# Stargazing А neutron star and a photon sphere

1. Feb 22, 2017

### daggab

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?

2. Feb 22, 2017

### Buzz Bloom

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

3. Feb 22, 2017

### Jonathan Scott

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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