Stellar%20Nucleosynthesis

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Stellar Nucleosynthesis

• Formation of heavier elements inside stars

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How old are your atoms?
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• Almost as old as the universe

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92 Natural Elements

• The periodic chart contains 92 naturally-occurring
  elements in the universe
• Elements 1 (H) to 26 (FE, iron) are synthesized
  in the core of normal stars like the Sun

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Most Abundant Element in the Universe

• Hydrogen, H, comprises 99 of the visible
  universe
• Hydrogen is the lightest element
• H has one proton

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Elements Heavier than 26

• The heavier elements are much less abundant than
  H
• Elements heavier than 26 are formed in supernova
  explosions

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• Supernovae are vast explosions in which a whole
  star is blown up. They are mostly seen in distant
  galaxies as new' stars appearing close to the
  galaxy of which they are members. They are
  extremely bright, rivaling, for a few days, the
  combined light output of all the rest of the
  stars in the galaxy.

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• A supernova is one of the most energetic
  explosive events known to man. Supernovas occur
  when a stars nuclear fuel is exhausted and thus
  no longer being supported by the pressure from
  the release of nuclear energy.

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• If the star is really massive, then its middle
  will collapse and release a lot of energy. Many
  supernovae have been seen in galaxies close by.
  They are considered rare occurrences in our own
  galaxy.

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• If the star is particularly massive, then its
  core will collapse and in so doing will release a
  huge amount of energy. This will cause a blast
  wave that ejects the star's envelope into
  interstellar space.

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• The result of the collapse may be, in some cases,
  a rapidly rotating neutron star that can be
  observed many years later as a radio pulsar.

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• Supernovae emit huge amounts of various types of
  radiation X-rays, ultraviolet, infrared, gamma
  rays, neutrinos, cosmic rays and radio waves.

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Type II Supernova Explosions

• A progenitor star that will explode must have 8
  or more solar masses
• Sun can never go supernova because it has only 1
  solar mass and is thus not massive enough

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Frequency of Supernovae

• About once per second in the universe
• Last one visible to the naked eye was in 1987 in
  the LMC (Large Magellanic Cloud)
• No supernova has been seen in our Galaxy since
  Kepler's (1604).

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Force of a Supernova

• The tremendous force of the explosion when a star
  goes supernova forces protons together and also
  neutron capture
• Protons have a like charge ()() and do not want
  to be next to one another
• Once the protons are forced together, the strong
  nuclear force (SNF) takes over and glues the
  protons together with gluons.

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Balance of Gravity and Nuclear Pressure

• A star like the sun assumes a natural spherical
  shape due to the 2 main forces that act upon it.
• The gravitational force due to the 8 solar masses
  tries to collapse the star while
• The tremendous pressure from the nuclear reaction
  occurring at the core of the star tries to expand
  the star

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Start of a Supernova

• Fuel in the core becomes Iron.
• Iron cannot produce fusion energy
• Fusion stops (Proton-proton chain)
• Nuclear pressure fails
• Gravity wins and star collapses
• Mass falls into the core crushing the electrons
  into the protons forming neutrons

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Iron Core Collapse

• It takes about 1 second for the iron core to
  collapse

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Rebound

• When the mass reaches the center and after it
  forms the neutron star, the mass rebounds and the
  supernova explosion ensues
• A nebula forms around the neutron star
• Crab nebula from the 1054 supernova
• A teaspoon of neutron star weighs a billion tons!

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If the Star is a Super Massive Star

• Then a black hole forms instead of the neutron
  star

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Elements in your Body

• Of the elements in your body, the only one not
  formed by stellar nucleosynthesis is H

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• Dont really want a supernova to occur too close
  to earth
• Radiation equivalent to 1 billion suns

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Type I Supernova

• Mass transfer to a white dwarf from a giant
  primary

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