Black Hole
A black hole is an astronomical object whose gravity is so strong that nothing—even light—can escape from within its event horizon. It forms when a massive star’s core collapses or through other processes, and may have an accretion disk of infalling matter that emits radiation. Supermassive black holes at galaxy centers influence stellar orbits, and mergers produce gravitational waves.
Source: nasa.gov
APODs including "Black Hole"
Chandra Resolves the Hard X-Ray Background
14/01/2000
It is everywhere but nobody knew why. In every direction at all times, the sky glows in X-rays. The X-ray background phenomenon was discovered over 35 years ago, soon after the first X-ray satellites were launched, and has since gone unexplained. Yesterday results were released using data from the recently launched Chandra X-Ray Observatory that appears to have resolved much of this mystery. The above photograph shows that about 80 percent of the apparently diffuse hard X-ray background can be resolved into very many very faint sources. The new question is now what are these sources? Early speculation, much of which predates these observations, holds that many of these sources are the active centers of distant galaxies, probably involving massive black holes. Still other sources may be of origins currently unknown.