  90% to 8% by number N.B. Deuterium is an isotope of hydrogen, also called “heavy hydrogen”, with a neutron and proton in the nucleus and an electron

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