Star Remnants: White Dwarfs, Neutron Stars and Quirky Stars

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Star RemnantsWhite Dwarfs, Neutron Stars and
Quirky Stars

• By Anthony Baldridge

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Overview

• Final Evolution of Low mass and High mass stars.
• White Dwarfs
• Physics of degenerate matter
• Neutron Stars
• Pulsars
• Quark Stars

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The Death of Low Mass Stars

• Hydrogen fusion in the core.
• Shell Hydrogen fusion around the core.
• Star will expand and become a Giant.
• Hydrogen shell adds to the helium core and allows
  for the core to begin helium fusion.
• Helium fusion begins with helium flash in low
  mass stars.
• Carbon and Oxygen can not fuse so helium shell
  fusion begins.
• Star becomes a asymptotic branch giant (ABG).
• Helium fusion takes place as a helium shell
  flash. Causes material to be ejected in a
  Planetary Nebula.
• What remains is Called a White Dwarf Star

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The Death of High Mass Stars

• Larger stars go up to helium fusion, but
  gravitational collapse can allow the carbon in
  the core to fuse.
• This can occur for massive stars until the core
  is composed of Iron.
• Fusion stops at iron because no more energy can
  be taken from the nucleus.
• As Layers around the core fuse the iron core
  collapses.
• The iron nuclei break apart in a process known as
  photodisintegration.
• Electrons are forced to combine with protons.
• When the core reaches nuclear density the core
  stops collapsing and pushes back in a process
  called core bounce.
• This core bounce forms a shockwave and causes the
  outer layers of the star to be ejected in what is
  called a supernova.
• The remaining core is either a neutron star or a
  black hole.

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White Dwarfs

• Mass ranges up to around 1.4 solar masses.
• Electron degeneracy pressure has stopped the
  collapse of the core.
• These star remnants are roughly the size of the
  earth.
• There are several types of white dwarfs
• DA white dwarfs show only hydrogen absorption
  lines in their spectra .
• DB white dwarfs hydrogen lines are absent.
• DC white dwarfs show no lines at all.

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White Dwarfs cont

• Most are composed of completely ionized carbon
  and oxygen nuclei.
• Low mass helium dwarfs may exist
• Oxygen-neon-magnesium white dwarfs have been
  detected.
• When white dwarfs accumulate matter on their
  surfaces and cause the core to collapse, fusion
  of the core can occur creating a type 1a
  supernova causing the dwarf to explode.

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Degenerate Matter

• At zero temperature all of the lower energy
  states and none of the higher energy states are
  occupied. A fermion gas of this nature is said
  to be completely degenerate.
• We can derive a condition that allows for
  degeneracy.

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Degenerate Matter

• We can use two important ideas from quantum
  mechanics to estimate the electron degeneracy
  pressure
• The Pauli exclusion principle, which allows at
  most one electron in each quantum state.
• Heisenbergs uncertainty principle.
• The electron degeneracy pressure is the reason
  that a white dwarf is in hydrostatic equilibrium.
  

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Neutron Stars

• Mass ranges from between 1.4 and 3 solar masses.
• Neutron degeneracy pressure is responsible for
  stopping the further collapse of the star.
• Neutron stars are roughly between 10 and 15 km in
  radius.
• This stellar remnant has an average density
  greater then the typical density of an atomic
  nucleus.

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Neutron Stars

• For a 1.4 solar mass neutron star of radius 10 km
  the acceleration due to gravity at the surface is
  190 billion times the acceleration near Earths
  surface.
• Normally, isolated neutrons decay into protons by
  standard beta decay with a half-life of 11 min.
  Since there is complete electron degeneracy the
  electrons have no place to go so this does not
  happen.

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Neutron Star Model

• The outer crust consists of heavy nuclei in the
  form of a fluid or a solid lattice and
  relativistic degenerate electrons. Iron near the
  surface.
• The inner crust has a lattice of nuclei, a
  superfluid of free neutrons, and relativistic
  deg. Electrons
• The interior is primarily superfluid neutrons.
• There may be a solid core consisting of pions or
  other elementary particles.

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Pulsars

• Pulsars are neutron stars that are rotating very
  fast.

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Relative Size of Stars
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Quark Stars

• It has been believed that a supernova explosion
  leaves behind either a neutron star or a black
  hole.
• A new possibility has emerged , the Quark Star.
• These stars are composed of quarks, fundamental
  particles, rather than atoms or atomic nuclei.

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Quark Stars

• The two stars that were observed both act and
  look like normal neutron star but one is too
  small and the other is too cold to fit the
  current theory.
• If they turn out to be quark stars this would be
  very big discovery.
• It could also mean that the current theory on
  neutron stars is inaccurate or incomplete.

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Quark Stars

• Quark Confinement quarks can not exist in a free
  state.
• Neutron collapsing into a quark would produce a
  strange quark. These particles are not found
  inside normal atoms.
• Strange Quarks got name from being able to slow
  down decay time for some elementary particles.
• Quark stars would be roughly 2 3 times more
  dense than a neutron star.

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Sources

• http//www.wired.com/news/technology/0,1282,51943,
  00.html (quark or quirky neutron stars)
• http//www.astro.umd.edu/miller/nstar.html
  (intro to neutron stars)
• http//zebu.uoregon.edu/soper/NeutronStars/nsands
  n.html (neutron stars and supernovae)
• http//imagine.gsfc.nasa.gov/docs/science/know_l1/
  pulsars.html (neutron stars and pulsars)
• http//imagine.gsfc.nasa.gov/docs/science/know_l1/
  dwarfs.html (intro to white dwarfs)
• http//www.astrophysik.uni-kiel.de/arbeitsgruppen/
  agkoe/h-jordan-wz.html (more white dwarfs)
• Carroll, Bradley W. and Ostlie, Dale A.
  Introduction to Modern Astrophysics.
• Comins, Neil F. and Kaufmann, William J. III.
  Discovering the Universe. Fifth edition.