The End of a High Mass Star

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The End of a High Mass Star
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Fusion of Heavy Elements

• Very massive stars can fuse larger and larger
  elements
• Hydrogen to Helium
• Helium to Carbon
• Elements after carbon are created by Helium
  capture
• O-16, Ne-20, Mg-24, Si-28, S-32, Ar-36, Ca-40,
  Ti-44, Cr-48, Fe-52, Ni-56
• Ni-56 is unstable and decays to Fe-56

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The Iron Core

• Iron-56 is the most stable of all nuclei
• Elements smaller than Fe-56 release energy
  through FUSION
• Elements larger than Fe-56 release energy through
  FISSION
• Once Fe-56 is formed, the energy production of
  the star completely stops

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Stellar Nucleosynthesis

• Hydrogen and Helium are primordial elements
• They have existed since the beginning of the
  universe
• Elements larger than these are created in the
  cores of stars and released when they die
• Fusion creates C and others (4, 4, etc)
• Elements and isotopes between H and Fe are
  created by decay, proton or neutron capture, or
  side reactions but are less common

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Elements larger than Iron

• Fusion is out of the question, because too much
  energy is required
• Heavier elements form from neutron capture
  followed by decay (usually beta decay)
• Called the s-process (the s is for slow)
• Large quantities are not produced
• Most of this is done during the impending
  explosion

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The Fate of a High Mass Star

• Once fusion produces an iron core, energy
  production ceases and the core contracts
• The heat caused by this contraction is so great
  (10 billion K) that photons have enough energy to
  split the iron back down until there are only
  protons, neutrons, and electrons
• Called photodisintegration
• The process absorbs some of the energy and causes
  the core to contract even further

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Neutronization

• Protons and Electrons are crushed together
• Forms a neutron and a neutrino
• The neutrino carries away a lot of energy,
  further reducing the cores support
• The disappearance of electrons eliminates the
  electron degeneracy pressure, so the core can be
  compressed all until the neutrons come in contact
  with each other
• Density of 1 billion kg per cubic centimeter

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The Bounce

• By the time the collapse is halted by the neutron
  degeneracy pressure, it has overshot its
  equilibrium
• The core bounces back
• Entire process from beginning of collapse to
  bounce lasts just seconds
• An enormously energetic shockwave travels out,
  blasting all layers into space
• Core-Collapse Supernova

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Novae vs. Supernovae

• Appear similar
• Both increase luminosity significantly
• Very different
• Supernovae are WAY brighter
• Novae arise from accretion around a white dwarf
  in a binary system
• Supernovae are the explosions of high mass stars

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Types of Supernovae

• Spectroscopy indicates that there are two
  distinct types of supernovae
• Type I is lacking in hydrogen
• Type II is hydrogen rich
• The two types even remain bright for different
  lengths of time
• Type II supernovae are caused by the
  implosion-explosion of a massive star

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Type I Supernovae

• In a binary system with a white dwarf, accretion
  can cause novae
• With each nova, not ALL of the material is
  ejected
• Can build up over time
• If the total mass of the white dwarf exceeds
  about 1.4 solar masses (the Chandrasekhar Mass),
  the degeneracy pressure is overcome and the star
  collapses
• Carbon begins to fuse everywhere at once and the
  white dwarf explodes
• Explains why the spectra are lacking in hydrogen

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Supernovae

• The probability of witnessing a supernova is low,
  but there are examples throughout history
• In 1054 AD Chinese astronomers recorded a bright
  star that could be seen during the day for nearly
  a month
• Tycho observed one in 1572
• Kepler observed one in 1604
• Hundreds have been observed in other galaxies
  since then

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Supernova Remnants

• We can see the remains of the star after it
  explodes
• The hydrogen envelope is ejected and excited by
  the energy produced
• A new kind of nebula

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The Crab Nebula Supernova of 1054 AD
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Tychos Star
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Vela Supernova Remnant
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Remnant N 63A
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Keplers Supernova
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N49 in the Large Magellanic Cloud
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Cassiopeia A
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Supernova 1987A In the Large Magellanic Cloud