Astronomy has traditionally played a major role in the advance of knowledge in the physical sciences. From Newton through Einstein up to the present, the contributions of astronomy to the development of such fields as mechanics, gravitation, relativity, atomic physics, nuclear physics, and elementary particle physics have been important and at times crucial. In many cases astronomical observations provide the only means for studying phenomena occurring in matter under the most extreme conditions of temperature and density. In addition, every so often astronomical discoveries are made that are so surprising and enigmatic that they serve as a stimulus for the further development of a particular branch of physics and sometimes lead to completely new theories. In the past two decades a spectacular series of discoveries of this type has occurred, primarily as a result of the development of techniques for exploring regions of the electromagnetic spectrum outside the "optical window."
Light, or optical radiation, is part of a large family of radiation called electromagnetic radiation. This radiation is usually produced when electrons, the tiny charges on the outer parts of atoms, undergo rapid changes in their state of motion. These rapid changes or accelerations, which can be thought of as vibrations, produce bundles of energy called photons that move away from the electrons at the speed of light. (All electromagnetic radiations move at the same speed in a vacuum.) In general, the more rapid the vibration or acceleration, the more energetic the photon. For example, to produce an X-ray photon requires a vibration several hundred to a thousand times more rapid than for an optical photon, which in turn requires a vibration millions of times more rapid than necessary for the production of radio photons.
Streams of photons behave in many ways like the water waves you can set into motion by moving your hand rapidly back and forth in a tub of water. Like water waves, electromagnetic waves have wavelengths and frequencies that depend on how rapidly the electrons producing the waves vibrate A high frequency wave is composed of high energy photons, a low frequency wave of low energy photons.
Electromagnetic waves come in all wavelengths, but our eyes are sensitive to only a very small portion of the total range or spectrum. To the long wavelength (low frequency) side of the optical portion of the spectrum we encounter the infrared, microwave, and radio radiations, in order of increasing wavelength (decreasing frequency). To the short wavelength side lie...
....the ultraviolet, X, and gamma radiations. It is certainly no accident that our eyes are sensitive only to optical radiation. For one thing, the strongest source of electromagnetic radiation in our cosmic neighborhood, the Sun emits most of its radiation in the optical range. Furthermore, water vapor, dust, ozone, and other molecules in the upper atmosphere absorb most of the radiation at other frequencies.
There is an opening in the atmospheric screen at radio frequencies, and it is there that the exploration of the invisible universe began in earnest in the early 1950s. Radio astronomers soon discovered, much to everyone's surprise, that stellar explosions, or supernovae, release tremendous amounts of energy in the form of high energy particles. Then followed the  discovery of radio galaxies and quasars, leading to the conclusion that explosive phenomena are taking place on a very large scale in galaxies as a whole and that high energy processes play a large and quite possibly a decisive role in the structure and evolution of our universe.
There had been a hint of this earlier, in the form of the cosmic ray particles that constantly bombard the top of Earth's atmosphere. Cosmic rays consist of bare hydrogen nuclei, or protons, together with a few heavier nuclei, also stripped of their electrons. They are distinguished from other particles that encounter Earth (in the solar wind, for example) by their high energy and by their arrival at Earth from all directions, which show that they cannot come from the Sun. Measurement of the numbers of cosmic rays striking Earth per second and of their energy indicate that high energy particles constitute a major component of the energetics of the galaxy.
Although it was realized in the 1930s that charged particles were coming into our atmosphere from space, it was not until the late 1950s that large high altitude balloons allowed experimenters to place their instruments above most of the absorbing atmosphere and study the basic properties of cosmic rays. Gamma rays and high energy X-rays (the distinction between the two is no more than a matter of terminology) can also be observed from high altitude balloons, but the more plentiful low energy X-rays are absorbed a hundred miles above the surface of Earth. It was not until the coming of the space age that experiments in the early sixties aboard rockets and satellites gave us our first glimpse of the X-ray universe.
The sky was found to be aglow with X-radiation, equally intense from all directions. The origin of this diffuse X-ray background is still a puzzle, but its uniformity over the sky implies that it was created at least ten billion years ago, about the time that galaxies were formed.
The early X-ray experiments also detected concentrated emission from several objects familiar to astronomers, such as the remnants of exploded stars and explosive galaxies. In addition, a new class of objects, X-ray stars, was discovered. These objects radiate almost all their energy in the X-ray band of the spectrum and have a total power output comparable to that of ten thousand suns. The nature of X-ray stars was a subject of lively debate and controversy among astronomers until a dramatic breakthrough was achieved in 1971. Scientists working with data from NASA's first X-ray satellite, UHURU, conclusively established that some and perhaps all X-ray stars are members of double star systems in which X-rays are generated by the infall of matter from a giant star onto a collapsed companion star. In many cases the collapsed star is apparently a neutron star. A neutron star is thought to be the collapsed remnant of an exploded star, a bizarre world of superstrong magnetic fields, and matter crushed to densities trillions of times greater than ordinary matter. (A grain of neutron star material 1/10 of an inch in diameter would weigh as much as an aircraft carrier.) Gamma ray observations indicate that neutron stars sometimes undergo cataclysmic  star quakes that release as much energy in a few seconds as the Sun releases in several years.
In some instances, the collapsed X-ray star is too massive to be a neutron star. In these cases it is believed that the stellar core has imploded or collapsed in on itself, forming a black hole in space, a region from which nothing can escape. Matter pulled off the companion star by the gravitational field of the black hole is compressed and heated as it swirls around the black hole, giving off a flash of X-rays before falling in.
As a result of these and other discoveries, high energy astronomy, that is, the study of X and gamma radiation and cosmic ray particles, has flourished as have few other fields in the history of astronomy. Physicists and astronomers have come to recognize it as an essential tool for investigating the laws of the universe. High energy observations give us access to regions of space that would be difficult if not impossible to study by traditional means. We can examine the surfaces of neutron stars, peer close to the event horizon of black holes, study the energy and chemical composition of particles produced in stellar explosions, probe the violent nuclei of galaxies to greater depths than ever before, trace the evolution of groups and clusters of galaxies by studying the hot gas they produce, and set limits on the size, structure, and evolution of the universe as a whole.
The sophistication of the tools of high energy astronomy has increased dramatically over the past two decades, culminating in the High Energy Astronomy Observatory (HEAD) program, which put three satellites into....
 ....orbit, each carrying over three thousand pounds of experiments. This program has transformed high energy astronomy from an interesting tributary or side branch to one of the main channels of astronomical research. In addition to providing information on phenomena that could be only poorly studied in the past, virtually every known astronomical object has been found to emit measurable quantities of high energy radiation. The HEAO experiments have made it abundantly clear that if we want to understand an astronomical object, we must look at its radiation at all energies, not just in the narrow optical band.
The HEAO program, then, has not only changed our knowledge of the astronomical universe, it has changed the way that astronomy is done. It has been, in the words of Herbert Gursky of the U.S. Naval Research Laboratory, "a landmark in NASA's space science program.' Of course, programs of that magnitude don't just happen. They require vision, planning, politicking, compromise, sacrifice, and years of hard work.