The X-Ray Universe

One of these important discoveries that eventually led to the detection of black holes was that the universe is virtually alive with X rays. These are an in- visible form of electromagnetic radiation that is much more energetic and penetrating than visible light. First discovered in laboratory experiments in 1895 by German physicist Wilhelm Roentgen, X rays are familiar to most people from their use in medicine. When a technician aims a beam of X rays at the human body, most of the rays pass through the body and strike a photographic plate, which renders a ghostly image of the patient's insides.

In the years that followed Roentgen's discovery, as artificially produced X rays became increasingly common medical tools, a few astronomers began to suspect that the Sun might produce this energetic radiation naturally. But they...