Q&A: Galaxies, Galaxy Clusters, AGN, and Quasars

Q:
Hello, I am quite interested in the X-ray photo of NGC 720 that is newly posted. Can you tell me what the 15 or so points of light are that show on the photo on the left and why they are not on the photo on the right?

A:
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.