Q&A: Black Holes
NGC 720 photo on the left and why they are not on the photo on the right?
The point sources are X-ray binaries, that is, a double star system in
which a neutron star or black hole that is pulling matter away from a
nearby companion star. As the matter falls toward the neutron star or
black hole it is heated and emits X-rays. These systems are very bright
in X-rays, but can be quite dim in optical light.