There is little doubt that most of the disc of the Galaxy is filled with cosmic rays. What is not certain is whether or not they are also found in large numbers hi the deeper reaches of extragalactic space, apart from such special places as quasars and radio galaxies. At present this question is controversial, but the more conservative view is that all except the most energetic cosmic rays are confined to and produced within the Galaxy. The main motive behind this attitude is the very large number of cosmic rays; at a typical point in the Galaxy there is as much energy hi cosmic rays as in starlight, with the result that cosmic rays play a significant role in the heating,
ionization and pressure balance of the interstellar medium. It is difficult enough to think of ways to produce the cosmic rays which fill the galactic disc; to make sufficient additional particles to fill the whole of the space between the galaxies is a daunting problem for all but the most courageous theoretical astrophysicists.

There may be several different kinds of celestial objects capable of producing cosmic rays, but it !H generally agreed that supernovae are probably the most important. The reason for this conclusion is the fact that supernova remnants are very strong sources of synchrotron radio emission and that they must therefore have the ability to generate their own high-velocity electrons. Some of these electrons, and presumably other particles such as protons, are ejected from the supernova remnant into the interstellar medium and contribute to the background of cosmic rays. The problem of how the particles become accelerated to such high velocities is un¬solved, but it is conjectured that either the electromagnetic fields of a rotating pulsar, or the shock waves in an expanding supernova shell may be powerful enough.

How long do the cosmic rays spend on their tortuous paths from their points of origin to the Earth ? An important clue is obtained from looking at the concentrations of the elements lithium, boron and beryllium in cosmic rays. We find that these elements are a million of times more common in cosmic rays than in the stars. This effect is the result of SPALLATION ; when a fast-moving cosmic-ray nucleus collides with a stationary atom of interstellar matter, nuclear reactions can take place. Most frequently, these reactions lead to the production of new atoms of the light elements. From the amounts of Li, Be and B found in cosmic rays it is deduced that an average cosmic-ray proton passes through 50kgnr2 of interstellar matter on its path to the Earth. The length of time it takes the proton to negotiate this much of the interstellar medium depends on the region of space it is confined to move within.. If the inter-stellar magnetic field restricts it to the disc of the Galaxy, a proton would have to live for 3 X 106yr and travel 106pc in order to pass through this much matter. If, as some astronomers believe, the cosmic rays travel in addition within the halo of our Galaxy occupied by the globular clusters then they must be more than 107 years old. In either case, the time is vastly greater than the few thousand years it would have taken a relativistic proton to travel in a straight line from a typical supernova to the Earth. This time difference of a thousand or more is an indication of the twistiness of the path that a cosmic ray takes as it transverses the Galaxy.

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