Stellar Fuel, Nuclear Energy and Elements

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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

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The Atomic and Sub-Atomic Zoo

• Atom ? protons, electrons neutrons
• Atomic number (protons)
• Atomic weight (protonsneutrons)
• Hydrogen ? 1H1
• Deuterium ? 1H2 (heavy hydrogen)
• Same element, different nuclei ? isotopes
• Nuclear reactions ? energy

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Nuclear Fusion H ? Hep-p chain (T gt 15 million
K)
neutrino
Deuterium
positron
Gamma-rays
electron
P.S. No gamma rays produced in the p-p reaction
itself
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Deuterium (Heavy Hydrogen) Hydrogen ? Light
Helium-3 gamma-rays (energy)
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Final Product He-3 ? Ordinary He-4 Energy
Helium nucleus is called alpha (a)-particle
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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

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Solar-type star
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Main Sequence Lifetime of Solar-type Star
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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)

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Low-Mass Stellar Evolution
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Evolution Beyond the AGB Phase

• He-burning via the triple-a fusion to carbon is
  highly temperature sensitive (T gt 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

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Central Star and Spherical Ejected Shell
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Cats Eye Planetary Nebula
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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.

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The Supernova Onion Rings (or Rings of Tree Stem)
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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

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Pulsating Variable Luminosity StarsInstability
Strip on the HR Diagram
Cepheid stars are Standard Candles
Cepheids used to establish the cosmological
distance scale
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Period-Luminosity Relation of Variable
Stars Apparent magnitude m vs. Period (days)
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Longer the period, more luminous the Cepheid
star Determine absolute luminosity M from
period Distance d from m-M 5 log d 5
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Summary of Stellar Evolution
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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
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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

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Three Pillars of Big Bang Theory

• Hubbles 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