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Matter at the core of a neutron star.

  1. May 7, 2012 #1
    Hello. I am wondering what are the properties of matter at the core of a neutron star. I read that it could be quark matter of strange matter, but overall uncertain. How can strange matter form if strange quarks decay very quickly into up quarks (the state of matter would last very shortly), and where would the strange quarks come from? Why do they start to matter at extremely high temperatures and pressures?

    Is it possible to experimentally detect and study the core of a neutron star to be sure?

    Thank you in advance.
  2. jcsd
  3. May 7, 2012 #2


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    The pressures inside the core of extremely massive objects such as Neutron stars cause matter to exhibit strange properties seen nowhere else. In the initial collapse, it is energetically more favorable for electrons to combine with protons than to remain as they are. Per the uncertainty principle, as the degenerate matter is compressed more and more, electrons have to attain a higher and higher momentum since their location is confined tighter and tighter.

    Neutrons normally decay into protons when free, but the enormous pressures make it so that it would take MORE energy to decay than to simply stay as they are, thus they do not decay. (Because of the above)

    It is the same for other exotic particles. The pressures are so high that normally unstable states can be favored over others that are normally stable.
  4. May 7, 2012 #3
    Here is a cross section of a neutron star's interior as provided by the website cited below.

    1. Outer crust 0.3-0.5 km ions, electrons
    2. Inner Crust 1-2 km electrons, neutrons, nuclei
    3. Outer core ~ 9 km neutron-proton Fermi liquid few% electron Fermi gas
    4. Inner core 0-3 km quark gluon plasma?

    This is inferred from mathematical models.

    The following Wikki article expands on this.

    Neutron Star

    Are there other ways to determine what lies at the subsurface of Neutron stars? There is asteroseismology, a method that is based on the star's occilations. This is comparable to the way we study the earth's interior by examining earthquake seismic data but in the case the neutron star it depends on the analysis of the light wavelengths that vary when such star seismic events occur.

    From Wikipedia, the free encyclopedia
    Last edited: May 7, 2012
  5. May 8, 2012 #4


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    The pressure at the core of a neutron star is less than the neutron degeneracy pressue, unless our models are wrong. If there is a further degeneracy limit - like a quark degeneracy pressure - this is the right place to look.
  6. May 8, 2012 #5
    Asteroseismology is one way, especially with magnetars. Magnetars can flare up in high energy emission. These guys are called anomalous X-ray pulsars and soft-gamma repeaters. This emission is thought to originate from star quakes: the magnetic field is so strong that twists and torques in the magnetic field can crack the crust as the magnetic field tries to get back to its original configuration.

    Simply measuring the mass and radius of neutron stars would also constrain the behavior of matter inside the core neutron stars. For instance, figure 3 on page 10 of "Shapiro delay measurement of a two solar mass neutron star" shows a plot of how different models for how matter behaves inside neutron stars as we vary the mass and radius.

    There are various ways of measuring the mass and radius. This paper used Shapiro delay, a particular general relativistic effect. Attempts have been made to use thermonuclear X-ray bursts to measure both the mass and radius.
  7. May 10, 2012 #6
    The quark matter theory fell out of favor when a neutron star was measured to have mass of 1.97 AU.

    As to what exactly is in the central core, no one knows. It is difficult to measure the radius of the star precisely, and it makes a big difference. I'll put my money on a central core containing viscous hyperons, each of which contains a strange quark. There could be kaons in there too.

    As to actual experiments, that's tough. CERN produces high-density matter, but it is much hotter than a typical neutron star.
  8. May 10, 2012 #7


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    Not exactly, what fell out of favor was the softer equations of state for condensed matter.
  9. May 10, 2012 #8

    Neutron star cores are composed of extremely hardened high density degenerate neutron matter on the order of the density of a neutron or an atomic nucleus.

    High temperatures and pressures is when nuclear matter starts to become degenerate and the stellar core properties start to become dominated by the core neutron degeneracy pressure.

    It is improbable that the core would be composed of quark-gluon plasma because the core temperature is not high enough and this would imply a softened core Equation of State, and such states with exotic particles has been ruled out by indirect observations of the Equation of State.

    Stellar models based upon General Relativity are constructed then compared to observation, typically if you know parameters such as total mass and total radius, the remaining parameters such as core pressure and core density fall into place.

    Neutron star - Structure - Wikipedia
    Degenerate matter - Wikipedia
  10. May 10, 2012 #9
    At high energies (temperatures and pressures), it's possible for some "normal matter" quarks (I don't remember which) to decay into Strange Quarks. So that answers that question.

    I think it's believed that the Strange Quarks will become more stable when bound into strangelets?

    And, again, this is all still pretty theoretical.
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