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Stargazing А neutron star and a photon sphere

  1. Feb 22, 2017 #1
    'This equation entails that photon spheres can only exist in the space surrounding an extremely compact object (a black hole or possibly a neutron star)'
    But how can a neutron star?
    I have a doubt because
    1. When а mass of а star increases, radius decreases.
    2. The radius of the most massive NS exceeds the schwarzschild radius.
    3. We know Oppenheimer-Volkoff limit roughly. It is not known exactly.
    I suppose that the existence of such neutron stars is questionable.
    Is it correct?
  2. jcsd
  3. Feb 22, 2017 #2

    Buzz Bloom

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    Gold Member

    Hi daggab:

    The Wikipedia article text indicates that at a neutron start of radius r,
    rs < r < (3/2) rs,​
    could have a photon sphere. Are you saying that the Oppenheimer-Volkoff limit is inconsistent with this?

  4. Feb 22, 2017 #3

    Jonathan Scott

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    Gold Member

    If you follow the link to reference [1] in that Wikipedia article, it links to a paper "Properties of ultracompact neutron stars" which mentions that certain equations of state would allow a neutron star to be less than 1.5 times the Schwarzschild radius and hence have a photon sphere.
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