Lecture 34 Stellar Evolution and Death (High Mass Stars)

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Lecture 34Stellar Evolution and Death (High Mass
Stars)
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High Mass Stars (Mgt5 M?)

• High mass stars have
• More mass
• Greater gravity
• Higher temperatures and pressures in the core.
• Fusion reactions do not stop with Helium burning
  in the core as for smaller stars.

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• Star becomes giant as for small mass star.
• Helium burning ends in core.
• Core contracts.
• Temp in core increase.
• He shell burning begins.
• Core continues collapse.
• Pressure increases
• Carbon fuses into higher mass elements.
• Process continues as core runs out of fuel.

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• Fusion of different elements continues through
  neon, oxygen, silicon and finally iron.
• Note all fusion occurs only in the very core.

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• Star expands to become a Supergiant.
• Star moves back and forth on the HR diagram with
  each type of fusion.

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• Each stage of burning lasts for a shorter period
  of time

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Death of High Mass Star

• Iron builds up in the core.
• Iron cannot be fused and produce more energy.
• What keeps iron core from collapsing?
• First electron degeneracy

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• After core has a mass greater than 1.4 M?
  (Chandresekhar limit) the electron degeneracy is
  not strong enough.
• Electrons are forced to combine with the protons
  to create neutrons.
• Core collapses until pressure from physical
  force of neutrons bouncing against each other
  stops it.
• Core rebounds and runs into outer material which
  is still falling inward.

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Supernova

• Collision produces huge shock wave pushing all
  material outward in an immense explosion.
• Explosion can be as bright as an entire galaxy
  (billions of stars) for a few days
• Some energy creates elements heavier than iron
  Elements produced are distributed to the rest of
  the galaxy.

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Supernova 1987a
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Eta Carinae (100 Solar Masses)
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Types of Supernovae

• Type 2 supernova extremely high mass star
  becomes a supernova as previously described.

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• Type 1 supernova
• Need binary star system with a white dwarf and
  red giant orbiting each other
• Red giant expands until it starts to lose some of
  its material to the white dwarf
• If mass of white dwarf becomes greater than 1.4
  M? its core will collapse and create a supernova
  explosion.

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Supernova remnants

• Supernova leave a large shell of slowly expanding
  material around a central core.

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Star Clusters

• Open Clusters --loose clusters of 10-100 stars
• Globular Clusters -- Old, tightly bound group of
  100s or 1000s of stars
• Age of a cluster can be determined by looking at
  what point the stars turn off of the main
  sequence
• Age of Cluster Lifetime of star at turn-off
  point.

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• Young Cluster -- Hyades cluster

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• Old Cluster -- 47 Tucanae