Class 17: Stellar evolution, Part I

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Class 17 Stellar evolution, Part I

• Evolution of stars of various masses
• Red giants.
• Planetary nebulae.
• White dwarfs.
• Supernovae.
• Neutron stars.

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• HR diagram gives clues to stellar evolution.
• Main Sequence (MS)
• Consists of stars living out the normal part of
  their lives
• Stars on MS produce energy via steady hydrogen
  burning (i.e., converting hydrogen into helium).
• Stars of different mass lie at different points
  on the main sequence.
• Mass-luminosity relation L ? M4.
• Eventually, the hydrogen fuel runs out and the
  stars begin to die they then leave the main
  sequence

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Life of a 0.05 M? star

• Star forms from rotating collapsing gas cloud
  (recall formation of solar system in class 2).
• Core heats up to few million K.
• Trace deuterium burns to form helium.
• Thats it
• Temperature never gets high enough to initiate
  hydrogen burning.
• So never really becomes a proper star.
• Object becomes a brown dwarf.
• This is the case up to about 0.08 M?.

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Evolution of the Sun

• Same beginning cloud collapses.
• This time, core is hot enough to initiate
  hydrogen burning (p-p chain).
• Steady hydrogen burning for 10 billion years (5
  billion years more to go).
• Then run out of hydrogen in core.
• Nuclear reactions slow then stop.
• Core gradually collapses outer parts of Sun puff
  up tremendously becomes red giant.
• He burning starts (forming carbon).

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• Once core helium is exhausted, the red giant
  blows off its outer layers into space.
• Produces a planetary nebula.
• Only the core of the star is left becomes a
  white dwarf. Cools forever like a dying ember.

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Evolution of a 10 M? star

• Star forms as before.
• H-burning much faster (CNO cycle)
• Only lasts a few million years.
• When H is exhausted, core contracts, gets hot
  enough for helium-burning (makes carbon).
• When He exhausted, core contracts and gets hot
  enough for carbon burning.
• And so on until the core is turned into iron
  (the most stable element).

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• Get shell or onion structure.
• No more energy available when core becomes iron.
• Catastrophic core collapse
• Core turns into neutron star
• Rest of star ejected in a supernova explosion.

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Core-collapse (type-II) supernovae

• Very powerful explosion
• 1044 J released as radiation (VERY bright!).
• 100? more released in a neutrino pulse.

SN1987A (LMC)
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Cas-A remnant
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Neutron stars

• The remnant of a SN explosion
• Typical mass of 1.5 M? but radius only 10 km!
• Made of densely packed neutrons (1018 kg/m3) a
  teaspoonful would weigh a million tons!
• Extreme properties
• Very strong gravity on surface.
• Very strong magnetic fields on surface.
• Can spin very quickly (hundreds of times per
  second) gives rise to pulsars.

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