The eventual fate of our Sun is to

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• The eventual fate of our Sun is to
• a) collapse into a black hole.
• b) form a neutron star.
• c) become a steadily cooling white dwarf.
• d) explode as a type I supernova leaving no
  remnant.

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Stellar evolution is controlled by just a few
knobs and a little physics

• Physics
• Ideal Gas Law
• Blackbody radiation
• Knobs
• Temperature in the core
• Composition of the core
• Location of nuclear fusion
• State of matter in the core

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Physics Ideal Gas Law

• If you compress normal gas, it heats up
• If you heat up normal gas, it expands

In general, this leads to a safety valve for
evolution
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Physics Blackbody Radiation

• The inner core of a star is where energy is made.
  This energy heats up the outer envelope of the
  star, making it a blackbody

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Hydrogen-burning pathways

• At low temperatures (5-15 million K), fusion
  follows the p-p cycle
• The energetic radiation from these nuclear
  reactions exerts pressure outward, counteracting
  gravity and keeping star in equilibrium (this is
  why its a sphere!)

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Knobs Temperature in the core

• Temperature translates into how quickly atoms are
  moving, which translates into how often and how
  hard they collide, which translates into how many
  fusion reactions occur per second.
• Core temperature controls the rate of nuclear
  fusion
• Core temperature controls the kind of fusion

What controls the core temperature?
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Knobs Composition of Core

• Hydrogen starts burning around 5 million K
• Helium starts burning around 100 million K
• Carbon starts burning around 600 million K
• Neon starts burning around 1 billion K
• Oxygen starts burning around 1.5 billion K
• Silicon starts burning around 2.7 billion K
• - Making heavier and heavier elements requires
  hotter and hotter core temperatures!

What if you have a lot of He in a core at 10
million K?
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Knobs Location of fusion

• Main sequence stars have a core of hydrogen
  burning
• As more helium is made, it sinks to the center
  and sits there.
• When enough helium is present, the core is inert
  helium and hydrogen burning occurs in a shell
  around the core.

What happens in this He core?
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The Red Giant star increases in size over 100x
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At the top of the Asymptotic Giant branch, the
star looks like
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• Eventually helium burning near the core produces
  thermal pulses which force a lot of the envelope
  off the star, sending the gas out into the space
  around the star.
• What is left is a very hot, exposed, degenerate
  carbon/oxygen core called a white dwarf, which
  ionizes the gas around the star creating a
  planetary nebula.