А neutron star and a photon sphere

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SUMMARY

The discussion centers on the existence of photon spheres around neutron stars, specifically addressing the conditions under which they can exist. It references the Oppenheimer-Volkoff limit, which defines the maximum mass of a neutron star, and the Schwarzschild radius, indicating that neutron stars can theoretically possess a photon sphere if their radius is greater than the Schwarzschild radius but less than 1.5 times it. The conversation highlights that certain equations of state may permit neutron stars to be ultracompact, thus supporting the existence of photon spheres.

PREREQUISITES
  • Understanding of neutron star physics
  • Familiarity with the Oppenheimer-Volkoff limit
  • Knowledge of Schwarzschild radius
  • Basic grasp of equations of state in astrophysics
NEXT STEPS
  • Research the Oppenheimer-Volkoff limit and its implications for neutron star stability
  • Study the Schwarzschild radius and its relevance to black holes and neutron stars
  • Explore the concept of photon spheres in astrophysics
  • Investigate various equations of state for ultracompact neutron stars
USEFUL FOR

Astronomers, astrophysicists, and students studying compact objects in the universe, particularly those interested in neutron star properties and their theoretical limits.

daggab
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Wikipedia(https://en.m.wikipedia.org/wiki/Photon_sphere):
'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?
 
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daggab said:
We know Oppenheimer-Volkoff limit roughly.
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?

Regards,
Buzz
 
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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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