Supernovae

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Supernovae
Lab 9

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Lets go Supernova!

• Stars which are 8x massive than our Sun end
  their lives in a most spectacular way they go
  supernova!
• First, the star swells into a red supergiant
• Then core yields to gravity and begins shrinking
• As it shrinks, it grows hotter and denser

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Sequence of events

• Then a new series of nuclear reactions begin to
  occur....temporarily halting the collapse of the
  core
• When the core contains just iron, there is
  nothing left to fuse and fusion ceases
• In less than a second, the star begins the final
  phase of gravitational collapse
• The core temperature rises to over 100 billion
  degrees as the iron atoms are crushed together

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Core Collapse!

• The repulsive force between the nuclei overcomes
  the force of gravity
• This combination, a process called "electron
  capture", creates a neutron and releases a
  neutrino
• The neutrinos escape from the core, carrying away
  energy and further accelerating the collapse,
  which proceeds in milliseconds as the core
  detaches from the outer layers of the star
• This is called core collapse!
• So the core compresses, but then recoils

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Ka-boom!!!

• The energy of the recoil is transferred to the
  envelope of the star, which then explodes and
  produces a shock wave
• As the shock wave travels to the star's outer
  layers, the material is heated, fusing to form
  new elements and radioactive isotopes
• The shock then propels the matter out into space
• The material that is exploded away from the star
  is now known as a supernova remnant
• All that remains of the original star is a small,
  super-dense core composed almost entirely of
  neutrons -- a neutron star
• If the original star was very massive (15 or
  more times the mass of our Sun), even the
  neutrons cannot survive the core collapse...and a
  black hole forms

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Implosion to Explosion!

• Even neutrons sometimes fail depending on the
  mass of the star's core. When the collapse is
  abruptly stopped by the neutrons, matter bounces
  off the hard iron core, thus turning the
  implosion into an explosion ka-BOOM!!!

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Remnant of Kepler's Supernova, SN 1604
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Temps of Fusion

• Min temp required for the fusion of Hydrogen is 5
  million degrees
• More protons in nuclei require higher
  temperatures - to fuse Carbon requires a
  temperature of about 1 billion degrees
• Lighter elements release energy when they fuse
  and heavier elements release energy when they
  split
• As iron "ash" begins to accumulate in the core of
  the star, gravity pulls more and more mass into
  the area of fusion, which, in turn, goes through
  all of the steps of fusion Hydrogen ? helium by
  the proton chain, He? carbon by the triple a
  process, C and He combine into oxygen, O fuses
  into neon, Ne into magnesium, Mg into silicon and
  Si into iron

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The Crab Nebula is an expanding cloud of gas
created by the 1054 supernova
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(No Transcript)
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Animation of a supernova

• http//heasarc.gsfc.nasa.gov/docs/snr.html

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Types of Supernovae

• Two distinct types of supernovae -- those which
  occur for a single massive star and those which
  occur because of mass transfer onto a white dwarf
  in a binary system
• Difference between the two types lies only in
  what gets the process started toward the
  explosion

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Supernovae Classification based on H

• The presence or absence of a line from hydrogen
• If a supernova's spectrum contains a hydrogen
  line, it is classified Type II, otherwise it is
  Type I

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A White Dwarf Goes Thermonuclear Type I

• A white dwarf star in a binary star system will
  draw material off its companion star if they are
  close to each other
• Once the in-falling matter from the companion
  star cause the white dwarf to approach a mass of
  1.4 times that of the Sun (the Chandrasekhar
  limit), the pressure at the center increases so C
  and O nuclei to start to fuse uncontrollably
• This results in a thermonuclear detonation of the
  entire star
• Nothing is left behind, except whatever elements
  were left over from the white dwarf or forged in
  the supernova blast
• Among the new elements is radioactive nickel,
  which liberates huge amounts of energy, including
  visible light

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Type II

• A much larger star, however, has enough gravity
  needed to create T and P to cause the C in the
  core to fuse once the star contracts
• The cores of these massive stars become layered
  like onions as progressively heavier atomic
  nuclei build up at the center
• An outermost layer of H gas, sinks down on a
  layer of H fusing into He, the He sinks down into
  a layer of He fusing into C, and the C sinks down
  to fuse into heavier elements

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Type II contd

• These stars go through progressive stages where
  the core will shrink, atomic nuclei which were
  previously unfusable start fusing then the core
  springs back into equilibrium with gravity
• This causes them to be irregular variables -
  each new burst of fusion pushes elements out of
  the fusing core into the "stellar envelope
• This dims the star, and causes gravity to pull
  mass back into the fusing core and begin the
  cycle over again

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Type II subdivisions

• Type II supernovae can be further classified
  based on the shape of their light curves into
  Type II-P and Type II-L
• Type II-P reach a "plateau" in their light curve
  while II-Ls have a "linear" decrease in their
  light curve ("linear" in magnitude versus time,
  or exponential in luminosity versus time)
• This is due to differences in the envelope of the
  stars.
• II-Ps have a large H envelope that traps energy
  released in the form of gamma rays and releases
  it slowly, while II-Ls are believed to have much
  smaller envelopes converting less of the ? ray
  energy into visible light

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Subdivisions

• Subdivisions according to the presence of other
  lines and the shape of its light curve
• Type I
• - No hydrogen Balmer lines
• Type Ia
• - Si II line at 615.0 nm
• Type Ib
• - He I line at 587.6 nm
• Type Ic
• Weak or no He lines
• Type II
• - Has hydrogen Balmer lines
• Type II-P
• -Plateau
• Type II-L
• - Linear

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Type Ia

• Observations of Type Ia supernovae reveal a
  picture of the cosmic environment in the way that
  the width of tree ring growth indicates the
  Earth's climatic environment over time
• Unlike the other types of supernovae, Type Ia
  supernovae are generally found in all types of
  galaxies, including ellipticals.

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Hypernovae

• There has been some speculation that some
  exceptionally large stars may instead produce a
  "hypernova" when they die
• Here, the core of a very massive star collapses
  directly into a black hole and 2 extremely
  energetic jets of plasma are emitted from its
  rotational poles at nearly light speed.
• These jets emit intense ? rays

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Are We Made of Stardust?

• Yes! (recycled, that is)
• The common elements are made through nuclear
  fusion in the stable cores of stars
• Supernovae are not stable, so they can make heavy
  elements beyond iron that require more energy to
  form.
• These are the elements that make up stars,
  planets and everything on Earth -- including
  ourselves