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