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Stellar Fuel, Nuclear Energy and Elements

Explore the fascinating world of nuclear fusion, stellar evolution, and cosmic abundances. Discover how stars shine and elements are formed through nuclear reactions. Unravel the mysteries of the Atomic Zoo and learn about the life cycles of stars, from main sequence to supernovae.

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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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