A high energy universe is almost by definition a violent one. That is too bad, you might say. Life would be far more pleasant in a nonviolent universe. Not necessarily. Violence has its uses, even for a species as fragile as our own. It is not simply a "force used so as to injure, damage or destroy," to give the dictionary meaning. It can be, indeed it is, a creative force in nature that has made life as we know it possible.
Without the violence of volcanos, life might never have spread across the face of Earth. Water, that essential ingredient for the development and sustenance of higher forms of life, might have remained forever locked deep in the interior of Earth if it had not been brought to the surface by volcanic eruptions and violent geysers.
Looking at things from a somewhat larger perspective, it is probable that the Sun and all the planets of the solar system would not be here but for violence on a much grander scale. The vast amorphous cloud of dust and gas from which our solar system was formed might still be a formless mass if an interstellar shock wave had not triggered the collapse of the cloud.
About once every 50 years, one of the massive stars in our galaxy blows itself apart in a supernova explosion. The force of the explosion hurls vast quantities of radiation and matter into space and generates shock waves that sweep through the arms of the galaxy. The shock waves heat the interstellar gas, evaporate small clouds, and compress larger ones to the point at which they collapse under their own gravity to form new stars. The recent discovery of meteorites with anomalous concentrations of certain isotopes indicates that a supernova might have precipitated the birth of our solar system more than four and a half billion years ago.
The cloud that collapsed to form the Sun and the planets contained an important legacy of stars that exploded even longer ago. Although composed mostly of hydrogen and helium, the cloud was enriched with carbon, nitrogen, and oxygen, elements that are essential for life as we know it. All the elements heavier than helium are manufactured deep in the interior of stars and would for the most part remain there if it were not for the cataclysmic supernova explosions that blow giant stars apart.
Supernovas are the creative flashes that renew the galaxy. They seed the interstellar gas with heavy elements, heat it with the energy of their radiations, stir it up with the force of their blast waves, and cause new stars to form. Some of these stars will be massive. They will rush through their life cycle, explode, dump a new supply of heavy elements into the gas, and...
...quite possibly trigger the formation of other stars to make other supernovas in a chain of violence that links the rise of life on Earth to events billions of years and half a galaxy away.
Once life was established on Earth, it might have been affected by supernova explosions in a more subtle but nonetheless crucial way. Supernovas produce clouds of high energy particles, the cosmic rays. Cosmic rays generated in supernovas and other processes form a background of high energy particles that continually rain down on Earth, producing many of the genetic mutations thought to be responsible for driving the evolution of the species.
The general picture that has been developed for the supernova explosion and its aftermath goes something like this. Throughout its evolution, a star is much like a leaky balloon. It keeps its equilibrium figure through a balance of internal pressure against the tendency to collapse under its own weight. The pressure is generated by nuclear reactions in the core of the star which must continually supply energy to balance the energy that leaks out in the form of radiation. Eventually the nuclear fuel becomes exhausted, and the pressure drops in the core. With nothing to hold it up, the matter in the center of the star collapses inward to higher and higher densities and temperatures until the nuclei and electrons are fused together into a superdense lump of matter known as a neutron star. As the overlying layers rain down on the surface of the neutron star, the temperature rises until,  with a blinding flash of radiation, the collapse is reversed. A thermonuclear shockwave runs through the now expanding stellar envelope, fusing lighter elements into heavier ones and producing a brilliant visual outburst that can be as intense as the light of 10 billion suns.
The shell of matter thrown off by the explosion plows through the surrounding gas, producing an expanding bubble of hot gas, with gas temperatures in the millions of degrees. This gas will emit most of its energy at X-ray wavelengths, so it is not surprising that X-ray observatories have provided some of the most useful insights into the nature of the supernova phenomenon.
More than 20 supernova remnants have now been detected in X-rays. Studies of these sources have provided the best estimates of the energy released in a supernova explosion. The powerful instruments aboard the HEAOs have gathered information that should clarify our understanding of the interaction of the expanding shell with the surrounding interstellar gas and the mechanism for producing cosmic rays. They should also help us to unravel such fundamental questions as how much of the ejected mass is in the form of heavy elements, and whether or not a neutron star is always left behind at the center of the remnant.
In the next few chapters the HEAO results on supernova research are discussed together with how they affect our understanding of such diverse problems as the creation of heavy elements, the origin of cosmic rays, the nature of pulsars, the formation of stars, and the large-scale structure of the galaxy.