Re: what is a photon sphere?

A "photon sphere" is defined by the set
of points surrounding a black whole where light which
is emitted tangentially (at right angles) to the
surface of such a sphere, would travel in a circle
around the black hole, forever.  If the light is
emitted just slightly toward the hole, it will
spiral in.  If just slightly outward, it will escape.
The radius of the photon sphere is three-halves
the size of the event horizon ("Schwarzchild radius"),
where no light can escape.

This is not to be confused with the term "photosphere",
which denotes the region on the surface of a star where
most of the visible light is generated.