In space, there is no horizon, unless you are on or near a suitably large planet. The horizon for our purposes is defined by a horizontal line which is drawn at the height of our eyes. So as long as there is an observer, you might say that a horizon exists. But in the three-dimensional swirly mix of a nebula, there isn't any structure strong enough to provide a horizon by itself.
An accretion disk around a black hole or neutron star, though, is a good equivalent of a horizon, since it forms a horizontal plane in space that is easily seen. There is also the phenomenon of the "event horizon" which is found at the junction between "ordinary" space and the highly altered space of a black hole. This may be what you see in this picture. The very bright star at the upper center may be an unstable very large star, emitting a lot of radiation and gas and just about to collapse in a supernova or even into a black hole. You can then see the beginnings of an accretion disk forming.
Should you find yourself in such a situation, I suggest that you leave the area immediately so that you will not be obliterated in a nova blast or possibly even a supernova. Your spaceship's warp drive may not be able to move that fast, but your Imaginal Drive should clear both you and your spaceship out of danger in no time at all.
"Horizon in Space" is....uh,....acrylimaginal on eventhorizon board, relativistic dimensions, November 1991.