An exploding star. To the sixteenth century mind this was as absurd as a flying elephant. It just did not happen. It was contrary to the order of nature according to which stars belong to the ". . . ethereal region of the celestial world which is free from change or corruption." The stars were symbols of the eternal and unchangeable, part of a system of permanence standing above the ever-changing, ever-corruptible world below.
Then, in November 1572, a star brighter than the planet Venus appeared suddenly in the constellation of Cassiopeia. It was noticed throughout Europe and in the Far East. The New Star or Nova of 1572 would shatter forever man's belief in the incorruptibility of the stars. The man most responsible for this rearrangement of the cosmic landscape was Tycho Brahe, a stormy, roisterous astronomer known for his acid tongue and silver nose.
Tycho, who wore a prosthetic silver nose to replace the one he had lost in a duel at age twenty, made accurate measurements of the position of the star relative to the other stars in Cassiopeia. For 18 months, though the brightness of the star declined steadily until it became invisible, its position remained fixed. This proved that the new star, or Stella Nova, belonged to the "eighth sphere" of the fixed stars.
Today, more than 400 years later, we use the word supernova to describe Tycho's object, even though we now know that it was not a new star at all. It had been there for tens of millions of years or more, invisible to the naked eye because of its distance of more than 6000 light years. It became visible at the end rather than the beginning of its evolution, as it underwent a catastrophic explosion.
The remnant of Tycho's supernova was singled out for study by the HEAOs because it is relatively young and is still glowing brightly in X-rays. We know when the supernova explosion occurred, and, thanks to Tycho, we know a good deal of its early history. It presents one of the best opportunities to study the interaction of the exploded star with the interstellar gas around it and to determine the mass and composition of the ejected material. Only through such comparisons can we check the theories that purport to explain the mechanisms of supernova explosions, the origin of the elements, and the origin of cosmic rays.
Most of the energy emitted by Tycho's supernova remnant falls in the X-ray band. There it gives off several billion times more X-rays than does the Sun and more energy than several hundred suns would emit at all...
...wavelengths. Relatively little energy comes from the supernova remnant in the form of optical radiation, and even less in radio waves.
That is not to say that the radio waves are unimportant. On the contrary, the sensitivity of giant radio telescopes is such that the remnant of Tycho's supernova was first discovered in 1952, using the Jodrell Bank radio telescope. Shortly thereafter, faint optical wisps in the same location were discovered using the 200-inch telescope at Mt. Palomar.
The radio discovery of Tycho's supernova remnant was a watershed in astronomy. It demonstrated the usefulness of radio telescopes in searching for the remnants of supernova explosions. The technique developed rapidly, until today no nebula is considered to be a certified supernova remnant unless it possesses the characteristic signature of radio emission of high energy electrons.
Observations show that the radio waves in supernova remnants are produced by the synchrotron process, named after a phenomenon first observed in synchrotron particle accelerators, namely, the emission of strongly polarized radiation by very energetic electrons spiraling along a magnetic field. The shells of supernova remnants are filled with magnetic energy and enormous numbers of charged particles bouncing around the shell at speeds very near the speed of light. This situation is not unique to Tycho's supernova remnant. Every supernova remnant somehow produces large quantities of extremely high energy particles. Clearly, supernova remnants are efficient particle accelerators and play an important role in producing the high energy particles, or cosmic rays, that constantly bombard Earth. How do they do it?
The initial explosion cannot explain the high energy particles observed in supernova remnants. Particles produced then will quickly lose their energy as they cool in the expanding cloud, or they would have escaped from the remnant. In two supernova remnants, the Crab Nebula and the Vela supernova remnant, a rapidly rotating neutron star, or pulsar, is generating large quantities of high energy particles. These objects appear to be the exception rather than the rule, however. In most cases the acceleration of particles to high energies must have something to do with the shock wave produced by the supernova explosion. It is here that X-ray astronomy has made an important contribution.
If the high energy electrons producing the radio emission are generated in a supernova shock wave, and the shock wave is known to produce a very high temperature X-ray emitting gas, then the X-ray and radio images of a supernova remnant should be very similar. The accompanying figures show that the X-ray and radio maps are strikingly similar. The X-ray and radio maps of two other supernova remnants known as Cassiopeia A, Puppis A, and SN 1006 also show a close correspondence between radio and X-ray images.
A close inspection of high resolution X-ray images of the supernova remnants shows that the X-ray emitting shell has broken up into many small fragments. It has been suggested that these fragments represent turbulent....
 ....magnetic eddies and that charged particles are accelerated to very high energies through collisions with the fragments. The acceleration process is analogous to that of a ping pong ball moving through a collection of randomly moving bowling balls. Over the course of many collisions, the ping pong ball will be accelerated to a very high speed. It is too early to say definitely that the size and number of fragments are adequate to generate the high energy electrons and magnetic field necessary to explain the radio emission and, ultimately, the cosmic rays that reach Earth, but a preliminary analysis looks promising.
By examining the radiation from young supernova remnants in detail, we can test other ideas about the role of supernova explosions in the scheme of things. For example, are the medium-heavy elements, such as oxygen, neon, silicon, sulfur, calcium and iron, produced there? If so, then we might expect to find an enrichment of these elements in the shells thrown off by supernova. Observations with the spectrometers of the Einstein Observatory have borne out this expectation. The gas producing the X-rays shows an excess of emission associated with the medium-heavy elements. The excess emission could be due to some peculiar nonequilibrium conditions in the shock wave. More likely, however, we are seeing a concentration of elements made inside a star and blown into the interstellar gas, where they will ultimately be mixed with that gas.
On Earth, these elements are so common-most of the dirt and sand is silicon-that we take them for granted. Yet they are not common at all in the universe at large and would never have been here, indeed we would never have been here, if it were not for a long chain of events that can be traced back to a star that exploded somewhere in the galaxy more than four and a half billion years ago. Today, we are gaining confidence that we understand some of the first and most fascinating links in this chain, thanks to the power of the HEAOs, and thanks also to the astronomer with the silver nose.