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Published online by Cambridge University Press: 14 August 2015
Cosmic rays were discovered in 1912, but it was only about forty years later that they were found to play an important role in astronomy. Firstly, cosmic rays (including the electron component) are an important source of astronomical information, namely the cosmic synchrotron radiation. Secondly, cosmic rays are essential as energetic and dynamical factors in the galaxy and also as a source of heating and transformation of the interstellar gas composition. Suffice it to remember, for example, that near the solar system the cosmic ray energy density is about the same as the thermal energy of the interstellar gas, and the cosmic ray pressure is likewise about the same as the interstellar gas pressure. Thus, there is every reason to believe that galaxies do not consist of stars and gas only, but of cosmic rays as well.