Reminder: the main sequence is a sequence in mass.  The lifetime of
  a star is set by the amount of fuel it has in its core, and the rate
  at which is it using it up.  

  So if a star ends its main sequence existence when it burns up all
  the hydrogen in the core, what happens next? 

  Consider a star as massive as the Sun.  When all the hydrogen in the
  core has been fused into helium, the furnace goes out.  The star
  collapses under gravity, becomes more dense and heats up, enough so
  that hydrogen fuses to helium in a shell around the core.  This puts
  out so much energy that the outer parts of the star blow up like a
  balloon: the star becomes enormous, very luminous, and (as it turns
  out), relatively cool on the surface: it is a red giant.  (Much of
  this story was set by Princeton's Martin Schwarzschild, in the
  1950's).  

After a star like the Sun become a red giant, with hydrogen burning to
helium in a shell, the helium in the core compresses more under
gravity, and new nuclear reactions take place, fusing helium into
carbon and oxygen.  The star enjoys a 'second main sequence' during
this phase.  When all the helium in the core is depleted, the star
collapses again, and shell burning of helium to carbon/oxygen happens
around the core; the star becomes even more luminous, and balloons
even larger to become a red supergiant, with a radius up to 10 AU.  

  This process does not continue.  The outer parts of the star are
very low density, and held to the star by gravity only tenuously.
With a bit more energy output from the nuclear reactions in the core,
the outer parts detach completely, and expand out in what is called a
'planetary nebula'.  The core of the star (now essentially pure
carbon-oxygen) is left behind: very small, very hot, and very dense;
this is a white dwarf.  Its photons excite the now expanding gas that
was the outer part of the star, and cause it to glow.  The planetary
nebula lasts only a relative short time (less than a million years),
before dissipating.  The white dwarf sits there, and very slowly cools
off over billions of years.  Indeed, the oldest white dwarfs are the
coolest, as old as 12-13 billion years old.

  For stars significantly more massive than the Sun, say, 10 times as
massive, the story is interestingly different.  First, we've already
seen that such a star has a main sequence lifetime *much* shorter than
the Sun, perhaps a few tens of millions of years.  They then go
through the red giant and red supergiant cycles as we saw above.  For
a very massive star, the core will be very massive, and have a
particularly large gravitational field.  Gravity will win again, and
the core can collapse further.  When it does so, it heats up even more
(now temperatures exceeding 10^9 K!), and further nuclear reactions
take place, C and O fusing in a complicated series of reactions that
yield neon, magnesium silicon, sulphur: the star develops an 'onion'
structure, with shells of different elements all burning away.  How
long can this continue?  Not forever; you can't continue to squeeze
energy out of atoms by fusing them.  300 years to burn carbon, 200
days for burning oxygen, and only 2 days for the elements heavier than
silicon.

  In particular, if we make a graph of the amount of energy released
in nuclear fusion, as a function of the atomic number of the atoms
involved, we find we get the most out when hydrogen fuses to helium,
somewhat less when helium goes to carbon and oxygen, less still with
the synthesis of neon... The buck stops with iron (atomic number 26);
you can't fuse iron with anything to get energy out.  

  So Iron is the most stable of the nuclei of atoms.  If you start
high up in the periodic table, say with Uranium or Plutonium, energy
is released when the nuclei split, or "fission".  This is what we
usually refer to as 'radioactivity'.  Starting there, you continue to
get energy out as you fission, until you reach iron again.

So once the core of the onion is pure iron, something dramatic has to
happen.   The energy source in the core is now gone, and the star
collapses catastrophically due to gravity.  Note, however, that only
the core is inert iron; all the onion rings around it are made of
material which can still undergo nuclear fusion and release energy.  
And indeed they do, explosively, as they are compressed and
dramatically heated as the star collapses.  This is a *dramatic*
explosion; it blows the star to smithereens in a Supernova.  Indeed,
lots of energy is available, and this energy drives nuclear reactions
that make all elements beyond Iron on the periodic table.

  This explosion spews the contents of the interior of the star,
including all those elements that were synthesized, back into the
Galaxy.  This material then gets incorporated into the next generation
of stars born.  

 Thus essentially all elements on the periodic table heavier than
helium are created in the cores of stars, and those heavier than Iron
were specifically formed in the supernova explosion itself.  We are
made of carbon, nitrogen, oxygen, potassium...  Our Earth is
principally Silicon, Nickel, Iron... Every atom was synthesized in the
cores of stars.

  

Notes for Lecture 13

© Copyright 2009 Anatoly Spitkovsky & Michael Strauss