Today in Astronomy 241: the fate of massive stars

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Today in Astronomy 241 the fate of massive stars

• Todays reading Carroll and Ostlie pp. 517-533,
  on
• Neutron star and black hole formation
• Supernovae of type II
• Young supernova remnants, their light curves and
  energetics
• Explosive nucleosynthesis
• Before and after Supernova 1987A (David Malin,
  Anglo-Australian Observatory)

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Late stages of evolution of massive stars

• For massive stars (those with core masses gt
  2Msun)
• AGB evolution
• Repeated core collapse - fusion reignition -
  nuclear fuel exhaustion occurs, including silicon
  burning to produce iron-peak elements.
• Each of the successive fuel exhaustions is faster
  than the last. For a 20 M sun star,
• hydrogen burning (main sequence) lasts 107 years
• helium burning (horizontal branch) lasts 106
  years
• carbon burning lasts 300 years
• oxygen burning lasts 200 days
• silicon burning lasts 2 days !

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Death of massive stars

• The latter stages happen so fast that there isnt
  time for the star to eject much of its envelope.
• The mass of the iron core is greater than the
  maximum mass for white dwarf stars electron
  degeneracy pressure cannot support the weight of
  the core.
• No further heat can be generated from fusion
  reactions.
• Thus the star collapses under its weight. The
  core collapses fastest, and goes from a diameter
  of about 10000 km to 50 km in about a second.
• As the core collapses, it gets hot enough for
  highly endothermic reactions to take place -
  nuclear disintegration and reverse b-decay - and
  it turns into a neutron gas.

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Death of massive stars (continued)

• Neutrons are capable of exerting greater
  degeneracy pressure than electrons, due to their
  larger rest mass.
• When the core reaches dimensions of tens of
  kilometers, neutron degeneracy pressure sets
  in, stiffening the core tremendously.
• The outer parts of the star, imploding on the
  core at high speeds, bounce off the degenerate
  neutron gas.

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Death of massive stars (continued)

• The bounce results in the explosion of the star,
  giving it an enormous luminosity for a while (109
  Lsun or so, for a few weeks) - a Type II
  supernova.
• If the core is less massive than 2-3 Msun, the
  collapse of the core will stop - a neutron star
  is formed.
• If the core is more massive than that, it will
  collapse to form a black hole.

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Supernovae

• Enough free energy is present in the blast to
  enable explosive nucleosynthesis
• rapid formation of lots of carbon and oxygen,
  among others, from helium and hydrogen.
• endothermic nuclear reactions (r-process,
  s-process) that form elements heavier than iron.
• The exploding shell is kept hot for a while, due
  to radioactive decay of heavy elements
• Its very bright for a month or so, after which
  the luminosity declines at the rate of a
  magnitude every couple of months.

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Supernovae (continued)

• Expanding shell encounters interstellar gas,
  eventually forming a nebula of the type we call
  supernova remnants.

Crab Nebula (Messier 1) Exploded 1054 AD 2kpc
away Strongest X-ray/Gamma-ray source in the
sky Expanding 1500 km/s Powered by
pulsar/rotating neutron star (30 Hz
period) Probably Type II (massive progenitor
of 10 Msun)
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Todays in-class problems

• Recall that, for a stellar atmosphere, the
  optical depth to the photosphere isUse this,
  along with the equation of hydrostatic
  equilibrium, to show that the pressure at the
  photosphere is(Here is the mean opacity in
  the photosphere, generally very different from
  that deep in the interior.)

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Todays in-class problems (continued)

• The Hayashi track, or the red giant branch.
  Suppose a star is fully convectiveSuppose
  further that its atmospheric opacity is dominated
  by bound-free transitions, for which the
  mean opacity can be (very crudely) approximated
  byFind a relationship between luminosity and
  effective temperature for stars of this type, by
  setting the pressure and temperature in the
  interior (I) equal to those at the photosphere
  (O, e).

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Todays in-class problems (continued)

• Sketch your result from Part B, in the form of a
  plot of Sketch also,
  on the same plot, the hydrogen-burning main
  sequence. Comment on the comparison between the
  two, and on the nature of the stars interior on
  the Hayashi track or the red giant branch.

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Todays in-class problems

• Just Problem 13.11.
• Answers and/or secrets to problems done last
  time
• A. The density declines exponentially with
  distance away from the photosphere, as we have
  seen and calculated ourselves. The enclosed mass
  changes negligibly out side the photosphere even
  the radius changes little over the range in which
  the density is significant. This makes it easy to
  integrate the hydrostatic equilibrium equation

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Todays in-class problems (continued)

• B,C We get for the
  fully-convective stars on the Hayashi track or
  the giant branches, luminosity decreases sharply
  with increasing temperature, quite unlike stars
  on the hydrogen burning main sequence.

Hayashi track/ giant branches
L
M.S.