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 invisible 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.

The bright patches in this X-ray image of the Sun’s outer atmosphere, or corona, are X-rays emitted from extremely hot gases.

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 had no inkling of the virtual sea of X rays originating from beyond our star. As Herbert Friedman puts it, “There was no hint of the enormous portent [clue or foreshadowing] for the future of astronomy. Astronomers remained oblivious to the potential of X-ray astronomy.”

The first confirmation that the Sun does indeed produce X rays came in 1948, when American scientists attached detection instruments to rockets developed by the Germans in World War II. Although the instruments did detect solar X rays, they showed that the Sun is not a strong X-ray source; the volume of X rays it produces is only about one-millionth that of the visible light it emits. Thus, it appeared to scientists that stars in general would not prove to be important sources of X rays.

Then, in June 1962, American scientists launched a rocket carrying instruments designed to detect possible radiation coming from the surface of the Moon. The experiment found no radiation on the

Launched in July 1999, the Chandra X-ray Observatory has discovered thousands of previously unknown X-ray sources.

Moon. But it did quite unexpectedly detect a powerful X-ray source located beyond the solar system. Later rocket launches confirmed the existence of this source and pinpointed it in the constellation of Scorpius, the scorpion. Astronomers named it Scorpius X-1, or Sco X-1 for short.

After that, other distant, very powerful X-ray sources were discovered, especially following the December 1970 launch of a more sophisticated X-ray satellite. This device, named Uhuru, and several even more sensitive probes launched later, revealed that the sky is literally filled with X-ray sources; some scientists began to refer to this previously unknown phenomenon as the “X-ray universe.”